During the off-season, the main goal of a bodybuilder is to increase muscle size while minimizing increases in fat mass through the use of resistance training and maintaining positive energy balance (1). In order to accomplish this task, it is not only imperative for bodybuilders to consume a relatively high-protein diet (2) but one that is high in carbohydrates as well (3). The reason for this is multi-faceted.
Bodybuilders traditionally utilize higher-repetition, moderate load training five to six times per week to promote skeletal muscle hypertrophy (4). This style of training primarily relies on glycolysis to provide ATP and results in great reductions in muscle glycogen stores. It has been reported that glycolysis provides ~82% of ATP demands during one set of biceps curls taken to the point of muscular failure (5). Furthermore, it has been shown that a single resistance training session can result in reductions in muscle glycogen stores by as much as 24-40% (5,6).
As a result, adequate carbohydrate stores appear essential to fuel bodybuilding-style training and inadequate carbohydrate intake could impair performance and skeletal muscle hypertrophy adaptations. Indeed, a very-low-carbohydrate diet can limit the regeneration of ATP and the muscles’ ability to contract with high force (7,8).
Research has shown that a very-low-carbohydrate ketogenic diet is inferior to a higher carbohydrate diet for increasing muscle mass over an extended period (9,10). A possible explanation for these findings is that low glycogen availability, which is commonly observed with a very-low-carbohydrate ketogenic diet, may reduce the hypertrophic response to resistance training via inhibiting the mTOR pathway (11).
A high carbohydrate intake seems to be advantageous for other reasons as well. This includes its impact on recovery. The combination of carbohydrates and protein following resistance exercise could help reduce muscle damage (12). Sufficient carbohydrate intake is also critical to maintain immune health during intense high-volume training periods commonly employed by bodybuilders (13). Another benefit is that a surplus of calories from carbohydrates, in comparison to dietary fats, may lead to less accumulation of body fat(14).
The Role of Insulin
Carbohydrates have a potent effect on insulin secretion and this hormone can positively influence skeletal muscle through reducing muscle protein breakdown (MPB) and increasing net balance protein acquisition (15).
Insulin is primarily known for its role in nutrient storage. Some of its main metabolic effects include increasing the rate of transport of some amino acids into tissues, increasing the rate of protein synthesis in muscle and other tissues, decreasing the rate of protein degradation in muscle, increasing the rate of glucose transport across the cell membrane in muscle and adipose tissue, increasing the rate of glycolysis in muscle and adipose tissue, stimulating the rate of glycogen synthesis in muscle and other tissues, and inhibiting glycogenolysis and gluconeogenesis in the liver (16). For these reasons, insulin is generally referred to as an anabolic hormone that supports critical processes related to skeletal muscle adaptations.
While a caloric surplus consisting of a high proportion of carbohydrates is necessary to maximize accretions in muscle mass during a bodybuilder’s off-season, this style of diet can result in significant increases in body fat when used chronically (17). Excessive adipose tissue is an undesirable consequence for various reasons in this population.
A primary concern of accumulating excess body fat is an increase in the duration or the severity of subsequent contest prep periods (1). Another concern of a chronic energy surplus is its impact on metabolic health. Higher levels of body fat coincide with an increased risk of type 2 diabetes (T2D) (18). This condition is characterized by a state of insulin resistance in skeletal muscle as well as impaired β-cell function.
Insulin resistance is defined as an experimental or clinical condition in which insulin exerts a biological effect lower than expected. In insulin resistance, the target cells fail to respond to ordinary levels of circulating insulin (19). There is an impairment of glucose uptake in muscle accompanied by impaired glycogen synthesis and protein catabolism (19). This manifests as hyperglycemia both in the fasted and fed state.
According to the American Diabetes Association, insulin resistance is evidenced by a fasting plasma glucose level ≥ 126 mg/dL, a two-hour postprandial plasma glucose ≥ 200 mg/dL following an oral glucose tolerance test of 75g of glucose, or an HbA1c ≥ 6.5% (20). Other markers of insulin sensitivity have been shown to be efficacious as well. This includes fasting insulin concentrations, which have been shown to significantly correlate with the metabolic clearance rate for glucose in healthy subjects (21).
It is generally accepted that the “gold standard method” to measure insulin resistance is through the hyperinsulinemic euglycemic clamp (22). This procedure assumes that at high doses of insulin infusion, the hyperinsulinemic state is sufficient to completely suppress hepatic glucose production and that there is no net change in blood glucose concentrations under steady state conditions (22). Under such conditions, the rate of glucose infused is equal to the rate of whole body glucose disposal or metabolizable glucose and reflects the amount of exogenous glucose necessary to fully compensate for hyperinsulinemia (22).
The Homeostatic Model Assessment of Insulin Resistance (HOMA-IR) and Quantitative Insulin Sensitivity Check Index (QUICKI) are other popular tools to measure insulin sensitivity. Both of these methods use fasting glucose and insulin concentrations to assess insulin resistance and have been reported to correlate reasonably well with the results of clamp studies (23).
Insulin Sensitivity Affects Nutrient Partitioning
Individuals with normal weight and glucose tolerance are highly sensitive to insulin. In this population, skeletal muscle takes the major part of glucose uptake during hyperinsulinemia (75-80%), whereas a decrease in muscle glucose uptake is observed in insulin resistant individuals (24,25).
It has been hypothesized that insulin resistance may protect skeletal muscle tissue from glucose overload by restraining glucose entry into cells (26). As a consequence of reduced uptake of nutrients in skeletal muscle, the storage of excess nutrients will be favored towards adipose tissue. Indeed, adipose tissue can act as a “glucose sink,” especially during periods of caloric excess (27). Very recent data has displayed a shift from insulin-induced skeletal muscle perfusion towards insulin-induced adipose tissue perfusion during the early stages of high caloric intake (28).
Mitrou et al., compared glucose uptake in adipose tissue and skeletal muscle between obese and nonobese subjects (29). It was estimated that total muscle glucose uptake in nonobese subjects was equivalent to approximately 41% of meal carbohydrate ingested, whereas whole-body adipose tissue uptake was only 7%.
In the obese subjects, only about 19% of ingested carbohydrate was taken up by muscle, while 17% was taken up by adipose tissue (29). This is in agreement with previous studies suggesting adipose tissue is not a major consumer of glucose in nonobese subjects (30), and an increase in the contribution of adipose tissue to whole-body glucose uptake occurs in those with obesity and/or T2D (31).
There is a strong link between body mass index (BMI) and body fat with abnormalities in glucose metabolism (32,33). Even in average fit, non-diabetic subjects, a correlation between fat mass and fasting blood glucose has been demonstrated (34).
It cannot be certain whether insulin resistance is primarily driving fat accumulation, or if the presence of more fat is causing insulin resistance. Regardless, the two are strongly correlated and insulin sensitivity may play a primary role in the composition of weight-gain.
In line with these observations, it appears an individual’s baseline body composition is highly reflective of the composition of weight gain with overfeeding and this may be largely attributed to differences in insulin sensitivity. It has been reported that individuals with a higher initial body fat gain a smaller proportion of weight from fat free mass (FFM) (30-40%), while the weight gain of lean individuals comprises a much higher proportion of FFM (60-70%) (35).
Related to this, it’s been shown that individuals with lower carbohydrate oxidation in response to insulin infusion during the hyperinsulinemic-euglycemic clamp experienced greater increases in fasting insulin, glucose, and HOMA-IR values after three days of overfeeding (36). These subjects also gained more total weight after 28 days of overfeeding (36).
Together, the above findings suggest that greater insulin sensitivity could lead to superior body composition outcomes, or a more desirable proportion of FFM to fat mass with weight gain. In addition, less insulin sensitive individuals may efficiently store a higher percentage of excess calories in adipose tissue.
During periods of positive energy balance, both FFM and fat mass will contribute to changes in body weight. In the ideal scenario for a bodybuilder, insulin sensitivity would be high in skeletal muscle, resulting in the bulk of calories being funneled towards this tissue, and insulin sensitivity would be poor in adipose tissue, making it more difficult to store calories there (37).
As such, for a bodybuilder to make the most of their off-season, it seems relevant to utilize a nutritional strategy that best manages insulin sensitivity.
The Dynamic Nature of Insulin Sensitivity
Insulin sensitivity has been observed to rapidly adjust in response to substrate availability. Significant changes in the action of insulin have been observed with small perturbations in energy balance and without substantial changes in body weight over short time frames.
One day of overfeeding by 30% of estimated energy requirements (EER) has been shown to induce hepatic insulin resistance and increase nocturna plasma glucose and insulin concentrations in overweight/obese men (38,39).
In another 24-hour overfeeding study, the effects of a more severe protocol (78% greater than daily energy requirements) in young healthy adults was examined. The results displayed a 28% decrease in whole-body insulin sensitivity as well as significant increases in plasma glucose and serum insulin in response to an oral glucose tolerance test (40).
Significant reductions in insulin sensitivity have also been observed with three days of overfeeding in healthy subjects, and these consequences were only partially rescued by a bout of exercise (41).
This dynamic nature of insulin sensitivity has been documented during acute periods of energy restriction as well. Total or modified fasting for 24-hours has been shown to significantly alter fasting glucose and insulin concentrations (42,43).
Considering the fact that insulin sensitivity rapidly adjusts in response to changes in energy intake, a nutritional strategy that modulates or “cycles” calories over the course of the week, such as intermittent fasting, could have a favorable effect on insulin sensitivity.
Impact on Insulin Sensitivity
In recent years, intermittent fasting has attracted widespread interest as a dietary strategy to improve cardiometabolic outcomes and manage body weight (you can read more about the different types of intermittent fasting here).
Over an acute timescale, severe energy restriction through complete fasting or modified fasting (energy restriction ≥ 70% of EER) elicits reciprocal shifts in fuel utilization in a seemingly dose-responsive manner; free fatty acid mobilization, fatty-acid oxidation, and ketogenesis progressively rise, favoring the conservation of glucose as its demand drops (44).
From a chronic perspective, it has been speculated that these acute disturbances in fuel management may augment improvements in insulin sensitivity via the preferential reduction in the ectopic accumulation of lipid and associated cytosolic intermediaries, which are causally implicated in the development of insulin resistance (44).
Recent data has emerged demonstrating that intermittent fasting can significantly decrease fasting glucose levels and lower HOMA-IR independent of weight loss (45). Prior research on the topic has also confirmed marked improvements in insulin sensitivity can occur with energy restriction long before significant weight loss takes place (46,47,48,49).
As previously alluded to, there are many different types of intermittent fasting, each featuring a different duration and amount of energy restriction. For these reasons, some variants might do a better job of improving insulin sensitivity than others. In order to examine the efficacy of each method, it’s important to assess their effects on insulin sensitivity in both the fasted and postprandial state.
While it has been observed that complete fasting can improve fasted markers of insulin sensitivity (43), similar improvements have been observed with less severe energy restriction (42). In a study by Antoni and colleagues, the early metabolic responses to varying degrees of energy restriction were examined. Similar improvements in fasted blood glucose and insulin with partial (75%) and total (100%) energy restriction were found (50).
In further consideration, complete fasting has been shown to result in a subsequent reduction in insulin sensitivity as evidenced by hyperinsulinemic-euglycemic clamp (51,52). A worsening of insulin sensitivity following a fast has also been evidenced by significantly increased glucose and insulin responses to an oral glucose tolerance test (51,53).
In combination, the above findings have raised concern about the frequency and severity of energy restriction. It has been hypothesized that approaches such as alternate day total fasting may present too great of a metabolic challenge and could lead to aberrant, rather than beneficial, changes in peripheral insulin sensitivity over time (44).
Indeed, intermittent fasting interventions in human and rodent subjects that have reported consequences such as impaired glucose tolerance, increased markers of oxidative stress, reduced GLUT content, and/or reduced mitochondrial markers in skeletal muscle, have all used alternate day total fasting protocols (44).
The “5:2” Diet as a Solution?
Another form of intermittent fasting, known as the “5:2” diet, has shown more encouraging outcomes and could be a viable strategy for bodybuilders in the off-season.
The 5:2 diet, commonly referred to as intermittent energy restriction (IER), features five days of regular eating patterns interchanged with two days of “fasting” or an energy intake of approximately 25% of EER (54). This method has been found to result in superior improvements in insulin concentrations as well as HOMA-IR compared to continuous energy restriction with equal weight loss between dietary interventions (55).
Evidence of the beneficial effects of IER primarily stems from research conducted by Harvie et al. (56,57). The first of these experiments was conducted in a population of 107 overweight or obese women without diagnosed diabetes or cardiovascular disease (56). The participants were randomized to a 5:2 diet (IER) or continuous energy restriction (CER) for six months.
At the end of the intervention, there was comparable weight loss between groups with a non-significant greater decline in waist measurement in the IER group. In addition, while both groups experienced declines in fasting insulin and HOMA-IR, greater reductions were observed in the IER group.
In another trial led by Harvie, 115 overweight women were randomized to one of three dietary interventions for a three-month weight loss phase (57). This was followed by a one-month weight maintenance period.
Each of the diets aimed to achieve an energy deficit of 25% of EER over the course of the week. This was done through either daily energy restriction (DER), a 5:2 format (IECR), or a dietary pattern very similar to the IECR protocol, but with unlimited protein and fat (IECR + PF).
At the end of the three-month weight loss period, there was similar weight loss between conditions. The IECR and IECR + PF groups experienced significantly greater reductions in body fat than the DER group. The greatest reductions in fasting serum insulin and HOMA-IR were seen in the IECR group, with similar reductions observed between IECR + PF and DER.
During the weight maintenance period, the two IECR groups reduced the frequency of their energy restriction days from twice to once per week and this was found to be sufficient to preserve improvements in insulin sensitivity.
In addition to these trials, Wegman et al. found that a diet alternating days of 175% of normal caloric intake with 25% of normal caloric intake was able to decrease fasting insulin during a period of weight stability (58). While these results are interesting, it is worth mentioning that this study did not feature a control or comparator condition. Other trials examining differences in insulin sensitivity between a 5:2 regimen and CER have failed to find any significant differences in insulin sensitivity between conditions (59,60,61).
It is difficult to extrapolate the findings of these trials to the average bodybuilder. The primary reasons for this include the populations studied, the goal of the dietary intervention, and heterogeneity in the markers used to determine insulin sensitivity.
In contrast to bodybuilders, the subjects featured in these studies tend to be overweight with low levels of muscle mass. Second, the overwhelming majority of interventions use IER as a strategy to induce weight loss. To the author’s knowledge, there are currently no published trials that use IER to facilitate a state of positive energy balance. Lastly, a number of trials reporting null findings only measured fasted blood glucose (62,63), which may be insufficient to determine changes in insulin sensitivity. With these issues in mind, it’s clear more direct research is needed to prove the efficacy of cycling calorie intake over the course of the week during periods of positive energy balance.
Nonetheless, concerning the data showing superior improvements in markers of insulin sensitivity with similar weight loss between dietary interventions (56,57), the ability to improve fasting insulin during weight maintenance (58), and an upregulation of beneficial pathways with caloric restriction that may be independent of changes in body fat (64,65,66), it remains conceivable that incorporating acute bouts of energy restriction could have advantages over a continuous calorie surplus.
For the aforementioned reasons, a 5:2 diet could better preserve insulin sensitivity over the course of a bodybuilder’s off-season, and ultimately, lead to superior body composition outcomes. The following sections will cover how to potentially set up a 5:2 diet. This includes recommendations for energy intake, macronutrient distribution, and food choices for the five energy surplus (high) and two energy restricted (low) days.
Current evidence indicates that an energy surplus is required to maximize increases in FFM with resistance exercise (67), but the size of the energy surplus should differ depending on the training status of the individual.
It has been demonstrated that untrained subjects can benefit from a very large energy surplus upwards of 2,000kcal per day (68). In the advanced lifter, a surplus of this magnitude will likely lead to significant increases in body fat without concomitant increases in FFM (69). As such, it has been recommended that novice or intermediate bodybuilders consume an energy surplus of 10-20% above EER and advanced bodybuilders consume an energy surplus of 5-10% above EER (1).
An intense resistance training workout results in the depletion of a significant portion of stored fuels and causes damage to muscle fibers. Following a bout of resistance exercise, exogenous nutrients are required to create an anabolic environment to support muscle hypertrophy (70).
The post-exercise period has been commonly referred to as the “anabolic window.” Theoretically, consuming the proper ratio (or a greater proportion) of nutrients during this time not only initiates the rebuilding of damaged tissue and restoration of energy reserves, but it does so in a supercompensated fashion that enhances both body composition and exercise performance (70).
Research in this area predominantly centers around the impact of different quantities and qualities of protein and carbohydrates following exercise, with protein playing the star role due to its profound effects on muscle protein synthesis (MPS) (71).
In regards to carbohydrates, provided dietary protein is supplied at a level that would optimize MPS, carbohydrate co-ingestion does not appear to have an additive effect on post-exercise muscle protein synthetic rates (71), though it may promote greater net anabolism via a suppression of MPB (72,73). In addition to what some may consider a trivial effect, there are definitive benefits of carbohydrate ingestion post-exercise for glycogen resynthesis.
After examining the totality of the evidence, the International Society of Sports Nutrition has reported that the scientific literature currently supports the consumption of carbohydrates plus protein in close proximity to a training session to optimize the adaptive response to exercise as well as recovery for subsequent training bouts (12).
Another reason to funnel a large proportion of the daily carbohydrate allotment towards the post-exercise period is related to increases in insulin sensitivity. An increase in glucose tolerance post-exercise may last up to 48 hours (74).
Following exercise, muscle glycogen synthase activity and glucose transport are increased, and an enhanced metabolic action of insulin has been reported (74). These changes appear to be closely tied to the amount of glycogen depletion that occurs during the exercise bout (74). Put simply, the lower the glycogen content post-exercise, the stronger the response to insulin.
For these reasons, it’s likely advantageous to dose a large proportion of daily carbohydrate intake towards the post-workout period, and this should scale upwards with the volume and intensity of the training session. Portioning carbohydrates towards this window of time should result in a better postprandial glucose profile, improved recovery, and a greater likelihood that this energy will be funneled towards skeletal muscle as opposed to adipose tissue.
Most bodybuilders perform resistance exercise about five times per week (4). In comparison to a continuous energy surplus, the utilization of a modified 5:2 diet that aligns training days with high days will lead to a greater number of calories to portion towards the anabolic window and thus, could maximize skeletal muscle adaptations.
As detailed above, bodybuilders should consume a relatively high-carbohydrate diet for a multitude of reasons. It has been recommended that bodybuilders consume approximately 4-7 g/kg/day (3) or roughly 55-60% of energy intake from carbohydrates (75).
In the context of adequate energy intake, a minimum protein intake of 1.6 g/kg/day seems sufficient to support increases in FFM (76). With that being said, for those looking to leave no stone unturned and ensure maximum skeletal muscle adaptations from resistance exercise, it’s probably worthwhile to pursue a higher intake of protein in the area of 2.2 g/kg/day (2,76). Within these guidelines for total protein intake, it is recommended that this amount be split between at least four meals, each containing 0.4-0.55 g/kg of protein (77).
Dietary fat should comprise roughly 20% of total daily energy intake (75) or a minimum of 0.5 g/kg/day (1). Fat is an essential nutrient vital for many functions in the body, but not much is known about its role in regard to skeletal muscle hypertrophy. Considering the significant impact of protein and carbohydrates on skeletal muscle adaptations, and the absence of data suggesting any positive effect of increasing dietary fat intake, it seems reasonable to consume the minimum amount of fat necessary to ensure an adequate consumption of essential fatty acids and fat-soluble vitamins (78) in order to displace the most amount of energy possible towards the other macronutrients.
Low days restrict energy intake in order to promote acute increases in insulin sensitivity. In theory, these acute increases could lead to improved management of insulin sensitivity in the long-term, and thus, help maximize body composition outcomes during a bodybuilder’s off-season.
There are two primary factors that must be considered when establishing the size of the deficit for low days: its effects on markers of insulin sensitivity and its effects on muscle protein synthesis (MPS).
In comparison to total fasting, it appears that a modified fast (~25% of EER) is not only sufficient for improving insulin sensitivity, but superior (50).
In a trial by Assali et al., subjects were randomly assigned to one of two hypo-energetic diets for six weeks. The low calorie diet featured an energy deficit of 33% of EER and the very-low calorie diet featured an energy deficit of 66% of EER (46). This was followed by a four-week weight maintenance phase and then six weeks on the alternate hypo-energetic diet.
As a result of the intervention, significant increases in insulin sensitivity occurred. Most notably, two-thirds of this increase was seen during the first week of each hypo-energetic period. This suggests that an energy deficit of 33% of EER may be sufficient to promote acute increases in insulin sensitivity.
In the previously mentioned study conducted by Harvie et al., it was found that the 5:2 intervention that allowed ad libitum protein and fat consumption (IECR + PF) resulted in an energy deficit of approximately ~50% on “fasting” days and these bouts of energy restriction were sufficient to evoke acute reductions in insulin resistance (57).
While the aforementioned energy deficits might be sufficient to induce acute increases in insulin sensitivity, this amount has also been shown to compromise the muscle protein synthetic response. It has been demonstrated that a small energy deficit of 20% of EER can decrease basal MPS by 19% (79) and increase muscle fractional breakdown rate by 60% (80).
However, numerous trials have now conclusively shown that resistance exercise combined with a protein intake of 1.6-2.4 g/kg/d can mitigate the decrease in MPS observed with energy restriction (81,82,83). Furthermore, increases in lean body mass have been documented in subjects undergoing a 40% energy deficit accompanied by a high protein diet (2.4 g/kg/d) and resistance exercise (84).
While direct research is needed to tease out the nuances of this subject, the above findings suggest that an energy deficit of approximately 40%, in unison with resistance exercise and a minimum protein intake of 2.4 g/kg, is likely sufficient to induce acute increases in insulin sensitivity without compromising gains in FFM.
Similar to this concept, Menno Henselmans has proposed the idea of utilizing a protein-sparing modified fast outside of the anabolic window (i.e. rest days) when elevations in MPS and insulin sensitivity are no longer evident (85). Anecdotally, he has reported that an energy intake of about 10x a protein intake of 2.2 g/kg is a sufficient minimum sustainable energy intake that does not possess an increased risk of muscle loss.
In order to counteract the reductions in MPS and increases in MPB observed with energy restriction, it is crucial to consume a higher protein intake on low days. The current evidence suggests that a minimum of 2.4 g/kg/day is likely sufficient to ensure the preservation of FFM (82,84), with an upper limit 3.5 g/kg/day being potentially beneficial as long as it does not displace sufficient fat or carbohydrate intake (86).
For the ratio of carbohydrate to fat, in contrast to high days, a large proportion of energy from carbohydrates should be replaced by fat. More specifically, by unsaturated fat.
In a study conducted by Gadgil et al., participants followed three different diets for six weeks each (87). The CARB diet provided 58% of energy from carbohydrates, 15% from protein, and 27% from fat. The PROT diet replaced 10% of carbohydrate calories with protein and provided 48% of energy from carbohydrates, 25% from protein, and 27% from fat. The UNSAT diet replaced 10% of carbohydrate calories with unsaturated fat and provided 48% of energy from carbohydrates, 15% from protein, and 37% from fat.
The increase in calories from fat primarily came from an increase in monounsaturated fat (MUFA) and featured sources such as olive, canola, and safflower oil as well as nuts and seeds. Each diet provided 6% of calories from saturated fat (SFA) and were designed to maintain a constant body weight.
At the end of the trial, the researchers found that the UNSAT diet resulted in improvements in insulin sensitivity (as evidenced by improved QUICKI and HOMA-IR) to a greater degree than the CARB and PROT diets. These improvements were deemed both statistically and clinically significant.
In further analysis of the results, it was revealed that normal weight participants experienced a greater increase in insulin sensitivity with the UNSAT diet compared to the CARB diet. This led the researchers to suggest that the choice of macronutrient to prevent insulin resistance is less important than overall weight loss in obese individuals, whereas partial replacement of carbohydrate with unsaturated fat can mitigate risk in those of normal weight.
The findings of this trial align closely with more recent reviews on the topic. In a systematic review and meta-analysis of randomized controlled feeding trials, MUFA consumption did not appear to significantly influence fasting glucose concentrations compared to other macronutrients. However, it was reported to reduce HbA1c and improve HOMA-IR in comparison to carbohydrate and SFA (88).
In addition, isocaloric replacement of 5% of energy intake from either carbohydrates or SFA with polyunsaturated fat (PUFA) resulted in decreases in fasting plasma glucose, fasting plasma insulin, and HbA1c, and improved HOMA-IR and insulin secretion capacity (87). The improvement of some endpoints, such as insulin secretion capacity, remained even when substituting PUFA for MUFA.
In another meta-analysis of randomized controlled feeding trials, the effect of plant-derived PUFA on markers of glucose metabolism and insulin resistance were examined primarily in non-diabetic subjects (89). It was found that plant-derived PUFA significantly decreased insulin concentrations and HOMA-IR. Moreso, subgroup analysis showed that studies with the highest PUFA dose (≥ 9% of energy increase) resulted in the greatest effects.
Corresponding with these data points, large cohort studies have shown that circulating plasma PUFA levels, specifically, omega-3 and the omega-6 unsaturated fatty acid linoleic acid (18:2 ⍵-6), are inversely associated with T2D (90,91).
The purported mechanism behind these findings is that PUFA can influence the action of insulin and modulate fuel partitioning in the cell. Increased PUFA intake may favorably alter cellular functions including membrane fluidity, ion permeability, and insulin receptor binding/affinity (92). PUFA can also upregulate the genes involved in lipid oxidation and downregulate the expression of genes coding for lipogenetic enzymes (93).
Liver fat content is a major determinant of insulin resistance and research on the topic suggests that the source of dietary fat determines its contribution to liver fat content.
In trials that compared consuming excess energy from SFA (palm oil) or PUFA (sunflower oil), significantly greater increases in liver and visceral fat content were found when additional calories were consumed from SFA, despite equal weight gain between groups (94,95).
Similar findings have been reported after ten weeks of an isocaloric diet either high in vegetable omega-6 PUFA or SFA mainly from butter (96). SFA has even been shown to result in a greater accumulation of liver fat than free sugars from candy and sugar-sweetened beverages (97).
The current evidence indicates improvements in insulin sensitivity through an increase in energy intake from unsaturated fat, with the most robust improvements observed from increases in PUFA (omega-3 and omega-6 linoleic acid, specifically). This finding withstands whether you replace SFA or carbohydrates with PUFA (88). Concerning the primary importance of sufficient protein intake on low days, energy from carbohydrates will be reduced to account for increases in dietary fat.
How Much Dietary Fat?
Current evidence suggests that a very-high-fat or ketogenic diet has detrimental effects on insulin sensitivity. More specifically, while these diets have been shown to improve fasted markers of insulin sensitivity (98), they have also been shown to consistently result in impaired glucose tolerance.
In a study by Numao et al., healthy men consumed either a normal diet (67% carbohydrate, 22% fat, and 11% protein) or a low-carbohydrate high-fat diet (20% carbohydrate, 69% fat, and 11% protein) for three days each (99). An oral glucose tolerance test was performed after each three-day dietary intervention.
The results displayed significantly higher plasma glucose concentrations during the oral glucose tolerance test in the low-carbohydrate high-fat condition.
In another trial conducted in healthy men, subjects consumed three experimental diets (low-carbohydrate high-fat, intermediate-carbohydrate intermediate-fat, or control) for three days each (100). An oral meal tolerance test was performed after each three day dietary intervention.
The control diet (C) consisted of 10% of daily energy intake from protein, 30% from fat, and 60% from carbohydrates. The intermediate-carbohydrate intermediate-fat diet (ICIF) consisted of 10% of daily energy intake from protein, 50% from fat, and 40% from carbohydrates, and the low-carbohydrate high-fat diet (LCHF) consisted of 10% of daily energy intake from protein, 70% from fat, and 20% from carbohydrates.
In comparison to C, it was found that the LCHF diet excessively increased postprandial glucose concentrations and the intake of ICIF and C had similar effects on postprandial glucose metabolism.
Other trials using an intravenous glucose tolerance test (101) or euglycemic hyperinsulinemic clamp (102), did not observe decreases in insulin sensitivity with a diet containing ~50% of daily energy intake from fat compared to a high-carbohydrate control diet.
Overall, it appears that a short-term intake of a diet containing ≥70% of daily calories from fat may aggravate postprandial metabolism, whereas a diet containing <50% of daily calories from fat may have little effect (100).
These results can be largely explained by changes in the enzymatic regulation of glucose disposal in human skeletal muscle. Glycogen synthase and pyruvate dehydrogenase are the rate-limiting enzymes that control glycogen synthesis and glucose oxidation in skeletal muscle (103).
Decreased skeletal muscle glucose disposal via rapid reductions in pyruvate dehydrogenase has been observed with as little as 56-hours of a high-fat low-carbohydrate diet (103). In addition, glucose intolerance may be related to hepatic fat accumulation, and as a consequence, hepatic insulin resistance, resulting in the inability of insulin to suppress hepatic glucose output (104).
The above information might not be relevant to dedicated followers of the ketogenic diet, as they would not be experiencing oscillations in glucose-stimulated insulinemic conditions, but it is particularly relevant in the context of a modified 5:2 diet where low days are followed by high days with large volumes of carbohydrates.
The long-term health effects of acute impairments in glucose tolerance are currently unknown, but research on the matter suggests an association between postprandial hyperglycemia and increased cardiovascular risk and mortality (105,106,107,108). Moreso, acute fluctuations in blood glucose levels can induce vascular endothelial dysfunction (109), oxidative stress, and inflammation (109).
In consideration of these findings, a dietary fat intake of approximately 35-40% of calories on low days could be best in order to prevent blood glucose excursions on the days following.
Food Choices: The Role of Polyphenols
A dietary fat intake of 35-40% of total energy closely aligns with what is found in the Mediterranean diet (MedDiet). The MedDiet is a plant-based, fat-rich dietary pattern characterized by a high intake of fruits, vegetables, legumes, fish, whole-grains, nuts, and olive oil; moderate consumption of dairy products and wine; and low intake of red and processed meats and foods that contain high amounts of added sugars.
Both epidemiological and intervention studies have shown promising results from following a MedDiet. This includes the diet’s ability to positively impact cardiometabolic outcomes and decrease the risk for conditions such as metabolic syndrome, cardiovascular disease, and diabetes (110). Higher adherence to a MedDiet is not only associated with a lower risk of T2D, but also with a 35% lower risk of impaired fasting glucose (111), and lower insulin concentrations in non-diabetic subjects (112).
In further advocacy of following a dietary approach similar to the MedDiet on low days, the primary carbohydrate sources in the diet have been shown to positively influence insulin sensitivity. Observational and interventional investigations provide evidence that habitual intake of vegetables and legumes enhances insulin sensitivity in non-diabetic subjects (113). In addition, a high-intake of insoluble cereal fibers, or whole-grain products rich in cereal fibers, may reduce insulin resistance and the risk of developing T2D by 20-30% (114).
The nutrient density in the complex matrices of plant foods, together with a low glycemic load, likely explain the lower risk of metabolic syndrome and T2D observed with higher adherence to the MedDiet (115). The rich composition of bioactive nutrients and phytochemicals within the diet also seem to positively impact cardiometabolic health via their synergizing effects on different metabolic pathways (115).
Polyphenols are a large heterogeneous group of phytochemicals containing phenol rings and are divided into flavonoids, phenolic acids, stilbenes, and lignans (116). Flavonoids are classified into flavones, flavonols, flavanols, flavanones, isoflavones, and anthocyanins (116). Polyphenols are the most abundant antioxidants in the diet and their intake has been associated with a reduced incidence of T2D, anti-inflammatory effects, and improvements in insulin sensitivity (117). Furthermore, there appears to be a dose-response nature between total intake of polyphenols and the magnitude of their effect in some instances (118).
Several potential mechanisms have been proposed to explain the hypoglycemic effects of dietary polyphenolic compounds. This includes the inhibition of glucose absorption, as well as the stimulation of insulin secretion, and protection of pancreatic β-cells against glucotoxicity (119). Polyphenols may also improve glucose uptake in peripheral tissues by modulating intracellular signaling (120).
Among all polyphenols, the beneficial effects of flavanols and their primary food sources (cocoa, chocolate, and red wine), have been the most extensively examined. In two separate meta-analyses, improvements in insulin sensitivity have been reported with increased intakes of cocoa and chocolate over a period of 2-18 weeks (121,122). Another source of flavanols, green tea, has been thoroughly studied. In a meta-analysis of 17 randomized controlled trials (RCTs), significant reductions in fasting glucose and HbA1c were reported with green tea consumption (123).
Following suit with other popular beverages, coffee has been shown to have positive effects on insulin sensitivity. Its benefits are primarily attributed to its chlorogenic acid content. Numerous reviews have associated the habitual consumption of caffeinated or decaffeinated coffee with a lower risk of T2D (124,125). Results from the Nurses’ Health Study II reported that, compared to nondrinkers, a daily intake of at least four cups of coffee was associated with a 47% lower incidence of T2D (126). The observed positive effects of coffee on T2D risk might also be independent of BMI (127).
Red wine is rich in phenolic compounds including flavonoids and stilbenes like resveratrol. In a randomized crossover trial comparing the effects of a moderate intake of red wine, an equivalent amount of dealcoholized red wine, and gin for four weeks, it was found that red wine with or without alcohol, but not gin, improved glucose metabolism as measured by HOMA-IR (128). In another study, it was found that consuming 150 mL of red wine with dinner modestly reduces cardiometabolic risk in type 2 diabetics (129).
Transitioning to foods rich in polyphenols, extra-virgin olive oil includes several phenolic components such as oleuropein and hydroxytyrosol, flavones, and lignans (115). Evidence from the PREDIMED trial has shown that a MedDiet supplemented with extra-virgin olive oil can reduce fasting plasma glucose, and improve insulin resistance and inflammatory biomarkers (130,131).
The impact of nut consumption on insulin sensitivity has been assessed in many clinical trials with a variety of subject characteristics and intervention durations. Several trials have mainly tested the effects of walnuts and almonds and mixed results have been reported (132). In comparison, a meta-analysis of RCTs found that the consumption of tree nuts (~50 g/d) can modestly decrease triglycerides and fasting blood glucose (133).
Total fruit and vegetable intake is inversely associated with a variety of adverse health outcomes (134) and specific plants may possess unique benefits. For example, an increase in the consumption of green leafy and cruciferous vegetables is associated with a significantly reduced risk of T2D (135,136).
Muraki et al. assessed the relationship of fruit intake with T2D risk and found the strongest inverse associations for blueberries, grapes/raisins, and apples/pears (137). Anthocyanins, a sub-class of flavonoids, appear to be especially potent.
After pooling the results of three large cohorts, a higher intake of anthocyanin-rich fruits were found to be associated with a decreased risk of T2D, even when adjusting for multiple potential confounders (138). Berries are a particularly abundant source of anthocyanins. Blueberries have been shown to significantly improve insulin sensitivity measured by hyperinsulinemic-euglycemic clamp (139).
In this article, a modified version of the 5:2 diet is proposed as a nutritional strategy for bodybuilders in the off-season. In comparison to a continuous energy surplus, it is hypothesized that incorporating acute periods of energy restriction, combined with shifts in macronutrient distribution, could better preserve insulin sensitivity during a phase of positive energy balance, leading to a more desirable proportion of FFM to fat mass with weight gain.
While this model is largely speculative, and as such, needs direct research to prove its efficacy and effectiveness, the body of evidence suggests that this could be a plausible way to maximize body composition outcomes. Furthermore, if this approach is in fact superior, the effects would likely be trivial, and only begin to surface over an extended period of time. For these reasons, this is a nutritional strategy reserved for advanced athletes looking to eek out every percent advantage they can gain on their competition, or those who simply prefer to cycle their calorie intake over the course of the week.
|Calories||Novice to intermediate: 10-20% above EERAdvanced: 5-10% above EER|
|Protein||Minimum 1.6 g/kg (aim for 2.2 g/kg/d if possible within calorie constraints)|
|Fat||~20% of total calories (minimum of 0.5 g/kg)|
|Carbohydrates||55-60% of total calories (at least 4 g/kg if possible within calorie constraints)|
|Diet Component||Recommendation||Food Choices|
|Calories||40% below EER|
|Protein||Minimum 2.4 g/kg|
|Fat||35-40% of total calories (emphasis on PUFA)||Vegetable oils (extra virgin olive, canola, soybean, safflower, etc.), nuts, seeds, dark chocolate, avocado, fatty fish (salmon, sardines, etc.), dairy (cheese, yogurt, etc.), egg yolks.|
|Carbohydrates||Remaining calories||Whole-grain starches (pasta, oatmeal, brown rice, etc.), legumes (peas, beans, lentils, etc.), fruits (berries, grapes, apples, etc.), vegetables (green leafy, broccoli, brussel sprouts,, etc.)|
Intermediate bodybuilder (energy surplus 15% above EER)
- EER = 2500 calories
- Total weekly calories = 20,125 calories
- Low day (2) = 1500 calories
- High day (5) = 3425 calories
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