Walk into any gym or simply come in contact with any person who has struggled to lose weight and within any given conversation, you may hear something like “it’s hard for me to lose weight because I have a slow metabolism.” Something I notice amongst those who make this remark is that they don’t understand all that actually comprises metabolism. Their statement is purely based off of the concept of resting metabolic rate (basically the same thing as basal metabolic rate, just measured slightly differently, but for the sake of consistency, I’m going to stick with using resting metabolic rate for the rest of this discussion). Resting metabolic rate represents the minimal amount of energy expended for homeostatic processes or core body functions, it accounts for approximately 60% of total daily energy expenditure (1). About 80% of the variance in resting metabolic rate is determined by body size (1),
In essence, the more a person weighs and the more muscle mass contained within this body weight, the higher their resting metabolic rate will be. If you take two individuals with the same body weight and muscle mass, their resting metabolic rate will be virtually identical. As a result, those who are overweight actually have a HIGHER resting metabolic rate due to their larger size.
For the sedentary population, resting metabolic rate is the primary contributor of energy expenditure over the course of the day, so it is important in that respect, but due to its static nature, more attention should be placed on other factors of metabolism. Even if someone gains a reasonable amount of muscle mass, resting metabolic rate hardly changes. For every pound of muscle gained, it has been suggested that resting metabolic rate increases between 6 and 12 calories per day (2,3,4). In a study on rugby players, despite an increase in lean mass of 2.0 +/- 1.6 kilograms over 14 weeks, resting metabolic rate did not significantly change. Before the 14 week intervention, the resting metabolic rate of the subjects was 2,389 +/- 263 calories, and at post-intervention, it measured as 2373 +/- 270 calories per day (5).
Metabolism includes not only resting metabolic rate, but the thermic effect of food, and activity thermogenesis.
The thermic effect of food, also referred to as diet-induced thermogenesis, is the energy expended during the process of digestion, absorption, and storage of food substances (1). It accounts for approximately 10-15% of total energy expenditure and does not vary greatly between individuals.
The last and most important component of metabolism is activity thermogenesis which can be divided into exercise activity thermogenesis and non-exercise activity thermogenesis. Exercise activity thermogenesis is defined as planned, structured, and repetitive physical activity that has the objective of improving health (1). Non-exercise activity thermogenesis is energy expenditure that we do not typically consider and it includes the energy expended maintaining and changing posture, fidgeting, cleaning, singing, and other activities of daily living (1).
Non-exercise activity thermogenesis represents the predominant component of daily activity thermogenesis and is the most variable. In society, the vast majority of people do not exercise, so exercise activity thermogenesis is zero (6). In addition, for those who do exercise, most perform less than 2 hours per week which results in an average expenditure of fewer than 100 calories per day (6).
Differences in NEAT
To reiterate, when equated for body size, resting metabolic rate does not differ much between individuals. To really cement this point, in a study which set out to compare differences in energy expenditure between lean, healthy, sedentary elderly (76 +/- 5 years of age) and young subjects (38+/- 10 years of age), it reported a very similar average resting metabolic rate between groups (7). To be specific, the weight of the elderly subjects was 69 +/- 9 kilograms and their resting metabolic rate was 1,356 +/- 201 calories per day. Similarly, the weight of the young subjects was 65 +/- 9 kilograms and their resting metabolic rate was 1,354 +/- 179 calories per day (7). Further, the thermic effect of food does not differ much between individuals, and the vast majority of the population does not exercise or exercise enough to create meaningful energy expenditure. So, how can two people of similar sizes substantially differ in the amount of food they are able to eat to maintain their weight? The answer is non-exercise activity thermogenesis.
It has been established that for two adults of a similar size, daily energy expenditure can vary by as much as 2000 calories per day (6). As previously alluded to, this primarily stems from differences in non-exercise activity thermogenesis. This difference in energy expenditure can result entirely just from differences in occupation. On average, any sort of seated work (desk job) burns about 700 calories per day. In comparison, strenuous work, such as agriculture, expends about 2,300 calories per day (6).
Consider another example of two office workers. Both return home from work at 5 p.m. and go to bed at 11 p.m. One chooses to spend their 6 hours of leisure on the couch watching television, which in terms of non-exercise activity thermogenesis, expends about 30 calories for the night (6). The other, bikes home from work instead of driving, paints an unfinished bedroom in the house, and goes out to the yard to pluck some weeds. Together, these efforts expend somewhere in the range of 750-1125 calories (6).
It’s easy to see the implications of energy expenditure from the above examples, but what about simply standing more often? In a study titled “Interindividual Variation in Posture Allocation: Possible Role in Human Obesity,” 10 lean and 10 mildly obese subjects agreed to have all their movements measured for 10 days. The obese participants were seated for 164 minutes longer per day than the lean participants, and the lean participants were upright for 152 minutes longer per day than the obese participants (8). It was concluded that if the obese subjects had the same posture allocation as the lean subjects, they would have expended an additional 269-477 calories per day (8).
What is particularly fascinating about this study is that it was extended in order to see if changes in posture allocation would occur if the obese subjects were dieted down and the lean subjects were overfed. Even when the subjects became either heavier or lighter, posture allocation remained the same. This infers that posture allocation is biologically determined rather than a function of an individual’s current body composition (8).
Clearly, small changes can lead to meaningful results. Finding ways to manipulate your environment to result in more standing or ambulatory time over the course of the day may be the key to kick starting weight loss. There are a variety of ways to promote non-exercise activity thermogenesis, small changes like switching to a standing desk or even a treadmill-linked desk at work can result in enormous increases in energy expenditure. Another strategy is to set step or standing goals for the day which allows an individual to partake in a sedentary activity they enjoy after completing a certain number of steps or standing for a specified amount of time.
The Neat thing about NEAT
As discussed, two individuals of similar body size can require vastly different amounts of food to maintain their weight due to large differences in non-exercise activity thermogenesis, but there seems to be another factor at play. When people are overfed or eat more calories than their body requires to maintain their weight, there appears to be genetic differences in how two people will respond to the same caloric surplus, even in twins.
In a study titled “The Response to Long-Term Overfeeding in Identical Twins,” 12 pairs of male monozygotic twins were overfed by 1,000 calories per day, 6 days per week (the 7th day was at maintenance calories), for 100 days. This comes out to 84 total days of overfeeding. After the 84 days of overfeeding, weight gain between subjects ranged from 4.3 to 13.4 kilograms (9). When interpreting the results of this research, it is important to understand that physical activity was limited, so differences in weight gain were merely a result of non-exercise activity thermogenesis because due to the same caloric surplus for each participant, the thermic effect of food would be similar, and resting metabolic rate does not change much with overfeeding (10).
In another study titled “Role of nonexercise activity thermogenesis in resistance to fat gain in humans,” 16 non-obese adults underwent measures of body composition and energy expenditure before and after 8 weeks of supervised overfeeding by 1000 calories per day.11 At the conclusion of the study, fat gained ranged from 0.36 to 4.23 kilograms. From pre to post intervention, the average change in non-exercise activity thermogenesis was about 336 calories per day, but this figure ranged from -98 to + 692 calories per day (11).
The literature seems to show that responses to overfeeding are very individual. When given a caloric surplus, some will compensate by sub-consciously moving around more while others won’t adjust whatsoever. This is another reason why two people of a similar size may have differences in their caloric intake, especially in terms of the number of calories required to gain weight. Worth noting, this tends to work in the opposite direction as well. When a calorie deficit is utilized to lose weight, many will sub-consciously move around less (decrease their non-exercise activity thermogenesis) which can undermine the size of the established deficit (12).
A Slow Metabolism, Huh?
Aside from any potential medical issues, if a person is struggling to lose weight, it’s not a result of a slow metabolism. The larger an individual is, the higher their resting metabolic rate is and the more energy they expend during exercise and activities of daily living. The answer to the problem is low activity levels, or potentially misreporting food intake (13,14), but I’ll save that for another time. Before chalking up your weight loss efforts to a “slow metabolism,” consider your environment and how much you are actually moving throughout the day.
- Chung N, Park M-Y, Kim J, et al. Non-exercise activity thermogenesis (NEAT): a component of total daily energy expenditure. Journal of exercise nutrition & biochemistry. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6058072/. Published June 30, 2018. Accessed March 14, 2019.
- Krieger J. The 50 Calorie Per Pound of Muscle Myth. Weightology. https://weightology.net/the-50-calorie-per-pound-of-muscle-myth/. Published May 26, 2010. Accessed March 14, 2019.
- Broeder CE, Burrhus KA, Svanevik LS, Wilmore JH. The effects of either high-intensity resistance or endurance training on resting metabolic rate. The American journal of clinical nutrition. https://www.ncbi.nlm.nih.gov/pubmed/1550062. Published April 1992. Accessed March 14, 2019.
- Wolfe RR. The underappreciated role of muscle in health and disease. The American journal of clinical nutrition. https://www.ncbi.nlm.nih.gov/pubmed/16960159. Published September 2006. Accessed March 14, 2019.
- MacKenzie-Shalders KL, Byrne NM, King NA, Slater GJ. Are increases in skeletal muscle mass accompanied by changes to resting metabolic rate in rugby athletes over a pre-season training period? European journal of sport science. https://www.ncbi.nlm.nih.gov/pubmed/30614386. Published January 7, 2019. Accessed March 14, 2019.
- Levine JA. Nonexercise activity thermogenesis–liberating the life-force. Journal of internal medicine. https://www.ncbi.nlm.nih.gov/pubmed/17697152. Published September 2007. Accessed March 14, 2019.
- Harris AM, Lanningham-Foster LM, McCrady SK, Levine JA. Nonexercise movement in elderly compared with young people. American journal of physiology. Endocrinology and metabolism. https://www.ncbi.nlm.nih.gov/pubmed/17401138. Published April 2007. Accessed March 14, 2019.
- Levine JA, Lanningham-Foster LM, McCrady SK, et al. Interindividual variation in posture allocation: possible role in human obesity. Science (New York, N.Y.). https://www.ncbi.nlm.nih.gov/pubmed/15681386. Published January 28, 2005. Accessed March 14, 2019.
- Bouchard C, Tremblay A, Després JP, et al. The response to long-term overfeeding in identical twins. The New England journal of medicine. https://www.ncbi.nlm.nih.gov/pubmed/2336074. Published May 24, 1990. Accessed March 14, 2019.
- Harris AM, Jensen MD, Levine JA. Weekly changes in basal metabolic rate with eight weeks of overfeeding. Obesity (Silver Spring, Md.). https://www.ncbi.nlm.nih.gov/pubmed/16741271. Published April 2006. Accessed March 14, 2019.
- Levine JA, Eberhardt NL, Jensen MD. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science (New York, N.Y.). https://www.ncbi.nlm.nih.gov/pubmed/9880251. Published January 8, 1999. Accessed March 14, 2019.
- Loeffelholz Cvon. The Role of Non-exercise Activity Thermogenesis in Human Obesity. Endotext [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK279077/. Published April 9, 2018. Accessed March 14, 2019.
- Mahabir S, Baer DJ, Giffen C, et al. Calorie intake misreporting by diet record and food frequency questionnaire compared to doubly labeled water among postmenopausal women. European journal of clinical nutrition. https://www.ncbi.nlm.nih.gov/pubmed/16391574. Published April 2006. Accessed March 14, 2019.
- Lichtman SW, Pisarska K, Berman ER, et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. The New England journal of medicine. https://www.ncbi.nlm.nih.gov/pubmed/1454084. Published December 31, 1992. Accessed March 14, 2019.