The case against metabolic rate

The case against metabolic rate

We are culturally enamored with the idea that metabolic rate is the unavoidable variable concerning body fat. Culturally, we’re wrong; and we don’t really understand ‘metabolism.’ One of the most misunderstood ideas about why we get fat is the concept of ‘metabolism’ or ‘metabolic rate’. Too many people assume that they get fat (or are fat) because their metabolism slowed down (or is just slow). To get this out of the way up front, let’s get a clear definition of what the word ‘metabolism’ means.

What is “metabolism”?

Your body’s metabolism is the total of all chemical reactions/processes going on in your body. In other words, your metabolism, or metabolic rate, is the energy needed for the countless little things your body is doing to keep you alive and healthy. This is generally measured as calories per minute—or the amount of calories (energy) required by the body over a minute. Because our bodies constantly change (e.g., healthy vs. sick, young vs. old, growing up vs. already grown, etc.), our metabolic rate changes as well.

Inasmuch as our metabolic rate is a function of what our bodies do, they have to ‘do’ more when they’re bigger. In other words, the bigger the body is, the harder it works to maintain normal function, which leads to a higher the metabolic rate. That’s right—bigger bodies tend to have higher metabolic rates than smaller bodies [1, 2]. This is important enough to explain again. A person who is bigger, even if it’s because of more body fat (as in the referenced study), invariably has a higher metabolic rate than a smaller, leaner person. Even more interesting—this fact is so consistent that when bodies become smaller (e.g., lose weight), their metabolic rate decreases [3, 4]! Lest you think it’s all a result of having to move the larger mass around (after all, if I had 50 lbs. more fat on me, I’d have to work quite a bit harder to get around), the same study above found that only 8% of the difference in metabolic between the obese and lean subjects was a result of the increased metabolic cost of moving the extra weight.

Metabolic rate vs. fuel

But can metabolic rate predict who will gain more or less fat over time? In other words, if you determined your metabolic rate, could you predict or anticipate how likely you are to gain fat or stay lean? The Baltimore Longitudinal Study on Aging, the nation’s longest running study on human aging, explored this very question. Tracking almost 800 men over a 10-year period, they found that metabolic rate was a terrible predictor of who gained weight [5]. In other words, a person with a high metabolic rate at the beginning of the study period was just as likely to be obese as a person with a lower metabolic rate. Or, stated differently, a person with a low metabolic rate had every chance to be or become lean over time as a person with a higher metabolic rate.
This study found another gem though—while metabolic rate wasn’t relevant to future weight gain, the respiratory exchange ratio (RER) was [5]. The RER is a number that tells us what our fuel preference is; in other words, is glucose or fat the predominant fuel for our bodies. People that were had higher RER values, indicative of a “glucose-fueled” body, were significantly more likely to gain weight over this 10-year period when compared with people that were “fat-fueled”.

So, how do you become fat-fueled? It’s simple—eat more fat and fewer carbohydrates. One study found that just four weeks of eating a low-carbohydrate, high-fat diet was enough to shift the body into “fat-fueled” mode, as evidenced by a significant shift in the RER value [6].

Take-away Thoughts

Don’t waste time worrying or talking about metabolic rate—it’s misunderstood and not very important. Rather, focus more on how the foods you eat are fueling your body and, importantly, how they affect insulin. Low-carb, high-fat eating puts you in the perfect place to use fat for fuel.

1 Ravussin, E., Burnand, B., Schutz, Y. and Jequier, E. (1982) Twenty-four-hour energy expenditure and resting metabolic rate in obese, moderately obese, and control subjects. The American journal of clinical nutrition. 35, 566-573
2 Scharf, R. J., Demmer, R. T. and DeBoer, M. D. (2013) Longitudinal evaluation of milk type consumed and weight status in preschoolers. Archives of disease in childhood. 98, 335-340
3 Amatruda, J. M., Statt, M. C. and Welle, S. L. (1993) Total and resting energy expenditure in obese women reduced to ideal body weight. The Journal of clinical investigation. 92, 1236-1242
4 Kratz, M., Baars, T. and Guyenet, S. (2013) The relationship between high-fat dairy consumption and obesity, cardiovascular, and metabolic disease. Eur J Nutr. 52, 1-24
5 Seidell, J. C., Muller, D. C., Sorkin, J. D. and Andres, R. (1992) Fasting respiratory exchange ratio and resting metabolic rate as predictors of weight gain: the Baltimore Longitudinal Study on Aging. International journal of obesity and related metabolic disorders : journal of the International Association for the Study of Obesity. 16, 667-674
6 Ebbeling, C. B., Swain, J. F., Feldman, H. A., Wong, W. W., Hachey, D. L., Garcia-Lago, E. and Ludwig, D. S. (2012) Effects of dietary composition on energy expenditure during weight-loss maintenance. JAMA : the journal of the American Medical Association. 307, 2627-2634