If metabolic rates have indeed declined in recent decades, as this study indicates, there may be potential consequences. The main concern is weight gain. Because weight gain is the result of consuming and subsequently absorbing more calories than are burned, a lower metabolic rate would seem likely to increase the chances of weight gain.

Consistent with this observation, the average person’s body weight, and consequently the prevalence of obesity, in the U.S. and Europe has increased since the 1980s.^[1] Although the most feasible explanation for this change is an increase in calorie consumption,^[2] a decrease in metabolic rates could be a contributor as well.

Still, while it makes intuitive sense that a lower metabolic rate would lead to weight gain, the evidence is not entirely consistent on this point. It’s certainly true that some interventions can affect body weight by altering energy expenditure. Stimulants like ephedrine can cause weight loss at least partly by increasing the metabolic rate.^[3] Likewise, higher levels of exercise can increase daily energy expenditure, supporting weight loss.^[4] But there is no guarantee that a drop in energy expenditure will inevitably lead to weight gain.

This was observed in a prospective cohort study with 757 participants, in which the investigators compared weight gain in people with the lowest (15th percentile and below) and highest (85th percentile and above) basal metabolic rates, adjusting for body composition (fat mass and fat-free mass) and age.^[5] Over an average of 9.7 years, there was no difference in weight gain between the low metabolic rate group (+0.3 kg) and the high metabolic rate group (+0.5 kg), despite a difference in metabolic rates of almost 500 calories per day.

Sleep should also be considered. Although the effect is generally small, getting less sleep tends to increase total daily energy expenditure because people burn more calories while awake.^[6] However, insufficient sleep has been shown to promote weight gain and is associated with a higher risk of obesity, seemingly because sleep deprived people consume more calories, well beyond any caloric “advantage” they get from being awake for longer. This underscores that in the real world, the key is what causes the metabolic rate to change, rather than just in what direction it changes.

This is why it’s also worth considering what could actually cause a population-wide reduction in energy expenditure. This is a speculative discussion because there does not yet appear to be any obvious explanation, although a few possibilities are worth exploring.

Physical Activity

Because physical activity is a major determinant of an individual’s total energy expenditure,^[7] it would seem reasonable to suspect less physical activity on a societal level could explain the findings of the study. However, as previously covered, there was actually an apparent increase in energy expenditure related to physical activity.

This may seem surprising, given that cultural, occupational, and technological trends would seem to favor people being less physically active since the 1980s and 1990s.^[8] However, research from the U.S., Spain, and the U.K. suggests that leisure-time physical activity has not decreased in these countries and may have even increased slightly.^[9]^[10]^[11]^[12] On the other hand, data from the U.S. suggest that people expend less energy at their jobs.^[13] Ultimately, it’s difficult to be sure exactly how physical activity-related energy expenditure has changed. Even the current study relied on an equation to estimate physical activity energy expenditure (0.9 x TEE – BEE), making the results less reliable. Still, existing data does not provide much reason to think that less physical activity explains the observed declines in daily energy expenditure.

Ambient temperature

When it gets cold, physiological processes (like shivering and nonshivering thermogenesis) are initiated to generate heat and keep the body at an appropriate temperature. Because creating heat requires energy, exposure to cold temperatures causes a person to burn more calories.^[14]

This is relevant because people may be less exposed to the cold than they were in the past. Evidence suggests that, since the 1980s, indoor temperatures during the winter have increased in the U.K. and U.S., perhaps due to changes in indoor heating technology, better building insulation, and lower fuel prices.^[15] Some researchers have suggested changes in indoor temperatures could be expected to reduce daily energy expenditure. However, evidence for a clear link between indoor temperatures and metabolic rates remains elusive.

Smoking

Since the 1980s, both the U.S. and most of Europe have had declines in the number of people who smoke cigarettes.^[16] This is undoubtedly good for public health by reducing the incidence of lung cancer, coronary heart disease, and several other diseases. However, in the context of energy metabolism, tobacco is a source of the stimulant nicotine, which may increase metabolic rate.^[17] Could a drop in smoking rates explain why people burn fewer calories?

A number of studies have looked at what happens when smokers stop smoking. By and large, these trials have found that quitting smoking results in a reduction of energy expenditure, with a couple of these studies finding that people who started smoking again saw their energy expenditure climb back up.^[18]^[19]^[20]^[21]^[22] However, these studies often used less precise measurement techniques, and even the longest study only lasted 60 days, making it difficult to say if the effects diminish over the long-term.

In contrast to the previous research, one observational study found smokers did not burn more calories throughout the day than nonsmokers, although only 47 current smokers were included in the study, so it is by no means the final word on the subject.^[23] Of course, even if smoking does affect energy expenditure, it seems unlikely that declines in smoking could fully explain societal declines in energy expenditure because the number of smokers has only declined by a small fraction in the time period analyzed.^[24]^[25]

Dietary fat

Many countries have seen decreases in animal fat consumption in recent decades. Data from the U.S. suggest that this decrease has resulted in a decrease in saturated fat intake, although perhaps not to a substantial degree.^[26] The authors of the summarized study suggested that this decline in saturated fat intake could at least partially explain their findings, based on experiments performed in mice. Still, multiple randomized controlled trials have failed to provide support for the idea that eating less saturated fat lowers a person’s metabolic rate, or, alternatively, that eating more raises it.^[27]^[28]^[29]^[30]^[31]

International declines in animal fat consumption have occurred in parallel with an increase in vegetable oil consumption. Vegetable oils, including soybean oil and canola oil, tend to be high in linoleic acid, one of the main omega-6 fatty acids. As a result, omega-6 fatty acid intake has increased throughout the world.^[32]^[33] Could diets enriched in omega-6 have led to lower metabolic rates? This also doesn’t appear to be the case, based on the few trials that exist.^[34]^[35]^[36]

Perhaps the studies on saturated fat or polyunsaturated fat are insufficient to measure real effects, given that most had small sample sizes and short durations. Nonetheless, research is currently lacking that supports the idea that changes in dietary fat consumption in recent decades would result in population-wide reductions in energy expenditure.

Metabolic rates may have slowed in recent decades Original paper