Have a Fast Metabolism? Decades-Long Study Shows Future Generations May Not

by Kaitlyn Hung

As a kid, I was told I had a fast metabolism—I’d eat a meal and be hungry two hours later. I was prone to frequent bouts of ‘hangriness’ and I always kept two granola bars in my backpack as an emergency snack. It was never clear to me, however, what exactly having a “fast metabolism” really meant. I imagined little molecular machines inside me quickly turning their gears, consuming the energy from my peanut butter sandwich. It made me wonder—is having a “fast metabolism” a fixed characteristic I was born with? Or is it something that I can change?

What is metabolism?

Metabolism is the process that turns the food we eat into energy. Most of this energy is spent on basic processes required for survival—ones we’re not consciously aware of—like breathing, circulating blood, and keeping our cells alive. Active processes, like exercise and digestion, make up the remaining 25-40% of the energy we use daily (Figure 1).

**Figure 1. Energy homeostasis requires balancing energy intake and energy expenditure.** 60-75% of energy expenditure is basal, representing the minimum energy required for organs to function at rest. The other 25-40% is active, including processes like digestion and physical activity.

The energy required for these basic processes is called basal energy expenditure (BEE), or the minimum energy we require in a day for our organs to function at rest. BEE is dynamic and changes across our lifetime. Our bodies are built to balance our energy intake (eating food) with our energy expenditure, a term known as energy homeostasis. Our BEE peaks in the first year of life, then declines in adolescence until it reaches a stable state that is maintained for most of our adult life (age 20-60), after which it declines again (Figure 2). These changes in BEE correspond to the changing energetic demands of our organs across a lifetime. If energy expenditure declines without a corresponding reduction in energy intake (i.e. consuming less food), we end up with extra unutilized energy, causing disrupted energy homeostasis and weight gain.

**Figure 2. Basal energy expenditure changes across a lifetime.** BEE is highest in the first year of life and declines in adolescence, reaching a stable level for most of adulthood before declining again after age 60.

Measuring the decline in basal energy expenditure

How is BEE measured? Our breathing—inhaling oxygen and exhaling carbon dioxide—can be used to calculate basal energy expenditure. To measure BEE, participants rest in an air-controlled room or wear a plastic hood while their oxygen and carbon dioxide levels are continuously measured for 30 to 60 minutes. Because burning energy produces the carbon dioxide we exhale, BEE can be inferred from oxygen consumption and carbon dioxide production.

Interestingly, a recent study looking at energy expenditure data from a database of 1,429 adults in the US and Europe between 1982 and 2017 found that the average BEE has declined over the past 30 years, meaning we’re using less energy to maintain our bodies at rest. This observed decline in BEE could potentially explain the growing prevalence of obesity, which has more than doubled in US adults over the past 30 years. The findings of this study showed that, in 2017 compared to 1982, BEE significantly declined by 14.7% in men, but did not significantly decline in women (Figure 3).This difference between men and women could reflect sex differences in BEE—on average, men have a higher BEE than women, due in part to differences in body composition and aerobic capacity.

**Figure 3. Basal energy expenditure (BEE) has declined over the past 30 years, a trend more pronounced in men than women.** BEE is calculated by measuring participants’ oxygen consumption and carbon dioxide production at rest.

Technical limitations in measuring BEE, however, could cast some doubt on these results. For example, data in earlier studies was more likely to be collected using mouthpieces to measure oxygen consumption and carbon dioxide production than more recent studies, and this difference in technology could affect the calculated BEE. Additionally, if the researchers excluded data from a single study, the decline in BEE in women became significant, highlighting potential technical differences between studies.

However, the main conclusions in this study are consistent with a second, unrelated study, adding to its validity. Rather than tracking oxygen and carbon dioxide levels, this team analyzed body temperature using data collected over the past 150 years. Because metabolic processes generate heat, body temperature is influenced by metabolism; this means that a decrease in body temperature can reflect slower metabolism. The researchers found that mean body temperatures in men and women have significantly declined. Taken together, these studies reinforce the surprising conclusion that basal energy expenditure has slowed over the past decades.

Why is basal energy expenditure declining?

You might be asking why basal energy expenditure has declined. Researchers aren’t certain why, but they have several hypotheses about what might be causing the decline (Figure 4).

1) The immune system: One of our body’s typical responses to infection is fever, and this increase in core body temperature requires a higher basal energy expenditure. Over the past decades, sanitation practices have improved, antibiotics and vaccines help us fight and prevent infections, and deaths from infectious diseases have declined. Improved healthcare and cleanliness today suggest our immune system might be less active than it once was, decreasing the average person’s BEE. However, the relationship between immune activity and energy expenditure is complex, and it is unclear if they share a causal relationship.

2) Diet: Our diet composition also affects BEE. The composition of our diet has changed over the past 30 years, providing a potential explanation for the decline in BEE. In the US and Europe, the types of fats we eat have changed: we’re eating less animal fats (which are primarily saturated fats) and more plant-based fats like soybean and seed oils (mostly unsaturated fats). Researchers found that, in mouse models, eating more saturated fats is associated with increased energy expenditure. Thus, the fact that we’re eating less saturated fats could play a role in the declining BEE. However, while saturated fat consumption in the US and Europe has declined, total fat intake has increased since the early 1990s, further complicating the reasons behind declining BEE.

3) Weight loss efforts: Humans evolved to withstand periods of famine by slowing our basal energy expenditure during periods of restricted food availability. As a result, consuming less food may not always cause weight loss, as BEE can slow in response to food restriction. The increasing prevalence of obesity means more people may be dieting, which can cause this adaptive slowing of their metabolism, reducing average BEE. Researchers are still trying to understand when this adaptive decrease in BEE occurs following weight loss, how long it persists, and what we can do about it.

**Figure 4. There are three hypotheses for why basal energy expenditure has declined:** 1) Reduced immune system activity due to improved sanitation and medical advancements 2) Changes in diet, including less saturated fats, and 3) Adaptation to weight loss efforts, with less food intake causing an adaptive decrease in energy expenditure.

Importantly, these possibilities are speculative, and the cause of BEE decline cannot be concluded from a single study. Because the BEE database used in this study did not include information about participants’ diet and immune function, no relationship can be directly drawn between these factors and BEE. Additionally, while the researchers accounted for differences in age, sex, and body composition among the study participants, there may be other factors they didn’t measure that affect BEE. Regardless, the study highlights the need for more research investigating what factors regulate BEE and how these are changing in our modern environment.

Consequences of lower basal energy expenditure

Is a lower basal energy expenditure “bad”? In the context of obesity, potentially, as the decline in BEE makes it harder to maintain weight loss. But a decline in basal energy expenditure is not inherently negative. For example, lower BEE may be associated with longer lifespan—people with genetically lower BEE are more likely to have parents that lived longer. BEE is just one of many factors that affect our health and well-being, albeit an important and far-reaching one. Having a “fast” or “slow” metabolism is not simply about how much you can eat while maintaining your weight, but is a dynamic characteristic that represents one way we adapt to our modern environment as we consume food, fight infections, and age.

Kaitlyn Hung is a PhD student in the Biological Sciences in Public Health PhD program at Harvard University.

Cover image by Elsemargriet on PixaBay.

For More Information

You can find the original research article identifying the decline in BEE here.
This book chapter provides an overview on energy expenditure, including the contributions of basal energy expenditure, physical activity, and digestion.
For more information on the energy expenditure database analyzed in this study and to see other studies using the database, check out this website.