HEALTH AND SCIENCE

New Discoveries That Redefine Our View of Diet, Weight Gain, Exercise, and Health

The constrained energy expenditure model and why we can’t exercise our way out of a bad diet

“I can have dessert because I was really active today.” Most or all of us have often thought something like this to ourselves. This reflects an intuition we all seem to share about the relationship between activity and weight gain.

We imagine a type of metabolic account where the currency is energy/calories (we’ll use these words interchangeably). If we fill our account too much, we gain weight; if the account is too empty, we lose weight.

We can always shrink a bloated account by reducing our calorie intake. But when we’re surrounded by delicious, affordable, calorie-rich foods, that option requires a lot of restraint.

Wouldn’t it be much more convenient if we could just eat what we want and burn off unwanted calories later with some extra exercise? That’s what most of us assume, but sadly, that intuition turns out to be wrong.

An imbalance in how many calories we can burn and consume in a day

Why is that? Because there’s a limit to how many calories we can burn in a day. There’s also a limit to how many calories we can consume daily. But the limit on the calories we can consume is far higher than the limit on the calories we can burn.

The consequence of this imbalance is that we can gain weight more easily than we can lose it. And because most of us don’t know about this imbalance, we keep trying to exercise our way out of bad diets and overeating, not realising the futility of our strategy.

What’s the evidence for this, you ask? It comes from groundbreaking studies on the biology of energy use. These studies used an approach known as the doubly-labeled water method, which provides the most accurate estimates of energy use that metabolic science can produce.

You may be surprised to hear that there’s compelling evidence to support all of these points. There’s always more to learn, but these are not wild claims based on a paucity of data.

Thanks to the doubly-labeled water method, metabolic scientists have made discoveries that reshape our ideas about weight loss, diet, exercise, and health. The main takeaway is that diet is for weight loss, and exercise is for health. But if you want more than a sound bite, I’ll briefly dive into the details below.

I’ll unpack some pioneering studies on energy use and relate these findings to parallel discoveries in the biochemistry of metabolism. To finish, we’ll link these points to current ideas about how to live longer, healthier lives.

How energy use is measured

For the science fans in the crowd, I’ll briefly describe the doubly-labeled water method and some information about how researchers measure energy use. If you’re not interested in these details, feel free to skip to the next section.

The doubly-labeled water method uses water that contains unusual but harmless isotopes of hydrogen and oxygen (i.e., doubly-labeled water) to gauge the rate at which people use energy (aka ‘burn’ calories). Samples are taken as a baseline (e.g., urine and/or saliva), and people then ingest the doubly-labeled water.

Because the hydrogen and oxygen isotopes are unusual, they can be detected as they leave the body (e.g., in urine, saliva, sweat, and/or breath). And based on the rate at which those isotopes leave the body, researchers can infer our energy use and hence metabolic rate. This can be done for around 4–21 days. For a detailed description of the process, including the underlying theory, see this published paper on the topic.

There are also other ways to measure energy use. For example, researchers can measure CO2 output in our breath as a proxy for energy use, as CO2 is generated as our cells metabolise fuel. However, there are limitations to this approach that mean the doubly-labeled water method is the gold standard in the science of energy use.

Various factors must also be accounted for when assessing energy use. That’s because different tissues (e.g., muscle and fat) differ in their metabolic needs, and people vary in the proportions of those tissues.

In addition, energy use varies depending on our size/weight. As a result, differences between people (e.g., in overall weight, and fat and muscle content) are factored into measures of energy use, thus preventing distortions of the results.

We burn the same amount of calories regardless of our activity

Armed with doubly labeled water, biologists and metabolic scientists asked whether active people burn more calories than sedentary people. They fully expected to confirm that intuition, but instead got an answer that left them stunned.

In study after study, people essentially burned the same amount of calories per day, irrespective of their level of activity. As an example, farmers in West Africa, Hadza hunter-gatherers in Tanzania, and office workers in Chicago all burn the same number of daily calories.

A brief uptick in energy use often occurs when a person sharply increases their activity. However, studies have found that this is only transient, and their energy use soon returns to its normal range, even if they maintain the same level of activity.

This implies that the body has a range of energy use that it tries to maintain, whether active or sedentary. Researchers were surprised by these findings and wanted to see how much activity the body could compensate for.

They looked for events involving truly extreme levels of activity. Amazingly, they found that the body could still compensate for even super-intense amounts of exercise. For example, one study measured energy use in a group of people running across the USA.

The people ran roughly a marathon per day, six days a week, for 140 days. Unbelievably, their metabolic budgets showed the same trend as a sedentary person who suddenly started to exercise. They increased briefly at the beginning, but soon returned to a normal range.

Diet for weight loss, exercise for health

In ways that are still not properly understood, these results show that our metabolic budget is tightly controlled. Somehow, even stupendous amounts of activity can be offset, thus keeping our energy use in a fairly narrow range. The theory describing this is known as the constrained energy expenditure model.

Why does our biology work this way? Honestly, the answer is currently unknown. But even so, these findings do provide clear answers to other important questions about diet, weight loss, health, and exercise.

Intermittent fasting

As we said already, weight loss is about diet. Because our body keeps our energy budget in a relatively tight range, we can’t exercise our way out of a bad diet. As the old saying goes, six-packs are made in the kitchen, not the gym.

To lose weight, you must ingest fewer calories than you burn. Many are currently experimenting with intermittent fasting for exactly this reason, as shortening the window for eating reduces overall calorie intake for many people.

(Intermittent fasting also makes a hormonal difference, like lowering insulin. You can learn more about insulin resistance from this story written by Dr Mehmet Yildiz.)

Most regimens involve around 8 hours of eating and 16 hours of fasting per day. But we’re all different to one extent or another, so if you want to try intermittent fasting, you should experiment until you find the right regimen for you.

Dietary protein versus carbohydrates

Research also indicates another dietary strategy that can help weight management: consuming high-protein foods. Studies on humans and non-human animals show that diets high in protein make us feel fuller than diets high in carbohydrates.

Dietary protein provides a rich supply of nutrients (e.g., amino acids) that send a strong satiety signal to the brain. Research indicates that this involves a region known as the hypothalamus, which plays a crucial role in hunger and satiety by regulating hormones such as ghrelin (hunger) and leptin (satiety).

By contrast, dietary carbohydrates (e.g., sugar) do not provide a strong satiety signal. As a result, we can easily overconsume foods rich in dietary carbohydrates and poor in dietary protein, leading to unwanted weight gain.

Sedentariness and health

As we also said, exercise is about health. Even if you eat a healthy amount of calories, sedentary lifestyles are a risk factor for disease and death, including cancer. It seems that we need to stay active to be healthy.

There’s currently no consensus answer for why this is the case. But we can make an educated guess based on what we already know. We know that the body tries to keep our energy use in a fairly tight range, whether we’re active or sedentary.

But if we’re sedentary, and our calories aren’t spent on movement, where are they spent? It turns out that most of our energy is not spent on movement.

Rather, bodily processes take up most of our daily energy use, usually accounting for at least 2/3 of our energy budget (unless you’re very active). This includes immune activity, regular organ functions, and basic cellular processes across the brain and body.

When we’re sedentary, even more of our energy budget can be devoted to these various bodily processes. Researchers speculate that this may lead to serious conditions, such as autoimmune diseases, as overactive immune systems may lose the ability to discriminate your cells from invading pathogens.

Another interesting idea relates to cancer. Researchers have shown that cellular activity can be supported by many different metabolic flux patterns. Sadly, some of these flux patterns appear to encourage cells to grow, increasing the risk that some will become cancerous.

So, how do we avoid the patterns of metabolic flux that increase our risk of cancer? Evolutionary biochemist Nick Lane argues that staying active is a big part of the solution.

That’s because sedentariness itself appears to drive our cellular metabolism in a more cancerous direction. People like Nick Lane suggest that sedentariness in old age may also be why our risk of cancer is negligible until we reach our 60s.

(Wondering why we’re ignoring genetic mutations while discussing cancer? To find out why the genetic-mutation theory fails to account for what we know about cancer, check out my articles about it here and here.)

Conclusions

These discoveries are fascinating, as they offer practical advice for managing our risk of serious diseases and death. Much remains to be learned, but the evidence we know today shows that we must diet to manage our weight and exercise to manage our health.

To lose weight, we simply have to ingest fewer calories than we burn. Most of us find this challenging, and humans are known as the fat ape for a reason. But with discipline, support, and preparation, we can all find ways to reduce our calorie intake.

To stay healthy, we must exercise, even if we’re not overweight. Few things in life are certain, but our best understanding of health and biology suggests that staying active will add many healthy years to our lives and reduce our chances of getting serious diseases like cancer.

We can’t eat whatever we want and live like couch potatoes, but since when was that the goal of life, anyway? The best predictor of happiness is our number of healthy relationships, exercise can be inherently rewarding, and being healthy feels good.

If the price for weight loss and good health is staying active, spending time with loved ones, and embracing the proper fasting schedule, that doesn’t sound too bad. To learn more about this topic, I recommend the book Burn by evolutionary biologist, anthropologist, and metabolic scientist Herman Pontzer.

Thanks for reading!