Researchers' Zone:

New research shows that calorie excretion varies from person to person. This could be one of the explanations why some people gain weight easier than others.

Photo: Shutterstock

Do you easily gain weight? It might have something to do with the calories in the toilet

Our bodies do not take up all the calories we ingest. New research indicates that certain people are protected from weight gain simply because a significant part of their calorie intake ends up going down the toilet drain.

Jens Lund Ph.D. Student, Univeristy of Copenhagen

Zach Gerhart-Hines Associate Professor, University of Copenhagen

Christoffer Clemmensen Associate Professor, University of Copenhagen

Published 8 October 2020 - 14:21

For decades, we have been told that we need to eat less and exercise more, if we are to lose weight. This, however, is a simplification of the truth.

The reality is that in order to lose weight, we need to absorb (and retain) fewer calories than we burn. And the same is true when it comes to weight gain.

We only put on pounds when we absorb (and retain) more calories than we burn.

Following a meal, the food we ingest gets mixed and digested within our stomach and intestines. Yet, the four energy-containing macronutrients, carbohydrates, fat, protein and alcohol, can only affect overall energy balance once they have been absorbed across the intestinal wall and reached the bloodstream.

In fact, absorption of macronutrients is imperfect and even if these nutrients have entered the blood, they can still escape the body. As an example, we know that a minor part of the alcohol we ingest leaves the body via our breath.

In addition, it well-known that sugar and proteins can be excreted via urine, as seen in people with untreated diabetes or kidney disease for example.

This loss of energy is an overlooked factor in obesity research. To shed some light upon this topic, we recently published a scientific article about human energy excretion.

This exciting area of research is highly relevant because excretion of energy via feces and urine potentially is part of the explanation why some persons are prone to weight gain while others stay slim throughout life.

Why is weight gain easier for some than for others?

Humans are born with different appetites and this is likely an important reason why certain individuals find it very easy to gain weight. Yet, inborn variations in food intake is not the only explanation for why we differ in terms of obesity susceptibility.

Several overeating experiments have shown that humans are affected very differently by the same amount of excess energy intake.

This interesting observation has been made for example in a classical study in which volunteers overate a total of 84.000 calories in 14 weeks (corresponding to an extra 1.000 calories a day for six days a week). In this experiment, weight gain varied from 4 kg to 13 kg!

But what explained that variation between people? Differences in energy intake? Differences in energy expenditure? Or, maybe, differences in energy excretion?

Differences in energy intake are an unlikely explanation in this particular study, given that the researchers controlled what was ingested during the experiment.

Differences in energy expenditure could explain parts of the variation but probably not all of it. This is because another well-controlled study later has shown that the human body only weakly increases energy expenditure and heat production as a defense mechanism against weight gain.

This leaves us with the intriguing possibility that calorie excretion is a key factor when it comes to weight gain variation.

Certain individuals lose a lot of calories via feces

New research shows that calorie excretion through the gut can vary quite remarkably between persons.

This has been demonstrated in studies, where researchers have determined the exact energy content of fecal samples using a bomb calorimeter (see here and here) - an apparatus that measures the heat released upon combustion and which is also used for determining the energy content of foods.

When this is done, in combination with careful tracking of the energy intake of volunteers, one finds that healthy humans, on average, excrete around 5 per cent of their energy intake via feces.

In itself, this number is not that interesting. Yet, a noteworthy variation is hidden behind it. While some people only excrete 2 percent of their energy intake, others loose up to 10 per cent of their ingested calories in the toilet (se here, here and here).

This difference between individuals are potentially very important when it comes to overall energy balance and can be illustrated by a study in which one female volunteer lost around 50 grams of carbohydrate per day in her feces.

This is equal to 200 calories and corresponds to losing half a liter of sugar-sweetened beverage. Notably, this loss was more than three times as great as another women female from the same study who only lost 15 grams of carbohydrate per day.

Another overeating experiment indicates that calorie excretion can affect energy balance and thus weight gain in those periods where we tend to overeat (like Christmas and other holidays).

Moreover, differences in energy excretion might also be part of the reason why certain people find it easier to lose weight than others. While we recently highlighted these thoughts in a scientific article, it is important to emphasize that upcoming studies needs to test whether these ideas are actually true or not.

Our body weight depends on the biology we inherit from our parents. And here is calorie excretion possibly one of the biological factors that explain why we do not all weigh the same.

Is ‘natural thinness’ due to a large energy excretion?

A small group of people have been thin since birth and have always found it very difficult to gain weight. These individuals are likely born with a condition that is called ‘constitutional thinness’ or just ‘natural thinness’ in plain language (for more, see here).

Their low body weight is a bit of a mystery to scientists like us and cannot readily be explained by their diet and exercise habits. Most of them have a great appetite and, they do not differ from ‘normal weight’ individuals in terms of physical activity.

One can, therefore, speculate whether this type of thinness is caused by a lower calorie absorption within the gut and/or a larger calorie excretion via urine.

One study shows that naturally thin individuals lose the same amount of fat in feces as other healthy humans do.

Another investigation, however, shows that carbohydrates and protein excretion often differ to a greater extent between individuals. Thus, more thorough investigations are needed to test if differences in excretion of carbohydrates and proteins explain the low body weight.

Stop weight stigma: Body weight is (almost) as heritable as body height

People with a large fat mass are often exposed to stigmatization and discrimination. This occurs from family and friends, from strangers on the street, in schools, at work, and paradoxically also within the health care system.

This unacceptable type of stigma stems from the idea that obesity is caused by laziness and gluttony; an idea which clearly conflicts with the scientific evidence.

Body weight is almost as heritable as height and neither body ‘thickness’ nor ‘thinness’ is about will power.

It is instead about e.g. the biology we inherit from our parents - and calorie excretion might one of the biological factors that explain why we do not all weigh the same.

Read the Danish version at Videnskab.dk's Forskerzonen.