I've read https://www.nhs.uk/conditions/obesity/causes/ containing the list of many causes of obesity. I'm interested in the numerical distribution, i.e. out of 100 obese people how many eat too many calories, how many have a poor diet and how many don't do enough physical activity. Is data about this available publicly?

More specifically, I'd like to get answers to these questions, based on scientific studies:

  • Out of 100 obese people how many eat more calories than the average healthy, non-obese person of their age and gender?
  • Out of 100 obese people how many have a poorer diet than the average healthy, non-obese person of their age and gender?
  • Out of 100 obese people how many do less physical activity than the average healthy, non-obese person of their age and gender?

By knowing these numbers governments would have a better idea how to fight obesity: e.g. should they promote food with fewer calories, or should they promote physical activity? (However, this is just an example, and in this question I'm interested in the numbers above rather than on the best ways for governments to fight obesity.)

I understand that obesity is caused by a combination of factors, and for helping a person become less obese, all factors should be considered personally for them. Nevertheless, I'm still interested in the numbers above.

I understand that even these numbers don't paint the full picture, because we don't know e.g. how many of them eat more calories and do less physical activity (at the same time).

I also understand that it's possible to eat (a bit) more and not become obese, e.g. if also doing more physical activity.

If there are similar factor breakdown numbers available publicly, I'm interested in them.

  • Obesity is caused by a myriad of reasons. Thyroid will control how many calories you burn along with other hormones. Hormones will decided whether muscle or fat are consumed during calorie deficits. Carbohydrate intake will affect hunger and satiation.
    – paulj
    Jul 30 '19 at 13:19

From the title of the question: What are most probable causes of obesity?

There is a single cause of obesity: greater amount of calories consumed then burned.

From the body of the question: Out of 100 obese people, how many eat more calories, how many are not physically active enough...:

This is not possible to answer because:

  • High calorie intake alone does not cause obesity. A known US swimmer said that during a certain period he was consuming 8-10,000 calories per day and, apparently, he did not become obese, because of regular training (USA Today).
  • A bedridden person with nearly zero physical activity does not become obese if he/she consumes only the amount of calories he/she spends (for example, about 1,500/day).
  • The poorest/unhealthiest/most fatty/most sugary diet on the world does not make you obese if it is not hypercaloric, so "poor diet" by itself is not a cause but can be a risk factor for obesity.
  • Individuals with genetic disorders, such as Down syndrome, become obese only if they consume excessive calories, so genetics alone is not a cause but can be a risk factor for obesity.

Some health organizations that are concerned about public health have made the estimations of the number of overweight/obese individuals by region, sex, etc. They know the risk factors (mentioned above) and they know the trends about how these factors change with time (see below). What they do not know is how many people eat too much and how many do not exercise enough. Such estimations are probably more meaningful on a personal than public level.

Statistics of increased weight at the world level (World Health Organization, 2016):

  • In 2016, more than 1.9 billion adults (39%), 18 years and older, were overweight. Of these over 650 million (13% of all) were obese.
  • Over 340 million children and adolescents aged 5-19 were overweight or obese in 2016.
  • 41 million children under the age of 5 were overweight or obese in 2016.
  • Worldwide obesity has nearly tripled since 1975.

Overweight: BMI = 25-30; obese = BMI >30. BMI (body mass index) = person's weight in kilograms divided by the square of his height in meters (kg/m2).

The following is the closest from what I found about the relationship between calorie intake and the number of obese individuals:

Share of adult men overweight or obese vs. daily supply of calories, 1975 to 2013 (Our World in Data):

See this graph: X axis is daily calorie supply (per capita); Y axis is the percent of overweight or obesity in a country/region. You can see that United States have more than 3,500 calorie supply per capita per day and that more than 70% of their adult males are overweight or obese.

The trends of risk factors of obesity, according to World Health Organization:

The fundamental cause of obesity and overweight is an energy imbalance between calories consumed and calories expended. Globally, there has been:

  • an increased intake of energy-dense foods that are high in fat and
  • an increase in physical inactivity due to the increasingly sedentary nature of many forms of work

They obviously care only about the trends and not the number of people with increased calorie intake or physical inactivity.

  • Thank you! Wasn't the quoted phrase an increase in physical inactivity intended as a decrease in physical activity?
    – pts
    Jul 30 '19 at 10:51
  • @pts, yes it was; both their and your suggestion have the same meaning but they decided to express it in the more awkward way.
    – Jan
    Jul 30 '19 at 10:53

What you are seeking cannot in scientific terms exist. The reason is that obesity is caused by a combination of the issues you are talking about.

The term "poor diet" is a general term which can mean different things in different circumstances and in different contexts. A diet high in sugars would be very poor for someone who is diabetic, for example, no matter what their weight is.

As the NHS page you cited points out in simple terms

Obesity is generally caused by eating too much and moving too little.

A poor diet in the context of fighting obesity levels is a diet higher in calories than required.

If you consume more energy in the form of foods and drink, than you burn off through exercise and physical activity, much of the surplus energy will be stored by the body as fat.

The energy value of food is measured in units called calories (kcal).

Generally speaking, the average physically active man needs about 2,500 calories a day to maintain a healthy weight, and the average physically active woman needs about 2,000 calories a day (See your NHS link).

If you are more sedentary than the average person, the amount of calories you consume will need to be lower to balance that out.

Some of calories are burnt off keeping you alive. It's the amount of energy required to maintain basic bodily functions while at rest, such as regulating body temperature, keeping the heart beating, and breathing. This is called the "basal metabolic rate" or BMR and it accounts for about 2/3 of the total calories burned each day.

It's true: just sitting on the couch staring into space requires that you burn some calories (Harvard Medical School, 2018).

The BMR varies from person to person.

Some people have higher BMRs than others (although this variability is not usually the reason someone is obese or lean). And BMR can vary over time; it may speed up when you're sick or if you've added muscle mass or it may slow down with age or when you're losing weight. In fact, a slowing metabolic rate is one reason dieters have such a hard time continuing to lose weight or tend to regain lost weight. Certain medical conditions (such as thyroid disease) and medications can affect BMR (Harvard Medical School, 2018).

The remaining calories consumed which is not burnt off at the BMR is what you need to burn off with physical activity, whether it is exercise at a gym or swimming pool, or walking around etc.

If you burn more calories than you consume, you will lose the excess body fat put on when not exercising.


Harvard Medical School (2018). Burning calories without exercise. Retrieved from: https://www.health.harvard.edu/staying-healthy/burning-calories-without-exercise

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    @pts That's not how things are studied and not an answerable question. Chris's answer explains why. Jul 28 '19 at 16:09
  • 1
    @pts, an obese person has the exact same probability to be obese because of consuming too much calories or not being physically active enough. If you think backwards: an obese person has the exact same chance to lose weight by consuming less calories or by becoming more physical active.
    – Jan
    Jul 29 '19 at 11:37
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    @pts It is still not possible to answer that question in an informative way, so people don't study it that way. Saying things like "Probability has a very specific definition in mathematics and statistics" sounds very rude while people are explaining to you why what you are asking for is not meaningful. Jul 29 '19 at 18:48
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    @pts It's that those three questions don't actually tell you much about the causes of obesity. For example: "Out of 100 obese people how many eat more calories than the average healthy, non-obese person of their age and gender?": if 50 eat more and 50 eat less, that doesn't mean eating more calories doesn't contribute to obesity, because the 50 who eat less are more sedentary. The question you want to ask is "if you eat more, are you more likely to be obese?" Jul 29 '19 at 19:56
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    @pts To cut to the quick, I have a strong suspicion that the answer to the question is "The data you seek don't exist." I would be very surprised to learn that anyone has spent the time and money to break down the numbers by the categories listed in your bullet points.
    – Carey Gregory
    Jul 30 '19 at 4:19

From both biological plausibility and observational evidence one can conclude that Western-style diets are prone to cause obesity due to having an unnaturally high energy density. A problem within the field of nutrition is that biologically unnaturally high energy density diets have become the norm in the civilized world. People who eat a natural diet with a biologically normal energy density can be found in indigenous populations who are isolated from modern civilization, the vast majority of the people in the civilized world don't eat that way. Almost all research on the relation between diet and health involves the diets the people in the civilized world eat.

The gap between these sort of high energy density diet most people in the civilized world eat and a biologically natural low energy density diet is so large that the data of people participating in studies who do eat the latter type of diet would be filtered out as "implausible outliers". E.g. many studies use a limit of 1 kg of a single food item as an upper limit. But if you eat 1000 Kcal worth of potatoes you do need to eat 1.3 kg worth of potatoes. That may sound like an impossibly large amount of potatoes to eat, but I (a small guy weighing just 53 kg) regularly eat that amount of potatoes for dinner without problems. Eating large volumes of food is the only way you can get enough calories in the natural world where high energy density foods like cooking oils, butter, cheese etc. don't grow from the trees.

While it seems plausible that only total caloric intake and energy expended due to exercise should be relevant for obesity, we can easily see that there is a problem with biological plausibility here. Consider a group of animals who have a stable body weight who due to some environmental perturbation suddenly need to expend more energy to get their food and the amount of food they get is also a bit less than they used to be getting. If body weight were to be a delicate fine-tuned balance between energy intake and energy expended as a result of exercise, they would starve to death. Even a small imbalance of just 100 Kcal a day less energy intake compared to what's expended would cause a weight loss of 40 kg in 5.5 years.

This calculus must thus be wrong, obviously the fault lies with ignoring feedbacks on the metabolism. As the animal loses weight, it will expend less energy in moving around, but there are also more direct feedback effects. As the fat cells become emptier, they produce different hormones compared to when they were full, and this impacts appetite and it modulates metabolism. A lot is still unknown about such feedbacks on the metabolism, but it's implausible that mammals who burn energy at a fast rate would not modulate their metabolism as a response to their energy reserves, especially at rest when it would affect physical performance the least.

Such feedback mechanisms then keep the body weight stable under natural conditions, they don't only prevent the animal starving to death due to trivial reasons, it also works the other way and and prevent the animal from gaining weight due to eating a bit more on the long term. Weight gain and becoming physically unfit as a consequence is also very dangerous to animals living the wild.

The reason why we are prone to getting obese must then have something to do with our unnatural diet, rather than the energy balance between exercise and calorie intake, unless this energy balance is out of wack much more than what the natural feedback mechanisms can compensate for. The latter cases do occur, our very high energy density foods make it easy to stuff our stomachs with thousands of Kcal worth of food more than would normally fit in there. But most people who complain about gaining weight and try all sorts of diets to lose weight, don't fall into this category.

The typical people who struggle with their weight are people who eat a normal amount of calories, say between 2000 and 2500 Kcal a day, they stop eating before they're full, they feel hungry during the day and try to tolerate that as best as they can not doing that would lead to them gaining weight. So, their problem is that their weight is by far not as stable as that of animals in the wild. The high energy density of the diet plays a direct role on a perception of fullness after a meal but this alone doesn't explain why they need to fine tune the calorie intake to prevent weight gain.

Another relevant property of high energy density foods is that they have a low nutrient density. While we make sure that we do get all the essential vitamins and minerals, we can't be sure whether we get enough to prevent problems that are not acute medical problems, such as the risk of getting obese. One very important component of the diet is fiber. A natural low energy density diet will contain 80 grams of more of fiber per 2500 Kcal. The RDA which takes a high energy density diet where you would get a substantial part of your calories from refined oils as the norm, is 40 grams a day. But most people get only half of that, about 20 grams a day.

What could plausibly go wrong of we eat less than a quarter of the natural amount of fiber our body has evolved to eat? Fibers are food for the intestinal microbes, the lack of fiber has been linked to obesity. The mechanisms proposed in he literature are i.m.o. too simplistic as they appeal to arguments that would bring back the calorie intake finite tuning problem. I.m.o., getting the required amount of fiber allows the body to have a microbiome that it can more easily control to help keep the body weight stable.

As I explained above a stable body weight without having to count calories or count the number of steps you take per day, is essential for long term survival. Mechanisms that keep the body weight stable must thus have evolved. Evolution doesn't care about how exactly such mechanisms are implemented, whether it's purely internally regulated via hormones or via the microbiome with microbes eating more of your food if you eat more. What matters is whether all mechanisms together produce the desired result in a robust way.

The fundamental cause of obesity thus has nothing to do with calorie intake. In fact, putting the blame on calorie intake has led people to reduce portion size which causes people to stick to the extremely unhealthy high energy density diets that cause obesity. To get out of the obesity epidemic we need get the entire population to get gradually used to eating far larger volumes of healthy low energy density foods.

  • Thank you for the insights! Unfortunately it doesn't answer my question, because it doesn't provide any percentages asked.
    – pts
    Aug 9 '19 at 10:12

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