What causes obesity?

Take-home points

• Obesity is caused by eating too much and moving too little. However, people’s lifestyle choices are influenced greatly by genetic, environmental, and socioeconomic factors.
• Changing eating behaviour and activity levels will most certainly promote weight loss, but the sustainability of these approaches is demonstrably poor for reasons that are often beyond people’s control.
• There are effective pharmacological and surgical obesity treatments when lifestyle changes fail to benefit an individual.

Eating too much, moving too little?

On paper, the cause of obesity is simple: eating too much and moving too little. This state is called being in a positive energy (calorie) balance, where excess calories are stored mainly as body fat.

If somebody does this consistently, they will likely gain weight until they become clinically obese (defined as a body mass index equal to or above 30). In turn, “eat less and move more” has become the go-to tagline for tackling obesity, which affects over 650 million adults and 340 million children and adolescents. People now consume ~20% more calories than when obesity rates were low in the 1970s, and such changes can explain the bulk (if not all) of body weight changes in most countries.^1,2 There also appears to be a continued reduction in activity levels over the last 20 years that only exacerbates dietary issues.^3,4

Nevertheless, despite growing recognition of the problem, obesity rates are still increasing worldwide and remain a global epidemic. To much surprise, targeted attempts to lose weight by dieting and exercising are also generally far less effective than we might expect. For example, in an analysis of 29 long-term weight loss studies, half of all weight lost is regained by two years, and 80% by five years.⁵ Even the best lifestyle changes have an average weight loss of only ~ 5% body weight, which just scrapes the cut-off for being ‘clinically effective’.

But how is this possible? How can eating less and moving not be more effective when the explanation for obesity is so clear? Well, simple explanations often shadow complex underlying causes. 

Underlying causes of obesity

If we turn to recent scientific reviews, the cause of obesity is not solely defined as a eating too much and moving too little; rather, there is broad acknowledgement that deeper causes of obesity rest with influencers of peoples lifestyle choices, such as genetic, environmental, and socioeconomic factors.^1,6,7 The ‘true underlying causes’ of obesity are more elaborate than a simple explanation of energy balance might imply. Changing lifelong habits is difficult.

Environmental influencers of lifestyle

By environmental, we are talking about the environmental influencers of eating behaviour and activity levels. Humans are not robots, after all; we are susceptible to changing our behaviours in response to what we are exposed to, often beyond our conscious awareness. For example, nationwide surveys have found that the number of calories people have access to (for the same price) has increased by 500–700 calories per day in the last 50 years. In addition, the type of food available is generally now more processed, satisfying, and nutritionally lacking, which makes it more difficult for people to self-regulate their food intake.

Intricate experiments show that people spontaneously consume around 500 more calories a day when eating ultra-processed foods compared to unprocessed foods.⁸ So not only is there more food available today than when obesity rates were low, but the type of food available is now more calorie-dense, more pleasurable, and less likely to keep people full. Regarding exercise, a shift to working from home, closer proximity to local facilities, and less outdoor playtime activities for children, are among some environmental changes that have reduced overall activity levels.^9.10

Unhelpful marketing ploys from food companies only exaggerate the harms of such changes to the food environment. Whether you look at TV adverts, billboards, delivery apps, or social media adverts, we mostly see images of donuts, cookies, and milkshakes more than broccoli, blueberries, and beans. With over 50% of calories now sourced from ultra-processed foods, prioritising the consumption of natural foods is now described as being “on a diet”.¹⁰ We simply cannot overlook that our innate food preferences are commercially exploited for financial interests, resulting in more seductive and rewarding foods than we are evolved to handle. One recent study showed that restricting advertising on high fat, sugar and salt products on public transport can reduce the household purchasing of these foods by 1000 calories per week.¹¹

Genetic influencers of lifestyle

Yet, in an equally problematic environment, how do some people stay thin while others gain weight? To a large extent, genetics explains these differences. Though differences in willpower and self-responsibility often get the blame, a series of family, twin and adoption studies have consistently shown that body weight is a highly heritable trait.¹² To put a figure on it, comprehensive studies have found ~70% of body weight differences between individuals is explained by genetic differences.¹³ That’s not to say people are born obese—rather, genetic susceptibilities to overeating and physical inactivity are present from birth. We might say that “genetics loads the gun, and the environment pulls the trigger”.

Most obesity-related genes are involved in the regulation of appetite, i.e. how much we think and feel the urge to eat. Individuals with obesity tend to experience a greater sense of reward and emotion in response to food.¹³ They also have weaker internal signals to stop eating and remain full compared to individuals without obesity.¹³ On top of dietary influences, though, recent studies have found that habitual physical activity is also genetically influenced.¹⁴

Socioeconomic influencers of lifestyle

Socioeconomic issues, particularly those affecting food choices, offer another explanation for why people have different body weights in similar environments. Due to financial inequalities, we must accept that people have various freedoms to buy, store, and cook healthy foods. In fact, a spending and income analysis in the United Kingdom found the poorest half of the population would have to spend ~30% of their disposable income just to meet the Eatwell Guide’s targets–in households in the lowest 10% of income, this figure was a high as 73.6%.¹⁵ So food choice is not always a matter of personal freedom, as choices are sometimes constrained by, well, life. Hence why the most socially deprived areas have the greatest exposure to fast-food chains and the most limited access to basic foods such as fruits, vegetables, tubers, and fresh meats.^16,17 Not a great combination.

Treatments for obesity

A range of obesity treatments exist, with varying degrees of benefit. The recommended first-line approach to obesity treatment is lifestyle change, i.e. changing eating behaviour and activity levels. Both of these changes have the potential to offer tremendous benefit. There is data to support weight loss success with a low-fat diet, low-carb diet, Mediterranean diet, plant-based diet, and certain dietary programmes, to name a few. All of these can help to achieve and maintain a low-calorie diet.¹⁸ Similarly, exercising at least 90 minutes per week at a moderate to high intensity can promote weight loss. ¹⁹

The problem, however, as discussed throughout this article, is the sustainability of these weight loss approaches. As there are many barriers to lifestyle change–genetic, environmental, and socioeconomic–the average weight loss from lifestyle interventions, even good ones, is only ~5% of total body weight. Some people lose a lot more, but others lose none, at least in the long-term. This benefit may be clinically significant but is unlikely to rid the obesity crisis without dramatic changes to the food environment and socioeconomic barriers to change.

But on a positive note, pharmacology and surgery offer alternative treatment options for obesity. These options have evolved over recent years and can benefit people struggling to maintain lifestyle changes, or who have an amount of weight to lose that is unlikely to be achieved with lifestyle changes alone. Some newer pharmacological and surgical options even have superior success rates and greater long-term weight loss compared to lifestyle changes. Surgical options can directly remove body fat (liposuction) or alter the physical structure of the gut to reduce food load (gastric surgery).^20,21

Newer pharmacological options tend to biologically regulate appetite, reducing one’s desire to eat. In any case, it is extremely important to discuss the benefits and risks of all weight loss approaches with a healthcare professional. 

References:

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