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What Do We Really Know about Obesity?

Updated: 8/14/21 10:00 amPublished: 2/27/13

By Alasdair Wilkins

Obesity is the most common chronic disease in the United States. Over a third of Americans are obese (a body mass index >30 kg/m2), and another third are overweight (a body mass index of 25-30 kg/m2). Treating the health complications of obesity costs the United States $147 billion in medical expenses each year. Obesity is of course a proven risk factor for type 2 diabetes, and it’s also linked to increased risk of stroke, heart disease, hypertension, high cholesterol, and kidney and gallbladder disorders.

And yet, for all the importance of understanding what causes obesity and how it can be treated, it’s surprising how much we really know about obesity, especially compared to how much we think we know. That’s the focus of a new paper in The New England Journal of Medicine, in which 20 researchers from throughout the United States and Europe examined “Myths, Presumptions, and Facts about Obesity.” The paper not only provides evidence of misinformation seen in news reports and supposed weight loss cures, but it also addresses plausible, but nonetheless unproven assumptions you may hear from even healthcare providers and nutrition experts. We should stress than none of this is meant to suggest we know nothing about how obesity works – indeed, there’s tons of great science about weight management. To that end, when we spoke with Krista Casazza, PhD, RD, of the University of Alabama-Birmingham, a registered dietitian and the study’s first author, she explained why she and her fellow researchers decided to set the record straight:

This came from years of thinking, talking and reading among colleagues. We would hear these things and question; ‘Did you know that was true?’, ‘How do they know that?’, ‘Where is the evidence that this is true?’ Eventually we as a group tried to bring some of this to light.

Just The Facts: What We Know About Obesity

The researchers point to two key facts in their paper. First, although the word “diet” has gotten a bad rap for its association with trendy weight loss programs with often-dubious credentials, diets do indeed work by definition. This is an important distinction the researchers make – on the most basic level, dieting is simply reducing a person’s energy intake, and reducing energy intake is an extremely effective way to lose weight. The problem, of course, lies in sticking with a diet long-term – it’s very challenging. The other issue is that people often adopt healthier eating habits – like eating more fruits and vegetables or eating breakfast daily – without also reducing their overall food intake. While the healthier habit is certainly commendable, in these cases, that person isn’t actually dieting.

The second key idea is that weight loss is possible through moderate changes to a person’s environment, regardless of that person’s particular genetic predispositions. In fact, environmental change alone can be just as effective as any obesity therapies currently available. The trick is, of course, identifying which environmental factors will actually lead to significant reductions in obesity at the population-wide level – things like taking the stairs instead of the elevator, parking farther away, and making healthier foods more prominently displayed in the grocery store.

Why Weight Loss Isn’t Everything

There are some lifestyle changes anyone can make that have been established to improve health, if not always lead to weight loss. For instance, increased physical activity and exercise can offset the health problems associated with obesity, although substantial physical activity is required before person’s weight changes dramatically (e.g., running a mile burns only ~100 calories, easily offset by eating a tablespoon of peanut butter). It’s important to remember that health and weight are two different things – while there’s generally a health benefit to losing weight, it’s possible to improve one’s health while maintaining the same weight.

Indeed, weight maintenance is a crucial concept (see our diaTribe dialogue with Retrofit CEO Jeff Hyman for more on this topic) – while the initial goal of a diet is to lose weight, the bigger key to long-term success is to maintain the lower weight. As the researchers explain in their paper, obesity should be thought of as a chronic condition, so any treatment plan is a long-term project. The way to ensure ongoing success is to pinpoint what conditions and changes allow you to lose weight and stick with them (e.g., a specific combination of diet and exercise).

Finally, while the medical treatment of obesity is relatively new, a few proven facts have emerged. First, pharmaceutical agents like Qsymia and Belviq do work, when partnered with calorie reduction and physical activity, and they can allow people to achieve meaningful weight loss and maintain it, but only for as long as such medications are used. The researchers argue that such medications are helpful while we try to better understand and find solutions for preventing obesity.

One last treatment-related obesity fact deals with meal-replacement products and the structuring of meals. While diets that incorporate variety and moderation might seem like a good idea, these can prove more challenging for people to maintain the right portions on an ongoing basis. Instead, it’s been proven that the more structured a meal plan is, the more likely it is to lead to weight loss.

What We Think We Know: Unproven Assumptions about Obesity

The beliefs we’re about to discuss have neither been proven nor disproven – all of them are plausible, and all have some amount of data to back them up, but the data is controversial or is not data that could be considered definitive proof. These ideas are not useless, but a weight loss plan based on these ideas might be less likely to succeed than one built on the proven facts discussed above. 

The value of eating a breakfast everyday as opposed to skipping the meal is one of the most commonly heard obesity beliefs. In theory, people who skip breakfast are more likely to overeat later in the day, whereas those who eat breakfast eat more moderately throughout the day and less overall. It seems reasonable enough, but there isn’t scientific data to support it – in randomized, controlled trials (RCTs, the gold standard of nutrition research), researchers found no difference in weight management between those who were assigned to skip breakfast and those who were assigned to eat breakfast every day. In fact, eating a larger lunch than usual (e.g., 200 extra calories) would still be less than eating a standard breakfast (500-600 calories). 

Another common belief is that eating more fruits and vegetables or cutting back on snacking will help lose weight. While fruits and vegetables have some significant health benefits, eating more of them won’t lead to weight loss or even reduced weight gain without an overall calorie deficit  – in that case, the extra fruits and vegetables mean increased food intake, no matter how healthy they are. On the other hand, snacking is often believed to cause weight gain, as people who eat between meals won’t reduce what they eat at breakfast, lunch, and dinner. But according to the research, there isn’t evidence that links snacking and obesity – neither RCTs nor observational studies have revealed a consistent link between snacking and either obesity or weight gain. Nutrition experts point out that it is challenging, of course, to fund expensive RCTs.

Proven False: The Biggest Obesity Myths

Some of the obesity myths discussed in the paper may seem obviously false to those unfamiliar with them – for instance, there’s no proof that breast-feeding children decreases their risk of obesity, although that myth has lasted over a hundred years. But there are some common obesity myths that might be surprisingly unfounded.

For instance, take the idea that it’s important to set realistic goals for obesity treatment plans. The myth comes from the belief that people who try for too much weight loss will get frustrated when results don’t match their expectations and consequently abandon their diet and exercise plans. It’s a compelling idea but is not backed up by the scientific data. No studies have shown a link between ambitious weight loss goals and increased risk of failure, and if anything there’s some evidence to suggest people who set very high goals actually lose more weight.

Along similar lines, it’s often said that slow and steady weight loss plans are preferable to those that involve rapid weight loss. There is some truth to this idea – certainly, crash diets that are mostly starvation carry serious health risks. But emphasizing the value of gradual weight loss may go too far in the other direction. According to the authors, there’s evidence that some people lose weight initially much faster than others, and this may not be a problem in and of itself. That said, since we don’t know yet which groups of people this applies to, it’s wisest to be cautious when it comes to embarking on food reduction plans and to discuss it with your healthcare provider. 

Another commonly repeated notion is that small, sustained changes to a person’s diet will add up to large weight changes in the long-term, and this can work in either direction: exercising for 15 minutes each day will lead to substantial long-term weight loss, while eating a daily small snack will add up to significant weight gain. As the researchers note, our national health guidelines and many respected websites champion this idea, and it’s not hard to see the myth’s appeal, since it implies that meaningful changes aren’t necessary to achieve significant weight loss. Unfortunately, the body’s energy requirements change in response to changes in weight, which means small changes – whether positive or negative – may not add up to much in the long run.

Why This Matters

Perhaps a takeaway is that there’s still a lot we don’t know about obesity, and what we do know often involves some complex exceptions. Neither this review nor the reseachers’ original paper is meant to suggest you should ignore all the well-proven facts about weight management just because some assumptions haven’t yet been proven or have been debunked. If all this can seem a little confusing, a good guiding principle is that there’s no one simple answer to weight loss. Some beliefs widely repeated by healthcare providers and nutrition experts might be associated with positive weight benefits, but further study will help more concretely link that belief to its benefits, or lack thereof. A healthy degree of skepticism is wise whenever you hear new “facts” about obesity, and the best approach is simply to find a weight management plan – which won’t always be a weight loss plan – that is healthy and successful for you.




  • Realistic goals are necessary for the success of weight loss or obesity treatment plans

  • Gradual weight loss is the healthiest and best sustained type of weight loss

  • Small lifestyle changes can lead to large, long-term weight changes

  • Eating more fruits and vegetables can cause weight loss

  • Eating breakfast every day can help prevent weight gain

  • Snacking between meals can cause weight gain 

  • Diets, or the reduction of energy intake, can cause weight loss, but not changing out one type of food item for another

  • Moderate environmental changes can cause as much weight loss as a pharmaceutical agent

  • Programs that involve parents can have a greater effect and more sustained weight loss for children than outside-of-home programs

Three of the myths, presumptions, and facts from the NEJM article; please see the article for the complete list. 


What do you think?