Opinion | Carbohydrates, not calories, may be blamed for obesity

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David S Ludwig is a co-director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital, professor of pediatrics at Harvard Medical School, and professor of nutrition at the Harvard TH Chan School of Public Health.

The usual way of understanding obesity is simple: If you consume more calories than you need to fuel yourself, the surplus is deposited into body fat, and you gain weight. Because, according to this approach, all calories are alike to the body, the only way to lose weight is to eat fewer of them or burn more off with exercise.

For a century, this “energy balance” notion has dominated obesity prevention and treatment, from the original focus on calorie-counting in the early 1900s, to the low-fat diet (targeting the most energy dense nutrient) of the late 1900s, to the recent emphasis on reducing consumption of modern processed foods high in fat and sugar.

If this theory is correct, though, it’s hard to square with the facts. After more than a three-decade increase, calorie consumption in the United States has plateaued or decreased since 2000, a new analysis concludes. But obesity rates have increased by more than one-third since then, to an astounding 42 percent of the population today. This paradox cannot be explained simply by our sedentary lifestyles — in fact, Americans have become somewhat more physically active over the past 20 years.

So what if the focus on calories and energy balance is simply wrong, reversing cause and effect? Writing in the European Journal of Clinical Nutrition this week, my co-authors — researchers, physicians, public health experts — and I argue that overeating isn’t the primary cause of obesity. Instead, the process of gaining weight causes us to overeat.

This is a different model of obesity, the carbohydrate-insulin model. This theory puts the blame for rising levels of obesity on the processed, fast-digesting carbs that flooded our diets during the low-fat diet craze — white bread, white rice, prepared breakfast cereals, potato products and sugary foods. It positives that consumption of these carbohydrates raises insulin levels too high and produces other hormonal changes that program our body to store extra fat.

Looked at this way, obesity isn’t an overeating problem, it’s a calorie distribution problem — too many calories from each meal being siphoned off into fat tissue and too few remaining in the blood to satisfy the energy needs of the body. Consequently, our brains make us feel hungry sooner after eating to compensate for those sequestered calories. If we try to ignore hunger and restrict calories, the body conserves energy by slowing metabolism. In this sense, obesity is a state of starvation amid plenty.

According to this theory, simply cutting back on calories doesn’t work over the long term, because it doesn’t address the underlying predisposition to store excessive fat driven by hormones and other biological influences. Instead, the focus should be on reducing the surge of blood glucose and insulin after meals with a higher-fat diet low in processed carbs. This way, fat tissue can be coaxed to release the pent-up calories, leading to less hunger. Weight loss occurs without the need for calorie restriction, increasing the likelihood of long-term success.

So, is the carbohydrate-insulin model more correct than energy balance thinking? Unfortunately, we still don’t know for sure. The definitive research needed to resolve this controversy has never been done, in part because alternative paradigms for obesity have not been taken seriously.

Two scholarly papers, in addition to our new one this week, have aimed to build the carbohydrate-insulin model from available scientific evidence. Yet there have been more than a dozen papers from critics claiming to have disproved the model based on weak evidence, such as small, short-term trials of two weeks or less.

Meanwhile, despite investing in many major low-fat diet trials (virtually all failing to show any benefit for the main outcomes), the government’s National Institutes of Health has yet to fund a single long-term low-carb trial of a similar scope. This has not been a fair contest of ideas.

One reason for this resistance might be cultural. For centuries, obesity has been viewed as a character flaw. Despite decades of research into the genetic and biological influences on body weight, people with obesity continue to be stigmatized, more so than those with almost any other chronic disease, as if their weight were their fault.

Energy balance thinking implicitly contributes to these stereotypes by blaming overeating on poor self-control. Although newer energy balance versions emphasize primal reward centers in the brain that drive food intake, either way, people with obesity are considered unable to resist tempting foods for conscious or subconscious reasons. If the alternative paradigm is correct, however, then deeply ingrained notions about obesity are simply wrong.

Scientists are supposed to be skeptical. But when variations of the same approach keep failing — and rates of obesity keep going up — it’s critical that new ideas are encouraged, not suppressed. And with the cost of just one weight-related complication, type 2 diabetes, almost $1 billion a day, we must consider different ways of solving the intractable problem of obesity and open our minds to a radical-sounding notion: Overeating is a symptom, not a cause.

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