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By Matthew Rees

Food and Health Fact #159: Why has the U.S. obesity rate been rising when Americans have been eating less?

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“It’s time to share a striking, and not widely appreciated, secret: we don’t have a clear explanation for the obesity epidemic.” 

That’s the bracing start to a fascinating article recently published by The American Journal of Clinical Nutrition. The author, Dariush Mozaffarian, is a former professor at Harvard Medical School, currently dean of the Friedman School of Nutrition Science and Policy at Tufts University, and one of America’s most thoughtful analysts of issues at the intersection of food and health.

He begins by highlighting an uncomfortable fact for the conventional theory that the rising U.S. obesity rate is simply a product of increased consumption: U.S. government data shows no increase food consumption for at least the past two decades (see image below for 1999-2008) – a period when the U.S. obesity rate has steadily crept upward.

In fact, consumption has fallen by a small but statistically significant amount across weight classes and among children.

Mozaffarian discounts the idea that food consumption data is so flawed as to be wholly unreliable. And as for lower levels of physical activity, he writes that, “there is currently little evidence that widespread, continually declining physical activity is a major cause of the obesity epidemic.”

The real culprits, he says, may be in three interrelated areas: the gut microbiome, metabolic expenditure, and inter-generational transmission of risk – with each one influenced by increased consumption of highly-processed foods.

First, here’s what he says about the microbiome:

“Over the last 50 years, changes in crop breeding, food manufacturing, and consumer choices have led to more processed starches and sugars in the diet. Such refined ‘acellular’ carbohydrates – lacking any natural, intact plant cellular structure – are rapidly and completely digested in the stomach and small intestine, causing a double insult of excess flux of nutrients to the host and insufficient nourishment of the gut microbiome.”

In other words, the human body is absorbing more and more of what’s being eaten, and the microbiome is absorbing less. This dynamic, he writes, “could be a contributor to the obesity epidemic.”

Second, he notes that changes to the microbiome, as well as evolving dietary compositions, can alter metabolic expenditure (which refers to how many calories the body burns). “For instance, compared with dietary fats, higher amounts and processing of carbohydrates can reduce metabolic expenditure after weight loss by ~200 kcal/d, with even larger effects among individuals with elevated carbohydrate-stimulated insulin secretion.”

In other words, the body burns fewer calories, which increases the likelihood of weight gain.

Third, his commentary on inter-generational influences maybe the most alarming:

“As successive generations become more obese, risk may be transmitted to the next generation that increases their susceptibility independent of energy intake. The composition of the microbiome, clearly linked to risk of obesity, is transmitted from one generation to the next.”

Thus his startling summary: “A toxic food environment begat a toxic biologic environment, creating a self-sustaining, difficult-to-reverse cycle.”

One possible explanation for these changes, writes Mozaffarian, has been the industrialization of food. “Changes in food production and processing in the latter half of the 20th century could have contributed the initial population hit’, increasing energy intake, shifting the microbiome, and disrupting the prevailing, relatively stable rates of obesity.”

All of this calls for more research, he says: “The interplay between persistent changes in our environment and persistent changes in our biology has not been sufficiently investigated nor quantified.”

Thus his sobering conclusion: “We don’t have a clear explanation for the obesity epidemic – and it's time to acknowledge, and to correct, this regrettable truth.”