Changes in your microbiome - the billions of bugs that live in and on you - are the cause of the yo-yo effect that sees dieters paradoxically gaining weight over time, researchers in Israel have discovered.
Over 40% of the world's population is now overweight, one third of them morbidly obese.
Now a new study published this week inNatureby scientists from Israel's Weizmann Institute, shows that the reason why 80% of dieters relapse after weight loss - and in fact rebound to a higher body mass index within 12 months - is down to their microbiomes.
There are thousands of different groups of microbes inhabiting the intestines. The composition and relative proportions of these make up a microbial fingerprint that is unique to each individual.
When a person gains weight, this introduces a shift in the bacterial spectrum, which in turn affects how that individual handles energy and contributes to further weight gain.
Shedding excess weight again reverses the metabolic consequences of being overweight, such as raised cholesterol and insulin levels, but, Christoph Thaiss and his colleagues have found, does not reset the microbiome.
Working with mice subjected to the rodent equivalent of a human yo-yo dieting cycle, where the animals alternately gained weight by eating high calorie foods and then lost weight again by eating normal chow for a while, the team were able to track the responses of the animals' intestinal bugs.
While weight loss did indeed revert the metabolic consequences of being overweight, only about 45% of the bacterial populations returned to their pre-obese statuses.
To see how long these changes would persist, the team kept their formerly-overweight animals on a healthy diet and at a normal weight while measuring their intestinal microbiome every 3 weeks.
It took over 6 months before their intestinal bacterial populations began to resemble what they had done before the animals first became obese. Crucially, exposure to an "obesogenic" environment in the interim led to very rapid reinstatement of weight gain.
This explains why humans who "crash diet" ahead of a wedding or holiday invariably rebound to a higher weight if they do not maintain the good discipline that helped them slim to start with.
The team speculate that this "lag effect" in the microbiome might be an evolved trait that, under normal circumstances, helps to maintain a stable state in the intestine.
It prevents short-term changes in diet, or bouts of ill-health, from grossly changing our metabolic milieu, but in the world of plenty we now inhabit, it's turning out to be a pain in the posterior.
The researchers do offer one solution, however. They noticed that their overweight mice had lower levels of antioxidant flavonoids, because these chemicals were being broken down by the intestinal bacteria.
Flavonoids boost energy consumption, particularly in human brown fat which is the major site of heat production in the body.
Supplementing their animals with two flavonoids, apigenin and narigenin reduced their susceptibility to rebound weight gain. Would-be dieters should therefore perhaps take heed and up their intake of apples, which are a particularly rich flavonol source!
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