Why Being Thin Doesn’t Always Mean Being Healthy

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If there’s a universal truth in health news lately, it’s that being overweight isn’t good for your health. Extra weight, especially in the form of fat, can lead to heart disease, diabetes and high blood pressure, among other problems.

But a new study hints that being lean doesn’t get you entirely off the health hook either. In a genetic analysis involving more than 75,000 people, an international group of scientists led by Ruth Loos at the Medical Research Council in the U.K. found that lean people with a specific genetic variant were at higher risk of developing type 2 diabetes and heart disease despite their lower body fat.

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The key, say the scientists, is to focus on not just the amount of fat, but the type of fat that you might have. A growing body of evidence shows that fat deposited just under the skin doesn’t contribute that much to the development of metabolic disorders such as diabetes or heart problems. But fat accumulated in deeper tissues and organs, within muscle and embedded in organs like the liver, for example, can put you at greater risk of these diseases. And that goes for lean people too: they might not have much visible fat under the skin, but may be sequestering so-called visceral fat inside their body.

What regulates where you store fat? Unfortunately, says Loos, much of it is out of your control — it’s largely genetic and gender based. Women tend to store fat under the skin, while men are more likely to deposit it deeper in tissues. As for the genetic factor, in the current study, the researchers identified one variant out of more than 2.5 million candidate sites on the genome linked to body fat. This genetic variant seems to predispose people to depositing visceral fat as opposed to the more benign subcutaneous fat. “We think the gene we found causes an error such that it’s harder for these people to store fat under the skin,” she says. “So the fat is stored elsewhere, around the organs and in muscle where it disturbs the normal function of these organs.”

What surprised the scientists was the power of this genetic abnormality on metabolic factors such as cholesterol and triglyceride levels and insulin resistance. Even people with low body fat but who possessed this version of the gene, consistently had higher levels of blood cholesterol and were more likely to show some resistance to processing insulin, one of the first signs of diabetes. Many of these individuals showed normal body mass index or BMI readings, which assess weight but don’t take into account the percentage of body fat.

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The findings highlight the importance of refining our understanding of the relationship between weight, body fat and disease, says Loos. Many people who look lean may assume they are healthy and may not be as likely to get regular blood tests to check their cholesterol and blood glucose levels. That may put them at higher risk of having a heart attack or suffering from tissue and organ damage due to diabetes because they never knew they were at risk. Getting regular checkups, regardless of your weight, she says, may help more people to stay healthy and avoid diabetes and heart problems.

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