I think we all know at least one person who is skinny as a rail but eats rubbish, is allergic to exercise and brags about how "I can get away with it...I must have good genes, or something."
If you care about that person, I suggest you say something like, "Whoa, Nellie!" or "Not so fast, Skippy!" What look like "good genes" may in fact be "lean genes." And those aren't good. They aren't good at all.
Last month, researchers reported on a newly identified gene that is linked to having less body fat AND and increased risk of heart disease and type 2 diabetes. Specifically, the gene, known as IRS1, leads to unhealthy levels of cholesterol and glucose in the blood. And it turns out the "lean" part of this "lean gene" only applies to subcutaneous fat, aka fat under the skin. People with this gene easily carry harmful levels of fat around their internal organs (visceral fat). It's long been known that while being overweight or obese can be harmful to health, it's excess visceral fat that can be especially dangerous.
This gene discovery fascinates me for many reasons, but especially because it may well explain why some skinny people get type 2 diabetes. Type 2 diabetes used to be known as adult-onset diabetes, because it typically developed in middle age or beyond (as opposed to type 1 diabetes, which is an autoimmune disease that almost always begins in childhood). But with the current obesity epidemic, even children are now developing type 2 diabetes, when their young bodies become so trashed by junk food, lack of physical activity and excess body weight that their cells don't respond to insulin properly, allowing an excess of glucose (sugar) to build up in the blood stream and wreak havoc.
Anyway, in spite of the fact that there are strong links between overweight/obesity and type 2 diabetes, not everyone who gets this form of diabetes is overweight. And that's been a puzzler. This "lean gene" may be the missing puzzle piece. [I don't know this for sure, but I eagerly await the additional research that will come out of this gene discovery.]
The take-away is this: While many of us do our best to eat right and be active so that we can reach and maintain a healthy body weight, the size of our jeans is not the only reason to live healthfully. None of us knows exactly what protections or potential time bombs our genetic code holds. Even if we did know, that code is not absolute. The habits we adopt and the actions we take each day affect our health, for better or for worse. A healthy lifestyle can, in some cases, stop a "bad gene" from expressing itself. An unhealthy lifestyle can stop a "good gene" from expressing itself.
What you do for yourself and your current and future health...it matters. And your size and shape only tells one part of the story.
If you'd like to read more, there are articles here and here. And for any science junkies lurking, I found a PDF copy of the actual study here.