Eli5: Why is being skinny healthier than being fat?

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I mean shouldn’t the extremes of both end be equally harming to a person? why do skinny/underweight people and fat/overweight people not have the the same amount of health issues?

In: Biology

You are creating a false dichotomy – there is a range of healthy weights. Over that, you are obese and develop health problems because your body didn’t evolve to carry that weight, perform under changed metabolic conditions and deal with literal fat clogging your blood vessels. Under the range of normal weight, you develop different types of problems as the body is dealing with muscle wasting and trying to keep your brain alive. If you go significantly under the normal weight, you die.

They do, speaking as someone who went through rapid weight loss with my depression. I was sickly, really weak and deficient in like every vitamin, I would feel dizzy if I stood up too quickly. Fortunately I was not severely underweight and so I didn’t have the worst issues. I’m pretty sure researching the consequences of eating disorders and extreme starvation could give you all the answers you need.

Both extremes are unhealthy and very likely lead to premature deaths. But in nearly all societies, there is a much higher proportion of overweight people than underweight people. In that sense, it doesn’t make much sense from a public health perspective to raise alarms and focus resources about being underweight.

Extremes are definitely both harming, and the further away you get from the average weight in either direction the worse off you’ll be. In fact, in relation to BMI, your health will **quickly and severely** drop off far faster if it’s too low, killing you from malnutrition/starvation. For reference, a normal ***adult*** (if you’re younger than 18 reading this, *this* ***does not*** *apply to you*) BMI is 20-24.9; go up 5 points to 30 and you’re obese and generally less healthy, go down 5 points to 15 and you’re in severe danger.

I’m happy to go into the details of the dangers associated with either, but the ELI5 version is that underweight people have significantly higher short-term risk for injury/death and overweight people have significantly higher long-term risk for injury/death.