Eli5: Why is being skinny healthier than being fat?


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.

Even when considering factors such as sex, age, and smoking: low BMI and high BMI both have a higher Hazard Rate than normal BMI. See any of the charts in [this study](https://www.thelancet.com/journals/landia/article/PIIS2213-8587%2818%2930288-2/fulltext). The Hazard Rate plotted by BMI looks like a U or J shape, meaning being underweight or overweight are both bad for your health.

Also, there is the statistic of “Anorexia Nervosa has the highest death rate of any psychiatric illness (including major depression).” While it isn’t required to be underweight to have Anorexia Nervosa, it is typical. It is incredibly deadly to have this mental disorder, even more deadly than obesity, but there are lots more obese people than people with Anorexia Nervosa, so obesity accounts for a larger share of the total deaths.

This post is a good example of the too-common “ELI5: Why is <false-statement> true?”. Also “ELI5: Why is <my personal experience> a fact of life experienced by everyone?”

Pure physics really, every extra pound means that your body has to work that much harder any time you move, stand etc.

Skinny is also rarely unhealthy by itself but rather when low weight is a symptom of some other issue. Skinny and healthy is possible. Fat and healthy isn’t really possible because of all the extra wear and tear it causes

Fat cells around your body is very important to the delivery of nutrients and serves as temporary storage for things like iron and hormones, regulating release of those throughout your body. Certain fat cells are responsible for creating some heat around your body as well.

Having too much fat also means having inflamed fat cells that can’t regulate proper distribution of needed nutrients or might start behaving in a hoarding fashion.

Think of it as a fat cell being a house. If you start putting more things in the house that is coming out, soon it is impossible to navigate in it and than at that point, you could empty the house (lose weight) or build a second garage, a shed, a third floor, etc. (gain weight)

If you start taking out things from the house that is necessary to live like electricity, running water, etc. (being too skinny) your house becomes unable to operate and unable to deliver more nutrients outs.

So let’s look at both.

Underweight people usually suffer from dietary deficits, like rickets, malnutrition, low bone density, damage to the esophagus (bulimics) and eventually death.

Obese people generally suffer from increased risks of diabetes, heart disease, depression, increased musculoskeletal strain, high risk of fall injury, liver failure, heart attack and stroke.

Now consider the severity of each potential set of health problems. Being overweight has more deadly diseases associated with it over anorexia (and bulimia).

Now just being skinny (not anorexic) doesn’t really have health problems, almost exclusively they are health concerns. So being skinny vs being fat the fat will always be more closely associated with preventable deadly healtheffects and problems.