Assumimg that being overweight is due to fat, not muscle.
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Because they won’t stay that way. Being overweight virtually guarantees that at some point, those tests will *not* be normal which means something is wrong. Being overweight is also a huge risk factor for many things that aren’t directly testable. In other words, being overweight virtually guarantees that a person will, at some point, experience health problems they would not otherwise experience.
When a person gains weight their skeleton and major organs aren’t designed to grow to keep up with the excess weight. Carrying extra weight around all day puts extra strain on joints and can cause aches and pain. Eventually the extra weight gives organs more work than they can handle and either the body’s cells suffer because there are too many cells for the organs to take care of, or the organs themselves suffer due to overwork.
Imagine a city with 100 people living in it. That city has things like a fire department, a water treatment plant, a sewage system, and a power plant. Sometimes the city sometimes has a big parade and 50 extra people come; all the city’s systems can handle this for a short time (the fire department works an extra shift, the power plant uses backup generators, etc.), but afterwards there’s a rest (in this analogy, this is what happens when you exercise hard, or when you’re sick — all your organs have to work harder). If those extra 50 people came and lived in the city full time, eventually the sewage system and water treatment facility would be overwhelmed and people would get dirty water, the firemen would all be exhausted because they were working non-stop and eventually they would all be so tired that a fire might be totally missed, the powerplant couldn’t keep up with peak hours and there would be blackouts.
It caused your body to work extra hard to pump blood all over the body. The more fat the more blood vessels. In addition you are prone to sleep apnea. If tests are normal now they won’t stay that way. I work in surgery and it’s is significantly more difficult to perform surgery on overweight patients. Plus there is more chance of complications
Assuming there’s lots of extra fat and all tests are still good, and that’s a big assumption, you still have at least three problems.
#1 The joints were not designed for all that weight. Knees and back will be the first problems. After that it could be anything. There are a million tiny muscles designed to keep joints aligned, they will be overstressed. The cartilage did not get thicker when you got bigger. The cross section of your bones did not increase. Your tendons did not gain any more leverage. Your skeleton is not in a position to adapt.
#2 Your heart will get bigger, but will not keep up. It has to supply a lot more tissue, pump through a lot more blood vessels. It will be strained. The same is true of other organs, but the heart is likely going to be the first thing to complain. If you try to blow water through a straw you can make it shoot out the other end, try it with a garden hose and you get nothing. Longer tubes require more pressure, similar to Ohm’s Law. Blood pressure will go up.
#3 Depending on your ~~gender~~ sex and age fat will accumulate in different places, but a lot of it will be in the belly. This gets in the way of the mechanical operation of the small and large intestines, at least. More diarrhea, more constipation, more flatulence. Things will be increasingly inefficient, you can run short of vitamins, get dehydrated more easily.
#4 Your lungs are now supplying oxygen to a greater mass of cells. They will get bigger, but google up some x-rays of overweight people: like the heart they will not keep up. You will get winded easily.
#5 Exercise becomes more difficult, so you don’t get enough.
#6 Getting good quality sleep becomes more difficult, you won’t get enough.
As an anecdote, my partner is overweight and constantly having back problems from the excess weight. They are knocked out on muscle relaxers on the couch now.
Because it’s been studied and found that even controlling for all the factors we understand and can test for, being overweight makes you more likely to get sick and die.
The exact mechanisms through which it causes you to get sick and die aren’t fully understood, but we know they exist because of many overweight people, whose test come back otherwise healthy, getting sick and dying.
The factor is time. A person can be heavy and healthy in that moment. But prolonged periods of obesity will eventually lead to issues with joints and organs. Being extremely overweight eventually catches up to people. Think of it as being a smoker for 6 months vs 20 years. The person who has smoked for 20 years would usually have more smoking-related complications than someone who just started smoking.
Because of the combination of these factors:
* “Unhealthy” and “overweight” are both simplifications of complex things.
* Most people with a high body fat content are at increased risk for many bad health outcomes over the course of their life, even if they don’t currently have any of those outcomes.
* Body fat is very visible and a culturally-loaded signifier.
* Body fat has historic and ongoing connections to culturally important things – food, lifestyle, labor, and sexual attractiveness.
Current research indicates that high body fat beyond a certain range is consistently associated with health risks. There’s a lot of fuzziness about what “beyond a certain range” actually means; there is no hard line and *cannot* be such a line, given what we know about how variable individuals can be. Nevertheless, there is clearly a risk gradient somewhere; being 20 pounds over a given metric might not actually be much (or any) risk, but 100 pounds over is going to very likely be a risk, and 250 pounds over that metric is unquestionably a risk.
Others have pointed out the *nature* of that risk (primarily joint and heart impact, with some stress on other organs as well); but it’s hard to actually quantify the *extent* of the risk that without lots of extra information. We can’t just say, for example, “25 pounds overweight = 10% less healthy”.
So, does that mean that it’s “Unhealthy”? Depends. Culturally, “unhealthy” is only partly related to actual health risks of a given trait/substance/action – and another part is the social perception of that trait/substance/action, as well as a personal-judgement element. For example, most people wouldn’t look at a college football player and naturally call them “unhealthy” despite the well-established health risks of college football. Being a football player is generally a greater health risk than being 20 pounds overweight – but not a greater health risk than being 200 pounds overweight. So which of those things should be considered “unhealthy”? That’s in significant part a subjective judgement.
A lot of good answers for biological reasoning here but the answer is also in the statistics.
It’s more complicated than just BMI as you allude to and there are numerous measures for unhealthy weight but the ELI5 is that ‘having elevated weight increases your risk of disease/impairment and death’ (morbidity and mortality) which is verified in countless studies (I could link some but they are really very easy to find). Even if we were biologically clueless as to why (we aren’t), this would remain an answer.
“Routine” blood tests are varied and don’t necessarily pick up on all risks conferred from being overweight. Everything in medicine ultimately comes down to risk / probability and trying to reduce risk. Yes, there is elevated risk for bad cholesterol/diabetes which would usually be picked up on a “routine” blood test but even if that’s normal the risk still remains as these tests aren’t perfect markers for all possible ‘badnesses’ in the body some of which remain beyond our complete understanding.
Because person A is overweight and person B is of a normal weight doesn’t necessarily mean person A won’t live a longer life than person B but we would say that person A is at higher risk of earlier morbidity and mortality than person B (regardless of what routine blood tests show). If you have bad cholesterol and/or diabetes that just additionally increases risk.
Same reason smoking is unhealthy. Just because you’re “healthy” at the moment, doesn’t mean things aren’t working under stressful conditions.
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