ehh, it's not a conspiracy. It's also not perfect by any means. It's like any broad population based statistical measure: may not apply to all individuals but is a strong risk indicator for most people.
It's basically telling you, based on your height/weight ratio, what the risk is of developing cardiovascular disease, specifically type 2 diabetes and heart diseases. That's it. In general, those with BMI's greater than 30 are at noticeably and significantly greater risk of developing type 2 diabetes and heart diseases.
Of course there are many complicating factors:
1. Our general idea of "physically fit" really may not have ANY connection with "Health," at least as measured by what the BMI recommendations are based upon (heart disease, diabetes, life expectancy). It's really a socially determined idea that is located in a specific cultural time and place and is now connected to things like huge muscle mass and achievement in certain athletic events. However, to my knowledge, there have been no studies that have tried to link that type of physical fitness to the outcomes linked to BMI, namely life expectancy, heart disease, and diabetes. Because there have been no studies, it may be the case (not saying it is, but i'm saying we can't be certain that it is not) that someone we see as incredibly "healthy" (physically fit), such as, say, a strong running back like Adrian Peterson, may, due to his high body weight, be putting a strain on his heart that will shorten his lifespan, despite being a modern day paragon of strength and fitness. I don't know. But I know that we can't mix our social ideas of "healthy" (i.e. i see healthy with my eyes and know what it is) with measured outcomes, if that is how you are measuring health (which is further debatable and ties to quality of life/goals of life). Statistically, body weight is extremely tightly correlated with developing heart disease, decreased life expectancy; the general correlation is so strong it's almost inarguable at this point.
2. Obviously, though, when using such a broad statistical tool, there are major exceptions, and clearly it will not be true for all people. That's why, when used correctly, the BMI is simply one of many many tools for determining overall health.
Here is what one BMI website said:
Risk Factors:
High blood pressure (hypertension)
High LDL cholesterol ("bad" cholesterol)
Low HDL cholesterol ("good" cholesterol)
High triglycerides
High blood glucose (sugar)
Family history of premature heart disease
Physical inactivity
Cigarette smoking
For people who are considered obese (BMI greater than or equal to 30) or those who are overweight (BMI of 25 to 29.9) and have two or more risk factors, it is recommended that you lose weight. Even a small weight loss (between 5 and 10 percent of your current weight) will help lower your risk of developing diseases associated with obesity. People who are overweight, do not have a high waist measurement, and have fewer than two risk factors may need to prevent further weight gain rather than lose weight.
Someone can have perfect BMI, but eat crap, smoke, drink, and have a genetic predisposition and be at HUGE risk for heart disease. Furthermore, as the above says, you may be overweight but have no family history, not smoke, etc, and you're probably at neutral risk.
Additionally, there are "cardioprotective" factors, aka "negative risks." These basically, in statistical terms, "cancel out" one positive risk or counteract a few pounds of weight. Such things include:
Small Waist
High HDL cholesterol
Heart Healthy foods
Regular Aerobic exercise
3. And, of course, there's huge individual variation. we all know and tell stories of ridiculously healthy people that just die, and of ridiculously unhealthy people that live a long time. Of course, psychologically, we remember those and use those as evidence for why the BMI is evil, and tend to push aside the far more numerous instances of people dying at 62 with heart disease. Confirmation bias/shock bias.
4. Outside circumstances are HUGE and clearly are not measured by the BMI. Couple examples:
-If someone has a history of anorexia, and is now slightly overweight, you'd better recognize that being slightly overweight is far healthier both physically (despite what media says) and mentally than being anorexic. Absolutely stress how healthy they now are.
-If someone has a mental illness and is on medication, some medications can cause serious weight gain (from 5-10 lbs for some antidepressants to rapid gains of 40+ pounds for some of the stronger ones). Obviously you want to be extremely careful in finding a balance here to find the best overall health for the person in question, factoring in quality of life, emotional balance/satisfaction, physical health, self harm from their illness, etc.
To conclude my thoughts on the BMI:
-The BMI alone can in no way tell you if you are "healthy."
-The BMI is one tool to help paint a picture of overall health RISKS. It is a strong tool, stronger than many want to believe, but it NEEDS other tools. Otherwise it's like trying to build a house with a really nice hammer and that's it. Not going to work.