We're not sure if being overweight really causes the health problems that have been linked to it.

It could be a mix of many different things. For example, people with high BMIs often have a history of dieting. Studies have shown that yo-yo dieting, or going up and down in weight, is bad for your health as a whole.

So, does having a higher BMI make you sick, or does dieting cause inflammation in the blood that makes you sick?

We've been told a lot of things about BMI that aren't true because "fat is bad" is such a common idea. Here are five "facts" that you should start to question.

Myth 1: Having a low BMI means you'll be healthy.

High blood pressure, high blood sugar, and high cholesterol have all been linked to being overweight. So it makes sense that a low BMI would help with all of those things, right?

Wrong. In one study, researchers looked at the BMIs and blood pressure, blood sugar, cholesterol, inflammation, and insulin levels of about 40,000 adults. They found that a lot of people in the "obese" category had perfectly normal numbers, and a lot of people with "normal" BMIs had high blood pressure, blood sugar, and cholesterol.

This shows how weight discrimination hurts health. Because doctors would see people with normal weight and probably not check for high cholesterol or blood sugar, those people wouldn't get the treatments they need.

Myth 2: A high BMI makes you more likely to have a heart attack.

Many studies have shown a link between a high BMI and the risk of heart disease, just like they have with blood sugar, blood pressure, and cholesterol. But a recent self-reported study of 4,046 sets of identical twins finds that having a high BMI does not increase these risks.

During the 12 years that researchers watched the twins, those with higher BMIs had 203 heart attacks and 550 deaths, while those with lower BMIs had 209 heart attacks and 633 deaths. Even twins with a BMI of 30 or more, which would make them obese, did not have a higher chance of having a heart attack.

Myth 3: Your BMI will always go down if you eat well and work out.

Fat weighs less than muscle. So someone who exercises and eats well might actually gain weight, which isn't a bad thing. People can be healthy at different sizes, but many doctors still judge and blame overweight patients, even if they eat well and exercise.

Myth 4: If your BMI is between 19 and 25, you're healthy.

If you don't already know, here's how the BMI scale works: With a BMI of 18.5 or less, you are underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight, and 30 or more means you are obese. But things weren't always like that.

In 1998, the US government changed the BMI guidelines by lowering the number of people who were overweight from 27.8 to 25. A woman who was 5 feet 4 inches tall and weighed 155 pounds was considered overweight all of a sudden.

In the end, BMI was made to look at the health of groups of people as a whole. It wasn't made for people to keep track of their own health.

Myth 5: BMI is a good way to judge health.

By now, this should be pretty clear. Since the guidelines were made without much thought to health and we know that BMI doesn't work for everyone, it's pretty clear that it's not a good way to measure health. Some experts think that weight is not a good way to judge health at all.