7 Reasons You Can't Always Trust Your BMI ...

1. Muscle Weighs More than Fat

Let's start off with an example: there's a purely hypothetical girl who is 5'6” tall, 142 pounds, and quite fit. Remember two things: the ideal BMI falls within the 18.5-24.9 range and her BMI – (142 / 66^2) x 703 – is 22.3. Based on her BMI, she's very close to being classified as overweight, which ranges between 25-29.9. Any lower than 18.5 and your BMI marks you as underweight; if it's over 30, you're considered obese. The problem is that muscle really does weigh more than fat, so although this hypothetical girl is close to the end of the “healthy” spectrum, she's also active and, thus, muscular. However, the BMI calculation doesn't consider muscle or its weight. It doesn't measure muscle so it lacks very important information.

2. Fat Weighs More than Fat

There are also different kinds of fat to consider. Not all fat is bad, or at least not totally bad, and understanding your BMI depends on your understanding of that. Visceral fat weighs the most; it's the unhealthy, hazardous fat that forms around the organs, making it a much deeper problem. Conversely, subcutaneous fat exists under the skin; almost everyone has it and it's no big deal. If you simply pinch an inch, you're not fat. Honestly, that kind of fat doesn't weigh as much as its visceral counterpart.

3. Many Ways to Measure Fat

Although the BMI seems like an adequate measure of body fat, it doesn't even measure fat or weight correctly. For instance, another way to measure your perfect weight is to take a base weight of 100 pounds for your first 5 feet of height. If you're below 5', then subtract 5 lbs for every inch you are beneath; if you're above 5', add 5 pounds for every inch over. So, Hypothetical Girl should ideally weigh 130. By that scale, she's only 12 pounds over the ideal, but she's muscular. She's not fat-but-skinny, or very slender with no muscles to speak of, nor is she overweight without any muscles.

4. Many Ways to Measure Health

That actually presents another reason not to trust your BMI: you have to measure your level of health instead, and there are lots of (better) ways to do that. Do you eat right? Do you stay active and get plenty of exercise? How's your blood pressure, your cholesterol levels, and your heart health? You can't measure your health solely by how much you weigh. To do so can actually make you unhealthy, because you might end up not getting all the nutrients, vitamins, and exercise you need.

5. Age Matters

Another problem with trusting your BMI has to do with your age. When you're young, such as in your teens, you're still growing and developing, so while you might end up with a BMI that places you on an extreme end of the spectrum, it can change within a year. Similarly, older people lose muscle mass; their BMIs may pinpoint them as underweight when they simply don't have any bulk from their muscles.

6. Height Relativity

Your height plays an important part as well. Although it's not true that “big bones” make you weigh more, your bones are quite vital. Although the BMI calculation takes height into consideration, it doesn't seem to consider the fact that if you're very tall, for instance, you probably have long arms and legs as well. If you're short, your limbs are shorter. Someone who is 6'1” and weighs 220 pounds is well within a healthy weight range, yet their BMI classifies them as almost obese.

7. Doctors Don't Buy It

Finally, if your doctors don't trust your BMI, why should you? Of course, some doctors still rely on a person's BMI to determine their level of health, but not all of them do. An increasing number of physicians are starting to admit that it simply doesn't tell you everything. Many either use it as a rough guideline or eschew it altogether. At most, it's just a clue.

Now, this doesn't mean that your BMI is completely pointless and obsolete. It's a good idea to pay attention to it because it gives you an idea about your body fat, but you need to keep it in perspective. It's just an idea and it isn't the only way to measure your body fat. It's not the only way to measure your health, either. Understanding your BMI is important but just realize it doesn't tell the whole truth. How do you measure your health?

Sources: fitjunction.com, shape.com, nhlbisupport.com