Body fat confusion

Posted on: 29 August 2018 by 50connect editorial

Is the Body Mass Index (BMI) the be all and end all of judging a healthy weight? No, but it is a good indicator of future weight issues. Find out more...

Over 50s managing weight and BMI

Over half the population is overweight, more than 26 percent of the UK population are obese, and it is increasingly evident that this growing girth is taking its toll on the health of the population. Overweight is associated with type 2 diabetes, heart disease, certain forms of cancer and joint problems like osteoarthritis. It is also clear that even modest weight loss can have profound effects on health.

But it may take more than a mirror or an arbitrary "goal weight" to get on the road to reducing health risk. A body of research has found that body mass index, or BMI may be a better indicator of health risk than absolute body weight. In addition, where you gain the weight may be equally important in determining your risk for certain diseases. Though it is only one of a number of considerations that health experts use to determine these risks, BMI is more highly correlated with body fat that any other measurement of height and weight.

Below, Anne Wolf, obesity expert and instructor of research at the University of Virginia School of Medicine, offers simple instructions for determining body mass index, and talks about the usefulness of BMI in assessing health risk and as a goal for weight loss.

What is body mass index, or BMI, and what does it measure?

Body mass index uses a person's height and weight to assess that person's risk of health problems.

BMI does not distinguish between mass due to body fat and mass due to muscular physique, nor the distribution of fat. In order to measure abdominal obesity, waist circumference is measured, and categorised into desirable, high and very high, by sex-specific thresholds (cm):

Men: Desirable = Less than 94, High = 94-102,

Very high = More than 102cm

Women: Desirable = Less than 80, High = 80-88,

Very high = More than 88cm

(source: NHS UK)

How is measuring BMI different from just asking someone to step on a scale?

Weight alone is not a very good measure of body fat. BMI is highly correlated with body fat, and, subsequently, health risk.

Is BMI a new way to assess a person's health risk?

We used to assess a person's "ideal" weight using height measurements, but we realized that it was sending a very bad message. It implied that each person has an ideal weight for their height, which is not true. In fact, there is a range. It is more useful to understand that there is a range where your risk for developing disease is minimized. BMI is based on health risk, not on how you look.

BMI calculator

How is BMI calculated?

The formula is a little complicated. The first step would be to multiply your weight in pounds by 703. Next, multiply your height in inches by your height in inches again. So if you are 65 inches tall, you're going to multiply 65 by 65. The third and last step is to divide your answer in step one-the weight by 703-by your answer in step two. That will give you your body mass index. For instance, if you are 124 pounds and 5 feet tall, your BMI would be 24.2.

And what is considered a healthy, and an unhealthy BMI?

A BMI of less than 18.5 would be underweight. A BMI between 18.5 and 24.9 would be considered a healthy body weight, overweight is categorized as 25 to 29.9, and obesity is categorized as a BMI of 30 and above.

Does BMI account for body proportion?

No. And in fact, one of the other measurements that the National Heart, Lung and Blood Institute recommends using with body mass index to assess health risk is waist circumference. We know that a high waist circumference-or high amounts of abdominal body fat-is very dangerous body fat to have, independent of a high body mass index.

So if someone has a body mass index in a healthy range but they have a very high waist circumference, then they still are at risk of developing disease states.

diabetes check

What sorts of health risks does a high BMI pose?

People with high BMI are at very high risk of developing type 2 diabetes and cardiovascular disease. That includes coronary heart disease, hypertension, stroke, and enlarged heart. It's also involved very much with the whole metabolic syndrome. You're at a higher risk for insulin resistance, infertility, and polycystic ovary syndrome. You're at high risk for certain types of cancers, and particularly cancers of the reproductive system-endometrial cancer, breast cancer, etc. Colon cancer for men. We've seen associations with gall bladder cancer, as well, and some other smaller types of cancer.

A high BMI is also associated with structural problems. A lot of people who have a body mass index over 30, which is very high, have many more hip and knee problems, osteoarthritis, and also problems in the hand.

So the health risks are pervasive. They're not just diabetes or heart disease. It also hits the structural system and is highly associated with cancers, and lower quality of life.

Is BMI genetic?

If your family is heavy, you are more likely-for genetic reasons-to be more heavy set than other people. However, genes don't change, and in the last 15, 20 years our body mass indexes have continued to increase dramatically. It's not a shift in our genetic makeup, it's a shift in our environment. We have less physical activity, we are eating more, and we are gaining more weight.

Is BMI a good measurement of health risk for everyone? Or are there certain populations for whom the measurement might not be an accurate assessment of health?

BMI is good for most of the general population. There are, however, people with a lot of muscle-athletes for instance-who have a high BMI but the high weight part of the equation is more a result of muscle, not fat, and they are not at increased risk of health problems.

There's common sense that has to come through. You can look at someone and say, "They're in really good shape. They have high BMI, but it's because they're fit." And you won't see a BMI over 30 if someone is in shape.

So what are the major factors that need to be considered in assessing someone's health risks?

We would measure their weight and their height, calculate their body mass index, and we would measure their waist circumference. A waist circumference lower than 35 inches for wom.

Calculate your BMI


content provided by NHS Choices


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