These are the best ways to measure your body fat

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This week, an office conversation turned to body weight. We all know that being overweight is not great for your health-it's linked to metabolic diseases like diabetes and cardiovascular problems. But weighing yourself won't tell you all you need to know about your disease risk.

A friend of mine is a super-fit marathon runner. She's all lean muscle. And yet according to her body mass index (BMI), which is a measure of weight relative to height, she's overweight. Which is frankly ridiculous.

I, on the other hand, have never been all that muscular. I like to think I'm a healthy weight-but nurses in the past have advised me, on the basis of my BMI, to eat more butter and doughnuts. This is advice I never expected to receive from a health professional. (I should add here that my friend and I are roughly the same height and wear the same size in clothes.)

The BMI is flawed. So what should we be using instead? There are several high-tech alternatives, but a simple measure that involves lying on your back could also tell you about how your body size might influence your health.

First, let's talk about fat-the most demonized of all body components. Fat is stored in adipose tissue, which has some really important functions. It stores energy, keeps us warm, and provides protective cushioning for our organs. It also produces a whole host of important substances, from hormones that control our appetite to chemicals that influence the way our immune systems work.

Not all fat is equal, either. Our bodies contain white fat, brown fat, and beige fat. While white fat stores energy, brown fat helps burn calories. Beige fat tissue contains a mixture of the two. And white fat can also be broken down into two additional categories: the type under your skin is different from that which covers your internal organs.

It's the visceral fat-the type surrounding your organs-that is thought to be more harmful to your health, if there's too much of it. Having more visceral fat has been linked to an increased risk of diabetes and cardiovascular disease. (That relationship isn't straightforward either, though; studies have shown that removing this excess" fat doesn't improve metabolic health.)

Either way, having a good idea of how much fat is in your body, and where it is, would be valuable. It might at least give us some idea of our risk of metabolic disorders. There are quite a few different ways of measuring this.

BMI is the most widely adopted. It's the official measure the World Health Organization uses to define overweight and obesity. On the plus side, it's very easy to calculate your BMI. Unfortunately, it doesn't tell you very much about the fat in your body or how it corresponds to your health. After all, your body weight includes your bones, muscles, blood, and everything else, not just your fat. (And as we've seen, it can lead well-meaning health practitioners to recommend weight loss or weight gain when it's really not appropriate.)

Scanners that can specifically measure fat are more useful here. Typically, doctors can use a DEXA scan, which relies on x-rays, to give an idea of where and how much body fat a person has. CT scanners (which also makes use of x-rays) and MRI scanners (which use magnets) can give similar information. The problem is that these are not all that convenient-they're expensive and require a hospital visit. Not only that, but standard equipment can't accommodate people with severe obesity, and people with some medical implants can't use MRI scanners. We need simpler and easier measures, too.

Measuring the circumference of a person's waist seems to yield more useful information than BMI. Both waist-to-hip and waist-to-height ratios can give a better idea of a person's risk of developing diseases associated with excess weight. But this isn't all that easy either-measuring tapes can stretch or slip, and it can be difficult to measure the exact same part of a person's waist multiple times. And the measure seems to be a better indicator of health in men than in women.

Instead, Emma Borgeson, who studies cardiometabolic disease at Aarhus University in Denmark, and her colleagues recommend the SAD measure. SAD stands for sagittal abdominal diameter, and it's a measure of the size of a person's belly from back to front.

To measure your SAD, you need to lie on your back. Bend your knees at a 90-degree angle to make sure your back is not arching and is flush with the floor. Then measure how much your belly protrudes from the ground when you exhale. (The best way to do this is with a sliding-beam caliper.)

In this position, the fat under the skin will slide to the sides of your body, while the visceral fat will be held in place. Because of this, the SAD can give you a good idea of how much of the more dangerous" kind of fat you have. The fat can be trimmed down with diet and exercise.

This measure was first proposed in the 1980s but never took off. That needs to change, Borgeson and her colleagues argue in a paper published in Nature Reviews Endocrinology a few months ago. SAD is simple, affordable, and easier to implement than waist-to-hip based measurements," the team writes. We would argue for its extended use."

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