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With so many people choosing to track health data nowadays, I get a lot of questions about the best way to measure body composition. Today I’m going to cover some of the most common methods, their pros and cons, and also tackle the question of whether there is an “ideal” body composition.
When it comes to fitness, body composition refers to the relative quantities of fat, muscle, bone, and water in your body. These are measured by weight or as a percentage of your total body weight. Clinically, body composition differentiates between fat mass or adipose tissue, muscle, bone/bone mineral content, and residual mass (organs, connective tissue). For the purposes of this post, I’m going to focus on “body composition” as it’s used for fitness and general health, not medically.
Body composition is not the same as body weight and height. Two people of the same height and weight can have vastly different body compositions. One might be a very lean, muscular fitness competitor, while the other has a corpulent figure. Nor is body composition the same as build—ectomorph, endomorph, mesomorph.
Body composition is a measure of the building blocks, so to speak. Colloquially, when people talk about body composition, they typically mean how much body fat and lean muscle they carry. Although bone density is technically a part of the equation, I’ve never heard someone say they were going to improve their body composition by increasing bone mineral density.
I’m going to assume that’s what you mean, too: When you say you’re interested in measuring body composition, you want to know your body fat percentage and possibly how much muscle you’re carrying. Likewise, when people ask me whether there is an “ideal” body composition, they want to know whether a certain amount of body fat and muscle are optimal for health and longevity.
A Quick Note On Fat and Muscle
I write about fat and muscle a lot on MDA, but I want to reiterate a couple important points before moving on to the meat of this post.
First, we talk about fat like it’s one uniform thing we carry around in our bellies and butts, but that’s not the case at all. For health purposes, we want to distinguish between white fat and brown fat, and especially between subcutaneous fat and visceral fat. The former is the fat stored under your skin—the stuff you can pinch—while the latter is the relatively more dangerous kind that packs around your internal organs. We’re also learning more about the health risks associated with excess intramuscular fat.
Second, I know I don’t need to convince you that lean muscle is important. Not only does it produce important proteins and metabolites, it correlates with overall health and organ reserve. Having sufficient lean muscle serves a protective role in aging.
From a purely functional perspective—setting aside the aesthetic question—it’s important to have enough lean muscle and not too much body fat. What do “enough” and “not too much” mean, though? I’ll try to provide some insight into that shortly. It’s not as straightforward as you might think. First, though, let’s look at how you measure body composition.
How to Measure Body Composition
There are a variety of ways to assess body composition. For today I’m going to stick to the ones that are widely available. The gold standard for body composition analysis is either CT scan or MRI, but you aren’t going to be using those to track your gainz. Hydrostatic weighing (aka underwater weighing) is also very accurate but not so readily available. Plus you have to be willing to be dunked underwater.
Body weight is not by itself a good measure of adiposity (fatness) or lean mass. Likewise, BMI, which is calculated by dividing weight in kilograms by the square of height in meters, provides a crude estimate of adiposity at best. In fact, BMI was never designed to be a measure of body composition in any given individual. It was meant to look at population-level trends in body size. Various researchers have proposed mathematical adjustments to make BMI a better predictor of fat mass, but it remains imprecise. Your doctor may care about BMI because it’s the anthropometric measure used in most medical research, but it won’t tell you what you want to know here.
Let’s look at some of the ways you can measure body composition relatively easily and affordably, starting with the most sophisticated and working down to the more basic:
Air displacement plethysmography (ADP, aka whole-body air displacement plethysmography)
ADP is similar in principle to hydrostatic weighing. With ADP, you sit inside a small chamber, and body composition is assessed by air displacement. This technology is commercially available as BOD POD®.
The pros of ADP are that it is relatively inexpensive and scores well in terms of reliability. It’s also easy. All you have to do is sit quietly in a big egg, and measurement takes approximately three minutes. It’s not available in all areas, though. The biggest drawback is that it gives you body fat percentage but doesn’t tell you anything about muscle volume. The report simply breaks down fat mass and “other.” It also may not be as accurate for lean individuals.
Dual energy X-ray absorptiometry (aka DXA or DEXA)
DEXA is currently the best option for people who want a comprehensive body composition analysis. In medical settings it is often used to measure bone density to screen for osteoporosis. Many medical offices now offer DEXA scans for general body composition analyses as well, and mobile scanning services operating out of vans are popping up all over.
When you get a DEXA scan, you lie on a table with your legs loosely held down by straps. An arm emitting low-dose x-rays passes over your body while another passes underneath. The scan takes about 10 minutes. The technician then generates a report that includes:
- A visual representation of your fat and muscle distribution
- Total mass broken down into fat, lean (muscle), and bone, plus visceral fat
- Bone density
- Detailed analysis of fat versus lean tissue in your left and right arm, leg, and torso
Repeated DEXA scans—completed every three to six months, for example—can be a good way to track your progress if you are actively trying to lose body fat while maintaining muscle, or if you’re trying to bulk up. While generally regarded as very accurate and safe, there are still questions about the accuracy of DEXA’s visceral fat measurement specifically. If visceral fat is your primary concern, try combining repeat DEXA scans with simple abdominal measurements (more below), and follow up with your doctor.
Bioelectrical impedance analysis (BIA)
BIA devices pass small electrical currents through the body. Muscle is a better conductor of electricity than fat or bone, so by measuring the resistance to the current, these devices estimate the relative amounts of fat, muscle, and bone in your body. Because the calculations are based on the amount of water in your tissues, BIA can be thrown off by how well hydrated you are.
BIA technology is popular to its widespread availability and ease of use. There are many types of BIA devices on the market, some more reliable than others. I don’t put much stock in those cheap bathroom scales that claim to tell you your body fat. The more sophisticated InBody machines have been shown to give readings on par with DEXA scans. Unlike scales that only offer body fat readings, InBody devices also offer skeletal muscle mass and, in the more premium devices, visceral fat estimates.
Unless you have access to one of the more expensive devices, I think BIA should be used in the same way as regular bodyweight scales: Use it to track trends over time, but don’t get too hung up on any single measurement. Just as with body weight, BIA can vary for reasons having little or nothing to do with your actual body composition.
Electrical impedance myography (EIM)
Handheld EIM devices like the Skulpt Chisel are another option. Like BIA, EIM works by passing an electric current through tissues at different depths and measuring impedance. EIM is used in medical settings to measure muscle quality, or muscle strength relative to size. The technology has more recently been adapted to fitness tracking.
The advantages of these devices are that they are easy to use at home, allowing for daily tracking of body composition if you want. The Skulpt device gives you full-body and 12 region-specific (right/left shoulder, chest, biceps, etc.) measurements of muscle quality and body fat percentage. One study showed it to be fairly reliable when compared to DEXA, at least in young, healthy individuals. It’s not clear how accurate the device is in people with higher levels of body fat.
Skinfold calipers, aka plicometers, are extremely cheap and simple. When used with proper technique, they are reasonably accurate, although not as accurate as any of the methods already listed. I’d guess that most people using calipers to measure their own fat at home aren’t properly trained to use them, though, which means they almost certainly aren’t getting accurate results.
Calipers can only measure subcutaneous fat. They tell you nothing about visceral fat or lean tissue. Measurements are also affected by age, gender, and fat distribution. Given that there are much better options available now, I don’t think skinfold measurements have much to offer anymore other than being inexpensive unless (a) all you care about is subcutaneous fat, and (b) a trained pro is taking the measurements for you.
Navy body fat formula: The Navy body fat formula simply requires two measurements for males (waist at navel and neck) or three for females (waist at narrowest point, hip, and neck). Plug those values into one of the many online calculators to get your fat mass, body fat percentage, and lean mass.
The Navy adopted this method because it was looking for something that was relatively reliable and easier to train personnel to do correctly compared to skinfold calipers. Although it has its detractors, the Navy validated this method against hydrostatic weighing and found it to be acceptable.
Waist-to-hip ratio, waist-to-height ratio, or waist circumference: None of these measurements provides information about body fat versus lean mass. However, they may be indicative of having more of the dangerous visceral fat. As a health marker, abdominal fatness is more directly linked to negative health outcomes than fat that is distributed across your entire body.
- If you care to have a very accurate, detailed reading, spring for a DEXA scan. From what I’ve seen, they are much more accessible and affordable now than they used to be, at least in the U.S. DEXA is also your best bet if you want to assess bone health.
- If you just want a body fat estimate that’s good enough for tracking fitness or setting your keto macros, the Navy body fat measurements will do.
- Tracking a simple waist measurement is a good idea for health reasons. If yours starts to climb, it’s time to dig deeper into diet or lifestyle variables that might be at play. (Stress causes abdominal fat accumulation, for example.)
Whichever method you choose, understand that it will have some degree of error. That’s not a huge deal in the context of tracking body comp for fitness and general health.
Know, though, that body composition assessments tend to be less reliable in obese individuals. Studies find small gender differences as well, and there may be systematic errors based on other demographic characteristics. This is all the more reason not to get hung up on the exact number. Instead, track trends over time to see how your body composition is changing or staying more or less the same.
Is There An Ideal Body Composition?
First we have to define what we mean by “ideal.” Let’s set aside the aesthetic question. It’s subjective, and it changes across time and cultures. Anyway, as much as I’m open about my desire to LGN (look good naked), I also maintain that function trumps form. That is, I care more about how my body functions than how it looks.
“Ideal” body composition should therefore reflect the amount of fat and lean muscle that leads to optimal functioning: staying healthy and strong for the longest time possible. The questions I’d like to answer here are:
- At what body fat percentage are people, on average, healthiest?
- How much lean muscle tissue is optimal for health?
The thing is, I can’t. The data just isn’t there. Oh, there are plenty of studies looking at the relationship between BMI and health risk factors, chronic disease, or mortality. As I’ve said, though, BMI is a poor substitute for actual measures of body composition.
There are also smaller but sizable literatures looking at specific measurements—waist circumference, waist-to-hip ratio, DEXA, BIA—and those same outcome measures. Trying to glean specific recommendations from the literature is tough, though, for a few reasons:
- Results vary by age, gender, probably race/ethnicity, and according to what outcome variable you’re talking about.
- Researchers frequently divide people into categories—low, medium, high body fat, or low versus high waist circumference—when assessing risk. That means we can’t pinpoint specific thresholds for body fat or muscle that might be ideal.
- Studies are inconsistent in what possible confounding variables the researchers control for. In particular, smoking is a known confound in this area of research because it is associated with both lower body weight and more health problems. Still, not all studies control for it.
The biggest problem, though, the one that even the best-designed study won’t overcome, is that there is a lot of individual variability. Take 100 people with the same body fat percentage, and their health could vary tremendously. There are just too many factors that interact to create health, including the likely interaction between body fat and lean muscle.
All that said, we can make some broad generalities. In general, risk of developing metabolic syndrome, type 2 diabetes, and cardiovascular disease seems to increase with increased body fat. Again, though, there is a lot of variance from study to study that makes it impossible to nail down any kind of firm cut-offs.
When risk increases more or less linearly, how do you decide where the cut-off is between “still ok” and “getting into the danger zone?” It’s tough.
If I had to make any kind of recommendation, I’d probably tell you to track simple waist circumference. It’s easy to assess, and this research showed that when waist circumference is used as a continuous variable—that is, not broken into categories like “normal” or “high,” which are always imperfect—it predicts risk of hypertension, dyslipidemia, and metabolic syndrome by itself. Adding BMI to the equation didn’t increase predictive power.
In the U.S., the NIH recommends a cutoff of 102 cm (~40 inches) for men and 88 cm (~34.5 inches) for women as a healthy cut-off. The International Diabetes Foundation recommends a cutoff of 80 cm in women, 94 cm in men of European descent, and 90 cm in South Asian, Chinese, and Japanese men. The European Union recommends a cutoff of 94 cm for men and 80 cm for non-pregnant women. So as you see, it’s still somewhat subjective.
Nevertheless, being in the ballpark or below is probably a decent goal. Perhaps more importantly, you want to track over time. If you’re slightly above the cutoff but stable and otherwise healthy, I’d be less concerned than if you were seeing waistline expansion year over year.
What about muscle, you might ask? Same thing, I don’t have a great answer. A recent review concluded that having less lean body mass predicted greater mortality until old age, at which point the association disappears or perhaps even reverses. I’m of the mind that when it comes to muscle, measuring amount is less important than measuring strength or functional capacity. The goal for most of us isn’t to be jacked when we’re old. It’s to be mobile and independent, with a firm handshake.
Finally, I’d be remiss if I didn’t mention that although we usually talk about risk at the higher end of body fat percentage, you can of course go too low as well. The minimum body fat percentage for survival, known as essential fat, is 2–5% for men and 10–13% for women. Many people, especially women, need considerably more to feel well and have healthy hormone balance. Low body fat is a risk factor for hypothalamic amenorrhea (losing one’s period), but there are no specific guidelines for what constitutes “too low.” It varies from woman to woman.
We don’t have the fine-grain data to definitively say what amount of body fat and lean muscle is optimal. Being at the extremes seems clearly associated with poor health and mortality. Beyond that, there is a significant range in the middle where any given individual could be considered healthy.
In the absence of specific and reliable recommendations, I’m going to say what I’ve been saying all along: Lifestyle is more important than any one measurable data point. If you’re checking all the boxes—eating plenty of plants and animals, moving your body, resistance training, getting lots of sleep and sunlight, managing stress, nurturing social connections—that’s the most important thing.
If you want to try to get from 20% body fat to 17% body fat for aesthetic reasons, hey, that’s up to you. Just understand that losing that 3% body fat doesn’t necessarily buy you significantly better health, especially if you use extreme measures to do it.
Do I think it’s worth having a general sense of how much body fat and muscle you’re carrying? Yeah, probably. If nothing else, it’s good to make sure that you’re not losing significant muscle mass as you age. A yearly or biannual DEXA scan plus checking in semi-regularly with your waist circumference seems like a reasonable plan. DEXA will also allow you to track your bone mineral density, which is important. If you need to know your body fat for the purposes of setting your keto macros, something like the Navy body fat calculator will do just fine.
Otherwise, I’m more interested in subjective markers like how I feel on a day-to-day basis and whether I can perform at the gym. My jeans and my mirror tell me everything I need to know about abdominal fat. I don’t need to know exactly how many pounds of lean muscle I’m packing to know whether I’m as strong as I want to be and whether I can get off the floor after playing with my granddaughter.
What about you? What measures do you use to track your body composition—if you even do? What other body composition questions do you have? Tell me in the comments below.
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