Body Mass Index (BMI) is the standard metric for determining if someone is a healthy weight. BMI is calculated as weight in kilograms divided height in meters squared, and specific cutoffs are set to categorize people into weight categories.
- <18.5 kg/m2 is underweight
- 5 – 25 kg/m2 is “healthy” weight
- 25 – 30 kg/m2 is overweight
- >30 kg/m2 is obese
BMI doesn’t tell you anything about body composition, which is far more important. Furthermore, even separate from body composition, you can be normal weight and extremely metabolically unhealthy – so called, “thin outside fat inside” or TOFI; and you can be overweight and perfectly healthy – so called, “metabolically healthy obese” or MHO, whether or not you are metabolically healthy.
The concept of body composition is quite straight forward: it’s not the weight that matters but whether that weight is muscle or fat. A classic example is that of Dwayne “The Rock” Johnson. One looks at the man and you can tell he is not over-fat, and yet his BMI of 34.3 kg/m2 makes him “obese.” Obviously, this is an extreme example, but it makes the foundational point that BMI is an oversimplified metric for health. In fact, high muscle mass is one of the best predictors of healthspan, especially in men. So, should we prescribe weight loss in individuals who are overweight but closer to the Dwayne end of the spectrum? Probably not.
TOFIs, a.k.a. skinny-fat people, are the converse of the “Dwayne’s.” Generally speaking, about 20-25% of people are “TOFI,” meaning they show signs of metabolic syndrome and insulin resistance at normal weights. In fact, about 15% of persons with Type 2 diabetes are a “healthy weight.” In fact, TOFIs who develop Type 2 diabetes even appear to be at greater risk of heart disease than overweight persons with the condition. Let’s unpack this a little bit more.
Some TOFIs simply have low muscle mass. As we said above, muscle mass is healthy metabolically active tissue that interacts with fat and is also the major sink for blood glucose. That’s why people with big muscles can consume far more carbohydrates while experiencing a smaller rise in blood glucose. But being TOFI isn’t just about body composition, it’s also about where you store your fat. Just like in real estate, fat is all about “location, location, location.”
The body has two main types of fat: subcutaneous fat and visceral fat. Subcutaneous fat is the type that is under (sub) your skin (cutaneous). Visceral fat, by contrast, rests inside your abdominal cavity and around your organs. Visceral fat is extremely pro-inflammatory and excess visceral fat can contribute to metabolic dysfunction and insulin resistance. And some TOFIs, despite being a “healthy” weight, carry a lot of visceral fat, predisposing them to adverse health effects.
Then there are the metabolically healthy obese (MHO). About 15% of obese people are MHO and not all of them look like Dwayne Johnson. In fact, some look quite fat. However, they can exhibit excellent metabolic markers and live long healthy lives. This is because, generally speaking, MHO carry their fat as subcutaneous fat rather than visceral fat. They might pack on the pounds, but the body stores the fat in a relatively healthy manner. In fact, you can even recapitulate the MHO phenotype in mice by simply overexpressing GLUT4 glucose transporters in the mice’s subcutaneous fat cells. These mice become impressively fat, and yet remain perfectly healthy. The takeaway point is that it’s less about how much fat you have, but where your body puts it. And we are all bio-individuals in this respect.
Other Markers for Metabolic Health
Now for the most important question, if BMI is an inaccurate measure for many, how do you assess metabolic health? There is certainly no one answer, nor a medical consensus on this topic. So, what follows represents my opinion, not medical gospel or advice.
The first and easiest measure is the “string test” for visceral fat. Take a piece of string the length of your height. Then fold the piece of string in half and wrap it around your waist. Waist circumference is a proxy for visceral fat, and if the two ends of the string struggle to meet, your visceral fat level may be too high. If this is the case, one of the easiest ways to reduce visceral fat is to eat a low carb diet that’s rich in healthy fats.
To track your health progress, you can cut the string to your current waist circumference and then, as you progress on a low carb diet, see if you can make the string shorter and shorter. Sometimes, I see people “pass” the string test week by week even if their weight barely changes or doesn’t change at all. And, if you want a more advanced test, you can get a DXA scan for body composition and visceral fat content.
There are also blood markers that might be relevant to track. On a standard fasting lipid panel, triglycerides and HDL cholesterol are the most relevant markers to metabolic health. High triglycerides (>150 mg/dL) and low HDL (<40 mg/dL) are markers of metabolic syndrome. (LDL is not a marker of metabolic syndrome). So, conversely, low triglycerides and high HDL are signs of good metabolic health. Being more conservative, I prefer to see triglycerides <100 mg/dL and HDL >50 mg/dL.
In those who don’t have diabetes, fasting insulin is a good marker of metabolic health. Elevated fasting insulin in non-diabetics suggests the development of insulin resistance and metabolic syndrome. Experts have different opinions on what constitutes and good fasting insulin, but myself and most medical doctors with whom I work suggest a level <5 uIU/L. If someone has Type 2 diabetes, HbA1c might be a better marker to track and levels <5.7%, which can certainly be achieved on a low carb diet, signify “reversal” of disease.
Finally, I like to assess energy. This is a subjective measure. Do you feel energized? Do you want to move your body? A metabolically healthy body is a body that likes to move, just like a stallion likes to run. Scans and blood markers can be informative, but your subjective assessment of physical energy can also be an excellent functional “test.” Plus, in the end, quality of life matters most.
Improving Metabolic Health
88% of American adults have at least one marker of metabolic syndrome. Randomized controlled studies have shown that, even controlled for calories and protein, low carbohydrate diets are better than low fat or mixed diets for reversing metabolic syndrome (Hyde et al. 2019). Therefore, some form of a well-formulated clean low carb or ketogenic diet might be a good option for those looking to regain metabolic health.
Resistance exercise also helps to promote metabolic health. Building muscle can not only improve body composition, but it can help to burn visceral fat and improve glycemic control. Two high-intensity full body weightlifting or resistance training sessions per week is sufficient for most people. Focus on quality over quantity. Aerobic exercise is another tool that may improve metabolic health. Although, it may not necessarily lead to weight loss.
BMI is a poor proxy for health. It effectively stereotypes people into health categories, without further considering how the weight is distributed, into muscle, subcutaneous fat, or visceral fat. Because we are all individuals, we all have different “healthy” BMIs. If you want true health, look beyond BMI. And, if you achieve metabolic health, it’s just possible that your BMI will drop as a side effect.
Lectures & Listening
Podcast of insulin resistance with Bret Scher, MD and Benjamin Bikman, PhD
Interpreting lipid panels with Paul Mason, MD on Youtube
LowcarbMD: Meeting of the minds on visceral fat with one MD and three PhDs