Don’t judge a book by its cover.

This old saying is true for books but it’s true for what people see in the mirror too. It’s easy to stereotype overweight people and automatically assume they’re sedentary and have a poor diet. But research shows that some overweight and obese people are more healthy than we would expect based on their body shape—and the opposite is true too: being at a normal body weight doesn’t guarantee good health.

Just because someone looks good on the outside doesn’t mean everything’s working well on the inside. In fact health and fitness professionals have a couple of names for people who look fit but who are actually out of shape and experience the health problems normally associated with obesity: skinny-fat or TOFI – thin outside fat inside.

Healthcare practitioners sometimes say that in an odd way overweight people are lucky. They have undeniable visible proof that something in their diet or lifestyle is affecting their health. On the other hand people who are thin may have a harder time connecting their bothersome symptoms to issues with their diet because they “look fine.”

One of the measurements often used to determine someone’s status as normal weight overweight or obese is the body mass index (BMI). However BMI measures only height and weight. It doesn’t take into account body composition—that is the percentages of body fat; lean muscle bones and organs; and water. This is a very flawed way to associate body size with health because better overall health is associated with more muscle mass and relatively lower body fat not just a lower total body weight. A more accurate way to gauge someone’s health status is to look at some of the things commonly screened for in standard bloodwork. The numbers don’t lie; they provide information an attractive reflection in the mirror might hide.

The main cause of TOFI is metabolic syndrome—a cluster of signs that mostly result from chronically high insulin levels indicating the body isn’t processing carbohydrates properly. These signs include elevated blood glucose and hemoglobin A1c high triglycerides elevated LDL (especially the small dense kind) low HDL and high blood pressure. They also usually include obesity—particularly around the abdomen—but people who are TOFI have many of the signs without the excess body fat. For reasons of genetics or just plain luck they avoid putting on weight but can still experience all the same negative effects of insufficient physical activity and a diet high in refined carbohydrates and unhealthy fats.

People who are TOFI or skinny-fat are probably happy with what they see in the mirror and this makes it difficult for them to see the link between their diet and their fatigue afternoon energy crashes mood swings brain fog irritability acne PMS aches and pains and other complaints. The metabolic syndrome—in obese and normal weight individuals—is also associated with hormone imbalances and poor cardiovascular health so being at a normal weight can give people a false sense of security when it comes to some very serious conditions. 

It’s helpful to keep an eye on the scale but for long-term health it might be even more important to pay attention to some of the markers in labwork—the ones that can tell us if we need to clean up our diet even when our weight and clothing sizes don’t change.