Used as part of the Creative Commons
Source: Used as part of the Creative Commons

In my last post, I looked at whether obesity is taking years off our lives, and concluded (once more) that the evidence does not support that claim. So let's look at the third big lie about weight and health.

Lie #3: Being fat causes heart disease, stroke, type 2 diabetes, and other serious illnesses.

Truth #3: It's possible that there's some cause and effect between weight and illness, but we don't understand enough about the science to know that yet. And if there is a causal relationship, it's a lot less powerful than most of us have come to believe. Some illnesses (especially type 2 diabetes) are associated with higher BMIs; some are not. 

One of the big challenges in considering weight and health is how to distinguish between correlation and causation. Both are ways of talking about relationships among sets of events or variables. Correlation is, essentially, an association; this happened and that happened. The events may be unrelated—for instance, your sister gets married and you get a new job. Or one may cause the other—you give birth to a child and your health insurance premiums go up. Or a third variable may influence both of them: people who have yellow teeth are more likely to develop lung cancer, because smoking causes both.

Causation, as the term suggests, refers to a cause-and-effect relationship: the gravitational pull of the moon causes the ocean tides. Without the moon’s force, the tides literally would not flow in and out. In medicine, causation is notoriously difficult to prove, since so many variables contribute to human health. Still, some relationships are considered causal. For instance, the fact that smoking causes lung cancer. This doesn’t mean everyone who smokes will get lung cancer, or that only people who smoke will get lung cancer. It does mean that for a certain percentage of the population, smoking will lead to lung cancer.

Mostly, when we talk about weight and health, we’re talking about risk factors, forces that increase susceptibility to a particular disease or condition. All risk factors are not created equal, though we often talk about them as if they were. For instance, both having yellow teeth and smoking are risk factors for lung cancer, but clearly smoking carries a much higher degree of risk than tooth color, which after all can be caused by all sorts of things. So we need to be careful not to conflate risk factors with causes, unless, like smoking and lung cancer, they actually do cause a disease.

So what do we actually know about the relationship between weight and disease? We know that obesity has been correlated with heart disease, gallbladder disease, and type 2 diabetes, as well as with risk factors for those diseases. The strongest correlation is found between obesity and Type 2 diabetes; one 2014 study found that people who were obese but also “metabolically healthy” (that is, their cholesterol and glucose levels were normal) were four times more likely to develop diabetes than metabolically healthy people who were not obese.1 The question is how to interpret that correlation, since many other factors also correlate with type 2 diabetes. (Repeat after me: correlation does not equal causation.)

One aspect of the weight–health relationship that’s rarely mentioned is the fact that definitions and cutoff points for many illnesses and risk factors have changed quite a bit over the last decade. For instance, diabetes used to be diagnosed when blood sugar levels hit 140 milligrams per deciliter (mg/dl); now the cutoff is 126 mg/dl. New disease categories like “prediabetes” and “prehypertension” have also emerged; a blood sugar level above 100 is now considered “prediabetes,” and is often treated aggressively.

Similarly, hypertension is diagnosed when blood pressure reaches 140/90, and now “prehypertension” begins at a reading of 120/80. There’s been plenty of controversy over these widening disease categories, but whether they’re valid and helpful or not, they skew the perceived medical relationship between weight and health by shifting millions of people into the “ill” category.

And then there’s the chicken-and-egg question. We assume that weight gain comes first and causes diabetes and other illnesses. But what if, as surgeon and nutrition researcher Peter Attia has suggested, weight gain is actually an early symptom of diabetes?2 Or what if weight gain and type 2 diabetes are both caused by an unknown third variable? Pediatric endocrinologist Robert Lustig, who’s written copiously about the evils of sugar, suggests that we actually can’t identify which comes first, weight gain or insulin resistance. “Behavior can alter biochemistry, but biochemistry can also alter behavior,” he wrote in a 2008 editorial in The Journal of Pediatrics. No one truly knows which comes first, the illness or the weight gain. What we’ve got right now is a limited correlation, not out-and-out causation, though if you read the news or talk to your doctor about it you’re unlikely to hear this perspective.

Finally, another factor that's rarely discussed in this context is physical activity. In Steven Blair, a professof of exercise science, epidemiology, and biostatistics at the Arnold School of Public Health at the University of South Carolina, has published a lot of research showing that being physically unfit is as much or more of a risk factor for heart disease and death as diabetes, obesity, and other weight-based risk factors. Since then he and other researchers have argued it’s healthier to be fit and fat than unfit and thin.


1. J. A. Bell, M. Kivimaki, and M. Hammer, “Metabolically Healthy Obesity and Risk of Incident Type 2 Diabetes: A Meta-Analysis of Prospective Cohort Studies,” Obesity Reviews  15, no. 6 (2014): 504–515.

2. Attia’s TEDMED talk on the subject is provocative and fascinating. See it at

About the Author

Harriet Brown

Harriet Brown is the author of Body of Truth: How Science, History, and Culture Drive Our Obsession with Weight—and What We Can Do About It, Brave Girl Eating, and other books. 

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