All Fat Isn’t Equal When It Comes to Health
Location, location, location. The mantra applies not only to real estate but also to body fat. Experts say that where fat lives—not just its quantity—affects health. Body mass index (BMI) is an estimate of total body fat, but it doesn't indicate where the fat is found. This has led to controversy over whether BMI is really the best predictor of who is at risk for obesity-linked diseases, including type 2 diabetes, or if some alternate measure that takes fat's location into account would be better.
Experts once thought fat was fat, and more was worse than less. In recent years, though, these assumptions have come under fire. "I was always fascinated that some of our study subjects were obese by weight standards, but then you would look at their risk-factor profile and they would have normal blood pressures and blood sugars," says Jean-Pierre Després, PhD, FAHA, FIAS, of the Quebec Heart and Lung Institute. And others, Després recalls, "would be barely overweight and have complications."
In studies of the relationship between fat and health, scientists typically use BMI to assess if a person is underweight, normal weight, overweight, or obese. BMI reigns partly because it's convenient. It's calculated with just two easily accessible measures: weight and height. The number that results is meant to be a proxy for body-fat percentage, the proportion of one's weight that's made up of fat.
|The Obesity Paradox|
|Obesity is supposed to be a killer, but some research suggests that a little extra weight may not be harmful; indeed, it may even be protective. In January, a study in The Journal of the American Medical Associationreported that people with low-grade obesity (BMI of 30 to 35) had the same risk of death from any cause as those of normal weight. Overweight people (BMI 25 to 30) had it even better, with a risk of premature death significantly lower than those of normal weight. So if body fat is bad for the health, why don't these data show that?
It may be an issue of categories. The results make sense, says Jean-Pierre Després, director of research at the Quebec Heart and Lung Institute, if one takes into account the fact that the researchers included people he'd consider underweight in the study's normal-weight group. "To use a BMI of 18.5 s the reference group is wrong. [That low of a BMI is] not healthy," says Després. "The optimal BMI is probably 24 or 25."
Then again, some researchers think that a little extra cushion is healthy, though they aren’t sure why. It may be that having those fat reserves helps stave off death in frail older people. Or that heavier people go to the doctor more and get better treatment simply because they are heavier. Experts agree, though, that gaining weight on purpose is unwise.
Many studies have shown that, in general, higher BMI equals more disease. (Some evidence suggests, though, that there is a lower risk of death from any cause at higher than normal BMIs; see "The Obesity Paradox," right.) As a result, BMI has become the chief measure to assess whether body size puts a person at risk for health problems.
Yet, when scientists look at individuals, they find that the health of people with the same BMI can vary a lot. "In the late '80s, I began to show that the alterations in the risk-factor profile are not related to total body fat, but to the amount of fat in the abdominal cavity," says Després. The data suggested that people who are obese according to BMI may be at low risk if their fat resides outside the abdominal cavity, while those with a normal BMI can be at high risk if their fat accumulates inside the abdominal cavity. Despré's results, initially met with skepticism, have since gained traction as evidence has mounted that body-fat location may be crucial.
The Good, the Bad, and the Visceral
Today, visceral fat, which is the name for the fat in the abdominal cavity, is one of the most vilified types of flab. (Its buildup may give people an "apple shape." Looks alone, however, aren't enough to establish where body fat is stored.) A 2012 study in The Journal of the American Medical Association found that visceral fat—but not total fat—increased the risk of developing prediabetes and type 2. The senior author of the study, James de Lemos, MD, of the University of Texas Southwestern Medical Center, is now looking at the link between body-fat location and heart disease. "There are associations with cardiovascular disease," he says, "but we haven't teased that out yet."
Ectopic fat may be even more sinister than visceral fat, says Després. "Ectopic" literally means "misplaced," and this type of fat hangs out in areas where it has no business being. It can intrude on the organs, marbling the liver and heart. As visceral fat increases, so does ectopic fat. That makes it difficult to pinpoint which type of fat should get the bulk of the blame for health problems. "Maybe it's not the visceral fat that is bad," Després says. "Maybe it's a marker of heart fat or liver fat."
On the flip side, the body may sequester extra fat in places where it is less harmful. Some experts think that people who store their fat in the legs and buttocks (and have "pear-shaped" bodies) have a health advantage simply because they aren't storing fat in their abdomens, though this idea remains controversial. The de Lemos study also found that fat just under the skin (subcutaneous) isn't correlated with risk for prediabetes or type 2. Després cautions, though, that "the more fat you have, the greater chance you have of having a health problem." But people whose fat is stored under the skin may be better off than those whose fat is in the liver, for example.
Scientists still don't know exactly why fat's location is important. "It appears that visceral fat promotes a vicious cycle of insulin resistance," says de Lemos. "There's thought that inflammation may play a role." Other research suggests that visceral fat increases the amount of fatty acids in the bloodstream, exposing the liver to dangerous levels of fat and contributing to the development of heart disease and type 2 diabetes. Ectopic fat may also cause problems. "Fat in the liver is extremely bad," says Després. "A fatty liver will pump more lipids [fats] into the blood and more glucose into the blood." And ectopic fat in the heart may directly damage the organ. In animal studies, excess heart fat led to high blood pressure and heart failure.
|Bioimpedance is a measure of how easily electric current flows through the body. A painless procedure uses bioimpedance to provide an estimate of total body water. Water is a good conductor, while fat conducts poorly; the difference allows a device to estimate your body-fat percentage. Dehydration and other factors can skew results.|
So what determines where your fat is stored? "I'd love to know the answer to that," says de Lemos. He says fat storage preferences may depend, at least in part, on genetics. For example, genes may dictate how much fat a person can store under the skin. Once the subcutaneous reservoir is filled, people may start to deposit fat in the abdomen. Another factor is hormones, says de Lemos. Sex hormones are thought to play a role in fat placement. One reason: Men are more likely to store fat viscerally than women. Plus, women with polycystic ovary syndrome (PCOS)—a hormone disorder—tend to accumulate fat in the abdominal cavity.
Because you can't change your genes, scientists are seeking effective strategies that can help people keep fat out of their abdominal cavity and organs. "There is evidence that regular vigorous endurance exercise can [reduce] visceral fat, even without losing weight," says Després. Cutting down on overall body fat by reducing calorie intake can also help keep fat under the skin and out of the danger zones.
To help cut down on obesity-related risks, doctors often focus on lowering patient BMI. But, if body fat's location is a key factor for health, then BMI may have a limited capacity to predict who is at risk for disease. Plus, BMI can be imprecise; it can show heavily muscled people, for example, to be obese because of all that lean mass. "People think that the BMI measure is too simplistic" and maybe not useful, says Andrew Rundle, PhD, an epidemiologist at Columbia University. Ideally, the location of fat in a person's body would be measured and used to predict risk. Advanced scanning methods such as magnetic resonance imaging (MRI) can look inside the body and map out fat, but these approaches are too expensive and time consuming to be done with any regularity outside a research setting. "The nice thing about BMI is I can do that in my apartment or gym," says Rundle.
The controversy over BMI has sparked interest in alternatives, such as waist circumference, waist-to-height ratio, use of calipers, and bioimpedance (a noninvasive measure of how easily electrical current flows through body tissue). Waist circumference might approximate visceral fat and thus predict disease risk better than BMI. But it's not that simple, explains de Lemos, because waist circumference "includes both abdominal and subcutaneous fat." The other measures have shortcomings, too. For example, bioimpedance can be inaccurate if ethnic differences aren't taken into account, and calipers are notoriously imprecise.
When assessing body-fat measures, Rundle says it's critical to keep the goal in mind: predicting risk for heart attacks, strokes, diabetes, and other health issues so preventive action can be taken. "My biggest concern is that the message of alternative measures being better and more technologically advanced is to ignore the BMI," says Rundle. He thinks that would be a mistake.
Rundle compared BMI with a host of fat measures in a study published in January in Obesity Research & Clinical Practice. He and his team analyzed data from a diverse group of over 12,000 adults to see which body-fat measure had the strongest ties to high blood glucose, high blood pressure, and high levels of LDL ("bad") cholesterol. They found that "BMI works as well as any of the other proposed measures," says Rundle.
The researchers also checked whether any fat measure worked better than BMI for a particular ethnicity or for men or women. "We didn't find that in one racial group, one measure was better. No matter your race [or sex], BMI is still a good measure," says Rundle, adding: "We aren't saying BMI is great; we are just saying that nothing else is great either. If your doctor says you have a high BMI, you should pay attention."
It may still be a good idea to keep track of waist circumference, says Després, particularly for people trying to shed excess body fat. "If [people with type 2 diabetes] become active, they will lose liver fat but gain muscle," he says. While their weight won't change, he adds, the change in waist circumference may reveal a significant reduction in unhealthy visceral fat. Evidence that the waistline is shrinking may help keep dieters motivated even if the number on the scale doesn't budge.