We’ve all heard the recent news story splashed all over the TV and internet: overweight people actually have a lower risk of dying than those of normal weight. (1) What a shock to the millions of Americans on a weight-loss diet, not to mention the hundreds of health professionals, research scientists, and government policy makers who are working around the clock to combat the obesity epidemic in this country! Could it be that we’re all wasting our time trying to lose weight if we want to maximize our life span? Do fat people really live longer?
As with most epidemiological research, this report needs to be taken with a big grain of salt. While there may be some level of protection to the “extra padding” that many Americans are trying to lose, there are also serious shortcomings when using BMI as a determinant of body fatness. Someone with a normal BMI may have a high body fat percentage, while your average bodybuilder can have a BMI that falls within the obesity range while still boasting a single-digit body fat percentage. So is being overweight really health protective, or is this another case of mass media jumping to unfounded conclusions?
Are you wasting your time trying to achieve a “perfect” BMI?
When body fat is protective
First, the authors of this study admit that its limitations include a narrow focus on BMI as opposed to body composition as a predictor, and an outcome of all-cause mortality without addressing morbidity or cause-specific mortality. (2) This generalized view of the data leads to an omission of certain factors such as body fatness, overall indicators of health besides mortality, and risk of dying from obesity-related disease as opposed to risk of dying in general. In other words, body composition likely plays a significant role in overall mortality, and overweight people may still be more likely than normal weight people to die from certain diseases attributable to excess body fat. This study does not control for these important factors, and therefore cannot be used to make general health recommendations to the public.
However, this study does bring into question the benefits of weight loss for those who are metabolically healthy. It is reasonable to suggest that having a bit of extra body fat may be generally protective. In an accompanying editorial to this report, Drs. Heymsfield and Cefalu of the Pennington Biomedical Research Center observed that the “obesity paradox” of all-cause mortality may have more to do with survival rates during both chronic and acute disease states, where a small excess of adipose tissue may provide the extra energy needed for recovery during catabolic illness. (3) A slight excess of body fat may also protect against death from traumatic injury, when metabolic rate skyrockets in response to inflammation and infection following the trauma, and body tissue is broken down rapidly.
In any situation where the body’s energy needs are high, having extra fat stores can help prevent the deadly tissue wasting that can cause deterioration of vital organs such as the heart. So if you’re trying to lose the last ten or fifteen pounds, you may be reassured to know that body fat can serve a purpose when it comes to protection against catabolic illness or traumatic injury. Thus, if your overall goal is to live a long, healthy life, that “extra padding” could actually be helpful rather than harmful in the long run.
BMI versus Body Fatness
On the flip side, BMI calculations pay no attention to a person’s body composition. As I mentioned before, a person can be quite lean but still fall into an overweight BMI category if they’re stocky or extremely muscular. And we’ve all heard about the danger of being “skinny fat”, where one’s BMI appears healthy but his or her body fat percentage is dangerously high. In fact, research suggests that the sensitivity of classifying a high body fat mass from a high BMI may be as low as 20 to 50%, demonstrating the potential inaccuracy of judging one’s health based solely on BMI. Therefore, we must consider body composition more carefully when determining the health risks or benefits of being in a higher BMI class. What does the research have to say about the role that body fatness plays in overall health and mortality?
A comprehensive analysis of NHANES data found that for men, an overall BMI of 27 was associated with the lowest risk of mortality, but that both fat mass and fat free mass were more directly correlated to mortality. (4) When body composition was controlled for, the data demonstrated the lowest risk for men at a BMI between 19.5 and 20.5, suggesting that mortality risk is more related to leanness than it is to BMI. Another study conducted on 60 year old Swedish men found that a high percentage body fat was significantly associated with a 40% increase in total mortality, compared with a low percentage body fat, suggesting that a high fat mass is more strongly associated with mortality risk than a high BMI. (5)
A 2012 study found that body fat is inversely associated with mortality in patients with already-established coronary heart disease (CHD), with low body fat being an independent predictor of an approximately 3-fold higher mortality in these patients. (6) This is surprising, considering that high body fat is generally associated with a worse overall CHD risk profile. The researchers also found that higher lean mass was associated with a better prognosis for survival. They theorized that those patients with higher body fat and/or higher lean mass had a better response during times of negative caloric balance (catabolic stress) and may have had more muscular strength than patients with lower body mass, protecting against death.
Another 2012 study found that there is a significant gender difference in survival rates for older adults (≥65 years) with varying BMIs. (7) In both men and women, lean body mass was associated inversely with mortality, where those with the lowest lean mass had the highest risk of dying. The results also showed that higher body fat in women was generally protective against mortality, provided that the waist-to-hip ratio was low. This is an important distinction, as healthy men and women have very different body compositions and must be studied independently from one another.
Take home message: Body composition matters!
These studies all have their limitations, but the general pattern seems to suggest that a slightly higher BMI with a higher proportion of lean mass is associated with better mortality outcomes for both men and women. For women specifically, healthily distributed body fat is actually protective against mortality, even at higher levels. There are several explanations for this finding, including a greater caloric reserve during catabolic illness, overall greater muscle strength, and possibly a more adequate food intake, particularly for older adults.
It’s important to remember that a person’s body fat may increase for a variety of reasons, and that BMI is a poor indicator of overall health in most cases. As a clinician, I believe certain indicators of health such as insulin sensitivity, markers of inflammation, and overall digestive function are far more important predictors of health than BMI or total body weight. If you’re a metabolically healthy person with a few extrapounds that just won’t budge, current evidence suggests that getting to your “perfect” weight may not increase your lifespan. In fact, if you’re a woman, it may even be counterproductive. This casts doubt – at least from a scientific perspective — on our culture’s (pathological?) obsession with skinniness. It also suggests that maintaining adequate lean body mass by eating a nutrient-dense diet and doing regular weight-bearing exercise may be more important than shaving off those final few pounds of fat.
These findings do not, however, detract from the large body of evidence linking significant excess body fat and positive calorie balance (i.e. overeating) with numerous health problems ranging from metabolic dysfunction to cardiovascular disease. So if you’re substantially overweight and have a higher than normal body fat percentage, nothing in this article gives you an excuse to abandon your fat loss efforts.
Eat high quality, nourishing food, find strength-building exercise that you enjoy, get enough quality sleep, and manage your stress, and you’ll set yourself up for a long and healthy life – even if you’re carrying a few extra pounds along the way!
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