Fat but Fit - Is It a Myth?

Summarized by Robert W. Griffith, MD
February 11, 2005

Introduction

We know that both obesity and lack of physical activity are risk factors for major chronic diseases and early death, but which is most important? Some studies have suggested that if you're physically active, it doesn't matter so much if you're fat - i.e. 'fat but fit' people don't have a shortened lifespan. This viewpoint has now been questioned by a new study reported in the New England Journal of Medicine.

What was done

Physicians from the Harvard School of Public Health analyzed data from the Nurses' Health Study, which started in 1976. At that time, over 125,000 female nurses aged between 30 and 55 completed a questionnaire about their health and lifestyle. Thereafter, they returned follow-up questionnaires every 2 years. Women with cardiovascular disease and cancer at baseline were excluded for this analysis.

Body mass index (BMI) was calculated for each participant, based on their weight and height. (Do you know your BMI? If not, see the first link below.) The nurses were grouped into 9 categories according to their BMI at baseline. People with a BMI of 30 or above were defined as obese.

From 1980 onwards participants were asked to report the average number of hours per week engaged in moderate physical activity (e.g. brisk walking) and in vigorous activity (e.g. strenuous sports and jogging). In later years they were asked to detail the actual sports they practiced.

The end-point for this study was death from any cause, reported by the next of kin, the postal authorities, or the National Death Index. Deaths were then classified according to their cause: cardiovascular disease, cancer, or 'any other'.

What was found

The 24 years of follow-up yielded information on approximately 2.7 million person-years. There were over 10,000 deaths during this period - roughly half from cancer, a quarter from cardiovascular disease, and a quarter from other causes. Because of the possibility that smoking was associated with the cases of cancer reported, further analyses were done in the women who had never smoked.

Mortality increased in association with higher BMI levels in a statistically significant way. This was largely independent of the level of physical activity; greater physical activity was beneficial at all levels of overweight, but didn't eliminate the increased mortality seen with obesity.

The analyses showed that, compared with a woman who was lean (BMI less than 25) and physically active (3½ or more hour's exercise/week), the likelihood of dying increased for the other participants as follows:

• Lean and inactive - 1.55 times
• Obese but active - 1.91 times
• Obese but inactive - 2.42 times

The authors of the study used these results to calculate that overweight (BMI over 25) together with inactivity could account for 31% of all early deaths (59% from cardiovascular disease, and 21% from cancer) among non-smoking women.

More importantly, perhaps, even slight weight gain, independent of physical activity, was clearly linked to a slightly higher risk of death.

What this study shows

Increased weight and low physical activity are important, but independent, risk factors for dying early. It's not enough, as previously hoped, to be 'fat but fit'. And it's also not enough to be 'lean but lazy'. Your resolutions to lose weight by dieting are good, but they need supplementing by exercise. And exercise is, after all, a good way to help lose weight. So, as we've said so many times, eat right and exercise, if you want to live longer!

Source

• Adiposity as compared with physical activity in predicting mortality among women. FB. Hu, WC. Willett, T. Li,  et al., N Engl J Med, 2004, vol. 351, pp. 2694--2703

Related Links
BMI Calculator
How Much Exercise Is Needed to Lose Weight?
Overweight Possibly Linked to Dementia in Older Women