Healthy BMI: the lower the better?

According to the World Health Organization and many medical societies, BMI should be maintained in the 18.5 to 24.9 kg/m2 range to achieve optimum health. However, data from some epidemiological studies suggest that being overweight, and possibly even mildly obese, may be protective and associated with a reduced mortality risk; findings which have created a lot of confusion in the medical community and in the general public.

The problem is that normal weight, defined as a BMI between 18.5-24.9 kg/m2, is a broad reference group containing not only non-smoking individuals, who are lean because physically active and eating healthy diets, but also those who have fallen in the normal BMI range for other reasons, such as smoking and metabolic alterations and underlying occult chronic diseases, often driven by unhealthy lifestyles, that accelerate the accumulation of metabolic and molecular damage.

To address this issue, in this large prospective cohort study, we examined the joint associations of BMI, diet quality, physical activity level (PAL) and smoking with total, cardiovascular and cancer mortality among 71,695 women from the Nurses' Health Study (NHS 1980-2012), and 38,219 men from the Health Professionals Follow-up Study (1986-2012) who were free of cardiovascular disease (CVD) and cancer at baseline. 

The major findings of this study are:

1. Our data confirm that in people with unhealthy lifestyles (i.e. sedentary, eating unhealthy diets and smoking) there is a U-shape relationship between BMI and mortality with an increased mortality risk in the 18.5-22.4 and ≥ 30 BMI range.
2. Our data show for the first time that in never-smoking men and women eating healthier diets and exercising regularly ≥30 min/d at moderate intensity, there is a linear relation between BMI and mortality risk with the nadir in the 18.5-22.4 BMI category. These individuals experienced the lowest CVD, cancer and total mortality risk (HR=0.39).
3. Even among overweight and obese individuals, adherence to healthy lifestyle (i.e. eating healthier diets, exercising regularly and avoid smoking) is associated with a drastically lower risk of mortality. 

We believe that our findings are important because they show that the U-shaped relationship between BMI and mortality found in some epidemiological studies is mainly driven by an overrepresentation in our societies of individuals who are lean because of chronic metabolic and pathological conditions caused by exposure to smoke, sedentary lifestyle and unhealthy diets. In this large study of US health professionals, only 4.4% of men and 5.3% of women, respectively, were lean and had all the 3 healthy lifestyle factors. Finally, our findings indicate that leanness induced by healthy lifestyles is the optimal way to promote healthy longevity and to reduce the risk of developing and dying prematurely of cardiovascular disease and cancer.

http://www.bmj.com/content/355/bmj.i5855.long