The first and rather obvious explanation is what we call reverse causality. It is a well-established fact that the sicker patients tend to lose more weight than those who have mild disease. Thus, it is not the obesity that is protecting people rather the loss of weight occurring as a consequence of a more severe illness. Secondly, those people who are carrying less weight with established illnesses such as cardiovascular disease, cancer, osteoporosis and type II diabetes, tend to have a stronger genetic predisposition to the disease. Could it be, therefore, the more genetic an illness, the more seriously it affects the body compared with lifestyle associated disease, which tends to be associated with obesity?

Another explanation is that smokers tend to be thinner than non-smokers but also suffer more serious diseases.

In the specific case of chronic kidney disease this is a chronic inflammatory state and also associated with protein energy wasting. There is a paradoxical association between traditional risk factors & cardiovascular disease in this setting and the weight loss itself may be a marker of more severe disease.

Also, BMI is not as good a marker for abdominal fat as waist circumference and it may be that those with the higher BMI have a larger muscle mass, which is felt to be protective. Also, there is the consideration of being fat but fit, which may include better cardiovascular and muscle strength.

It is my strong belief that we should all try and maintain a healthy body weight but over the age of 50, I would suggest you should be aiming for a BMI between 25 to 30 with the waist circumference around 90 to 95 for a male and 75 to 80 for a female. I heard an interesting concept the other day that your waist circumference should be less than half your height.

I advise all my patients beyond the age of 50 to focus more on healthy eating which is basically to eat less and eat more naturally, along with maintaining 3 to 5 hours of testing exercise per week which should include two thirds cardio and a third resistance training. This is much better for your health than obsessing about weight. Regardless, there is certainly no strong science that once you are over the age of 50 being thin or possibly even more concerning trying to lose too much weight is of any major health benefit. Focusing on eating high-quality food and regular exercise is clearly much better for you and should be one of our major health goals.