I used to be thrilled that my blood level of HDL cholesterol, the so-called good cholesterol, was high, the likely result of my devotion to daily physical activity. After all, HDL, for high-density lipoprotein, acts like an arterial cleanser, removing cholesterol from blood vessels and preparing it for removal outside the body.

An ample supply of HDL cholesterol in blood serum has long been linked to protection against coronary heart disease and stroke, so what could be bad?

I now know that, as with many other good things in life, there can be too much of this usually helpful protein. The best available evidence has shown that most of us would probably be best off aiming for moderation with regard to serum levels of HDL.

A series of studies, large and small, in this country and abroad shows that the relationship of HDL levels to heart disease and overall mortality forms a U-shaped curve: too little and too much of this blood lipid are, on average, associated with an elevated risk of heart disease, cancer and premature death.