Letter: Weighing the evidence on healthy fatness

From Jon Arch, Welwyn Garden City, Hertfordshire, UK

Claire Wilson explains that as many as a third of people who are overweight have good scores for blood glucose, cholesterol and blood pressure (29 September, p 20). But why is this?

Many factors apart from body fat affect these scores. Another, underappreciated factor is that not all fat is equal.

The main component of stored fat, triglyceride, is not a problem (other than for weight-bearing joints) so long as it is tucked safely away in lipid droplets in small, subcutaneous fat cells. It is not the direct link between obesity and metabolic disease.

The problem comes when triglycerides are stored in large fat cells called adipocytes around the gut in adipose tissue, or even worse in non-adipose tissues, notably skeletal muscle and liver. Here triglyceride produces metabolites that cause resistance to insulin, a feature of type 2 diabetes and other features of “metabolic syndrome”, such as high blood pressure and low-HDL (good) cholesterol.

This helps explain why the glitazone drugs, which shift fat from large visceral to small subcutaneous fat cells, can increase total body fat but improve insulin sensitivity.

Those rare individuals who are unable to make adipose tissue and direct their fat to muscle and liver are insulin resistant and often have symptoms of metabolic syndrome.

Exercise is more beneficial than you might expect from its effect on body fat content, because it burns off troublesome metabolites.

None of this means that losing fat has no benefit in an overweight individual. If other factors remain unchanged, it does.