Researchers have found middle-age spread occurs in two distinct phases - casting doubt on the merits of using weight as a guide to health.
They found a thickening waistline in early middle age is accompanied by a rise in weight.
But although waists continue to expand with age, weight gain levelled off in later years as muscle turned to fat.
The study, by the Medical Research Council, appears in the Journal of Clinical Nutrition. The findings challenge the wisdom of relying solely on Body Mass Index (BMI) to assess whether a person is a healthy weight.
BMI, now used widely by health workers, is calculated by dividing a person's weight in kilograms by the square of their height in metres.
But it can only measure weight gain, and not deposition of fatty tissue, which, the latest study shows, might not be accompanied by an increase in weight. Muscle loss
Geoff Der, from the MRC's Social and Public Health Sciences Unit in Glasgow, said: "As people get older it seems that their bodies change - they lose muscle and get fatter.
"This explains why middle-age spread might not be reflected on the bathroom scales."
He added that BMI was a good measure of lean body tissue, but an expanding waistline might be a more reliable measure of the amount of fatty tissue a person has gained.
"Although the people in the older middle age group in this study appeared to put on less weight than the younger people, their waist circumferences continued to grow over time. What appears to have been happening is that the increase in fat was being obscured by a loss of muscle mass."
Source - BBC