The Doctor Weighs In

By Dov Michaeli MD, Ph.D

Ever wondered if loss of weight causes a reduction in the number of your fat cells? Wouldn’t it be wonderful if that was true? You go on a diet, you lose weight and a bunch of cells, and you’d never gain weight again. Except it ain’t true, as anybody who went on a diet knows; unless you stick with your diet forever you will gain back the weight you had lost. Why is that?

Your fat cell allowance

In theory,there are two ways you can increase your body fat: you can increase the number of fat cells in the body, and you can store more fat in each cell. The second way, increase of the fat content per cell, has been amply documented; the reason we have so much evidence for that mode of weight gain is that it is quite easy to document. All that needs to be done is take biopsies of adipose tissue before the and during the weight gain and measure the fat content. And if you want to really nail it: take an additional biopsy after a weight loss diet, and document the drop in lipid content.

But what about the number of fat cells? That’s much tougher to measure, for obvious reasons: you can’t do a total body fat cell count. Or can you?

In animal studies, this question can be addressed by labelling DNA nucleotides with radioactive isotopes such as ¹⁴C. Differentiated fat cells do not divide, and so radioisotopes, incorporated in their DNA in the last round of division before differentiation, remain there throughout the cells' life. The time of radiolabel incorporation, is therefore the 'birth date' of these cells. But the potential toxicity of radioisotopes means that such studies cannot be performed in humans.

Kirsty Spalding and her colleagues at the Karolinska Institute in Stockholm cleverly thought of the next-best option. Atmospheric levels of ¹⁴C have remained relatively constant for centuries, with the only major increase occurring between 1955 and 1963, when nuclear bombs were being tested above ground. A chain of reactions ensures that, at any given time, the radioisotope content of human DNA matches that of the atmosphere. The authors could thus follow fat-cell dynamics in individuals born around 1955–63.

Spalding et al studied the dynamics of fat-cell number in some 700 adults, both lean and obese, and combined their data with previous observations in children and adolescents.

As the results show, fat cells have a high turnover: new cells are continually being born to replace their dead predecessors. The average age of a fat cell seems to be about 10 years in both lean and obese individuals, and the number of fat cells as a proportion of all cells remains constant in each weight group. But the total number of new fat cells was higher in obese subjects, suggesting that they are replenishing an existing larger pool.

What’s the take home lesson for lean people? and for obese people?

Do the lean among us need to worry about our diet if we have fewer fat cells? Yes, we do: our fewer fat cells can still store large amounts of fat. Also, can obese people do anything about their weight? After all, they've already accumulated a large pool of fat cells in childhood and adolescence? Again, the answer is yes. Again, the answer is yes. They can still reduce the volume, if not the number, of their fat cells. But there is another tantalizing message here: researchers should uncover the mechanisms underlying fat-cell turnover. If they do, one could conceivably slow down the replenishment of fat cells that came to the end of their ten-year life span. End result: progressively lower fat cell mass. This is still not a panacea; as we know from studies of people who had undergone liposuction--they slowly regain their previous weight by storing more fat in the remainig cells.

Liposuction: it's futile, lady.

Oh well, pass the fois gras!