Eight weeks of extra ice cream shakes, king-sized Snickers bars, and Boost Plus energy drinks resulted in greater fat mass in the upper body, compared with lower body fat, but thighs developed more body fat cells, according to findings published in the Proceedings of the National Academy of Science (PNAS).
The study, by researchers from the Mayo Clinic, suggests that the mechanisms for fat accumulation are different for upper and lower body fat, with quick increases in lower body fat – through the formation of new fat cells – associated with a reduced accumulation of fat in the upper body.
"The accumulation of abdominal fat happens largely by individual cells expanding in size, while with fat gain in the femoral or lower body, it's the number of fat cells that increase…The cellular mechanisms are different," explained lead author Dr. Michael Jensen, of the Mayo Clinic.
“Greater capacity to form new lower body fat cells in response to overfeeding may confer protection against accumulation in [the] upper-body …, potentially mitigating adverse metabolic consequences of weight gain,” wrote the researchers.
Body fat distribution is an important predictor of the metabolic consequences of obesity.
The accumulation of fat in upper body tissue is associated with insulin resistance and obesity-related metabolic abnormalities, whereas lower-body fat gain is reported to have a protective effect.
However, the cellular mechanisms regulating regional fat accumulation and distribution are unknown and are of “considerable interest”, stated the researcher.
Previous studies have suggested fat-cell numbers remain stable after the age of 20 –implying that fat gain during adulthood is the result of fat cell (adipocyte) growth and not the development of new fat cells.
The new study assessed the changes in adipocyte size and number in response to overfeeding in upper and lower-body subcutaneous fat deposits.
The researchers observed that an average gain of 1.6 kg of lower-body fat resulted in the creation of approximately 2.6 billion new fat cells.
In contrast, the researchers found that, on average, the size but not the number of abdominal fat cells increased significantly in response to fat gain
It was reported that women with small abdominal fat cells gained abdominal fat largely through adipocyte growth, whereas women with larger adipocytes showed a reduction in average fat cell size, with researchers indicating that they “must have recruited new, smaller adipocytes.”
The researchers reported that inherent differences in fat-cell development and, potentially, in the production of mediators that recruit new fat-cells – together with differences in circulation, nutrient supply, and local hormonal factors – may contribute to the distinct responses of fat development.
The results “demonstrate that lower body fat-cell progenitors can develop rapidly into mature adipocytes in adult humans in response to overfeeding,” concluded the authors.
The researchers stated that the data “challenges the contention that total number of body fat cells remains constant in adults.”
The findings also support the idea that an increased capacity to produce lower-body fat cells increases protection to the upper body and can potentially help prevent metabolic diseases. The authors said that their findings “potentially provide an explanation for the purported beneficial health effects of leg fat”.
They added that future studies would be necessary in order to explain the mechanisms behind the new findings.
Source: Proceedings of the National Academy of Science (PNAS)
Published online before print, doi: 10.1073/pnas.1005259107
“Regional differences in cellular mechanisms of adipose tissue gain with overfeeding”
Authors: Y.D. Tchoukalova, S.B. Votruba, T. Tchkonia, N. Giorgadze, J.L. Kirkland, M.D. Jensen