Body Shape Changes with the Seasons

June 12, 2006 — Seasonal changes cause fat to shift locations in our body, thus altering the shape of our figures at certain times of the year, according to a new study.

Varying testosterone levels drive the shape changes, the study suggests. The hormone, often associated with brawn and aggressiveness, fluctuates over the seasons in both men and women.

The most evident changes occur within the waist and hip region, the study determined. When testosterone levels rose, women became less curvy as fat shifted toward the waist.

Other research has determined that the opposite happens in men, who retain more fat in the abdominal region when testosterone levels fall.

The scientists examined seasonal testosterone fluctuations in the saliva of 220 women and 127 men. They also measured the waists and hips of the female study participants over the seasons.

"We found that women’s and men’s testosterone is highest in the fall," said Sari van Anders, who led the research. "As well, women’s waist-to-hip ratio (how big the waist is relative to the hips) is highest during the fall, and central measures of fat deposition, like abdominal fat, were also somewhat higher in the fall (for women)."

Women also had high testosterone levels in the summer. Men showed lowest testosterone levels in spring.

Van Anders, a doctoral candidate in behavioral neuroendocrinology at Simon Fraser University in Canada, added, "This suggests that patterns of fat deposition, but not overall fat, are slightly different depending on the season, with more fat being deposited in the central waist region in the fall (in women)."

The results have been accepted for publication in an upcoming issue of the journal Psychoneuroendocrinology.

The study suggests women look more curvaceous in winter and spring. Men, explained Van Anders, look manlier during spring since waist and hip size becomes more uniform and less feminine.

"While we didn’t test this, a large body of previous research has found that men find lower waist-to-hip ratios in women more attractive in Western nations," van Anders told Discovery News. "As well, a large body of research finds that lower waist-to-hip ratios are associated with better fertility parameters and health parameters. This suggests that women should be perceived as slightly less attractive in the fall."

The researchers don't know whether the changes are apparent on a conscious or subconscious level, since they occur gradually over time and do not dramatically alter appearance.

Randy Nelson, a professor of psychology and neuroscience at Ohio State University, and one of the world's leading experts on how the seasons affect humans and animals, told Discovery News that the study "is a novel and important finding."

He added, "There have been previous demonstrations of seasonal changes in hormones, including testosterone, in humans, as well as seasonal influences on the timing of conception. About 6 years ago, a graduate student of mine even showed seasonal changes in cognitive performance among people!"

The scientists are not sure why these fluctuations in body shape exist. Van Anders said the changes might merely be byproducts of testosterone changing seasonally for reasons unrelated to waist-to-hip ratio, such as health, immune function, fertility and behavior.