Men and women accumulate fat in different parts of their bodies.
More and more adults in the Western world are obese, and the Western high-fat diet might be to blame.
However, men and women react differently to a high-fat diet.
These differences were the focus of scientists at the University of California, Riverside (UCR), who set out to examine the health consequences of obesity in male and female rodents.
Djurdjica Coss, an associate professor of biomedical sciences in the UCR School of Medicine, led the study, which is now published in the journal Frontiers in Immunology.
Coss and colleagues examined the role of the female hormone estrogen in accumulating fat, as well as the health consequences of being overweight in male versus female rodents.
Do ovaries protect females from weight gain?
As the study authors explain, previous studies have suggested that women tend to be lean when they are young but gain weight after menopause because estrogen helps them keep excessive weight at bay.
Ovaries are the main source of estradiol, which is a form of estrogen, in premenopausal females. So, Coss and team wanted to test the protective role of ovary-produced estrogen by removing ovaries in female rodents.
The researchers fed the ovariectomized females a high-fat diet and compared the effects with those of male mice that were fed the same diet.
How body fat affects males and females
Coss summarizes the findings, saying, "We found that the mice proceed to gain weight when fed a high-fat diet, suggesting that ovarian hormones are indeed protective against weight gain."
"But we found, too, that these female mice exhibit neither neuroinflammation, nor changes in reproductive hormones, suggesting that they are protected by factors other than ovarian estrogen. This is a novel finding."
She comments on what these results mean for humans, saying, "Mice on [a] high-fat diet develop metabolic syndrome — a constellation of pathologies that includes type 2 diabetes and insulin insensitivity — similarly to obese humans."
However, the health consequences of carrying excess body weight do not stop here. "Obese men have lower testosterone levels, contributing to low libido, low energy, and reduced muscle strength," explains Coss. "We see this in mice, too; obese male mice showed nearly 50 percent decreases in testosterone and sperm number."
On the other hand, "Obese women have difficulty with their menstrual cycles," she adds. "They don't ovulate. Obese female mice show the same, contributing to decreased fertility."
Pear-shaped bodies and brain inflammation
However, the research also suggests that women are more protected against the harmful effects of being overweight, a phenomenon that may have a lot to do with women's body shape and their body fat distribution.
"Our results agree that males have larger visceral fat depots," say the researchers. Also, the results seem to confirm that premenopausal women tend to deposit fat subcutaneously, or under the skin.
Accumulating fat around the abdomen leads to the "apple-shaped" body, while the fat that gathers around the hips leads to the "pear-shaped" body. Women are likely to develop the latter, and men the former.
Visceral, or abdominal, fat can reach and affect internal organs. Additionally, this type of fat "gets more inflamed with a fat overburden," as Coss explains. "This fat then recruits immune cells from blood circulation that get activated."
Previous studies have shown that neuroinflammation is a side effect of obesity, but this study specifically found that in male mice, macrophages — which are a large type of immune cell — penetrated the blood-brain barrier and reached the brain.
According to Coss, "The brain has been considered an 'immune protected site,' but we show that peripheral inflammation 'spills over' into the brain, which, in turn, may cause neuronal problems."
However, the authors cannot yet fully grasp the mechanisms that are behind this obesity-induced neuroinflammation, and they do not know why women are protected against it. So, future studies will be required.
"While overweight, women are more protected than men where neuroinflammation is concerned," Coss says. She ventures an explanation, saying, "This could be an evolutionary protection for women, who need to experience more change in weight due to pregnancy."