In a paper published recently in the Proceedings of the National Academy of Sciences, Mary Jane West-Eberhard, emeritus scientist at the Smithsonian Tropical Research Institute, describes her theory about visceral adipose tissue (VAT)—fat inside the abdominal cavity.
“Pathogenic obesity is an advantageous process gone awry,” West-Eberhard says. “Very early in life the body makes decisions about where to store fat. It makes sense for poorly nourished fetuses to invest in VAT rather than in fat under the skin because VAT evolved to protect us from infections, but this choice sets us up for disaster if we have access to too many calories later in life.”
A VAT-rich structure called the omentum—a loosely hanging fold of the membrane lining the abdominal cavity—sticks to wounds, foreign objects, and infection sites like a bandage full of antibiotics. In fact, surgeons sometimes use pieces of omentum to control severe postoperative infections. VAT surrounds the small intestine, defending the body from ingested pathogens and toxins.
“The fact that visceral fat tissue evolved to fight visceral infections provides a causal hypothesis for how high fructose sweeteners and saturated fats contribute to chronic diseases such as type 2 diabetes,” West-Eberhard says. “They influence which bacteria grow inside the intestines, making the intestinal walls more permeable and releasing more toxins into the bloodstream, stimulating the visceral immune system and potentially leading to chronic inflammatory disease.”
In the past, the role of visceral fat as part of the immune system may have been more widely important than it is today because starvation and infections were more common. West-Eberhard proposes that in fetuses subject to nutritional stress, more energy may be stored as fat around the abdominal organs rather than as subcutaneous fat (SAT)—fat under the skin. She notes that childhood catch-up growth, a better predictor of obesity-associated disease than low birth weight, may be a sign of the mistake the body has made as it assigns energy to VAT, producing the apple shape of abdominal obesity rather than the pear shape of lower body fat distributed in the hips, buttocks, or thighs or more evenly under the skin.
In overweight individuals, a dangerous feedback loop may develop: Increased VAT leads to increased chronic inflammation, which, in turn, leads to increased insulin resistance leading to further VAT storage and increased susceptibility to disease. Eventually, the ability to produce insulin is reduced and these individuals may need injected insulin.