Until fairly recently, fat was thought to be inert, evolution’s wobbly way of letting humans store energy for lean times. And we’ve long known that it’s better to be slightly overweight than underweight, as a recent study in the Journal of the American Medical Association reiterates.

Starting in the 1990s, though, scientists began to realize that fat is best understood as a single huge endocrine gland, one that wields powerful influence over the rest of the body. “For a typical North American, their fat tissue is their biggest organ,” says James Kirkland, M.D., director of the Robert and Arlene Kogod Center on Aging at the Mayo Clinic.

Not everything about fat is bad, of course. Fat tissue under the skin, known as subcutaneous fat—the kind that makes young people look succulent and ripe—is essentially padding that protects the body from injury, and it also helps fight infection and heal wounds. “Sub-q” fat produces an important hormone called adiponectin, which appears to help control metabolism and protect against certain cancers, notably breast cancer.

The bad news is that, as we age, we gradually lose this good fat, which is one reason why our hands get bonier. Instead, men and women alike tend to build up blobby fat on our midsections. Over the past decade or so, Kirkland and other scientists have discovered that this so-called visceral fat infiltrates our vital organs, bathing them in a nasty chemical stew that wreaks havoc in the body. Visceral fat produces an array of cell-signaling proteins called cytokines, including interleukin-6 (IL-6), which causes chronic inflammation, and TNF-alpha, for tumor necrosis factor, which has been linked to cancer.  DAMMIT!

(So there's an obese guy named Phil who started to work out, strenuously)

WITHOUT KNOWING IT, PHIL had kicked off a war for control of his body, with fat on one side and muscle on the other. Just as fat was long thought to be neutral, muscle was considered a passive organ that did what the brain told it to do. But muscle is now known to be one of the most dynamic systems in the body; when it contracts, it undergoes huge changes at the cellular level. And its mortal enemy is fat.

In any sedentary, inactive person—including people who aren’t actually obese—fat invades the muscles, slipping in between muscle fibers like the marbling in Wagyu beef. Worse, fat infiltrates individual muscle cells in the form of lipid droplets that make the cells sluggish. According to Gerald Shulman, M.D., a prominent diabetes researcher at Yale, these pools of fat, which occur in both the liver and the muscles, block a key step in the conversion of glucose, leading to the insulin resistance that’s a prerequisite for diabetes. This explains why some sedentary people of normal weight are still at risk for the disease. “It’s not how much fat we have but how it’s distributed,” Shulman says. “When the fat builds up where it doesn’t belong, in the muscle and liver cells, that’s what leads to Type 2 diabetes.”

"Fat is best understood as a single huge endocrine gland, one that wields powerful influence over the rest of the body."

That is fascinating.

Fat is tricky, if you deprive your body too much of calories/fats, it will actually hold onto the weight and fat more, your body goes into survival mode, very efficient in lean times of the past, but mostly unnecessary in a modern first world country.  I think our human lives have changed so much in a short period of time in the grand scheme of things, and our bodies have not adjusted/evolved to the changes.

Janill, I think you're right.

Human history until very recently has been characterized by scarcity.

Now our bodies need to learn to adapt to abundance.