Thirty years ago, we understood very little about body fat. It was assumed its sole purpose was to store excess energy then supply it when food was scarce, plus give us a bit of padding. Frankly, scientists thought fat was pretty boring. That view has now been transformed.
“Today, we know that adipose tissue produces and secretes hundreds of proteins and other substances into the bloodstream, which regulates a wide variety of processes,” says Auckland endocrinologist Rinki Murphy.
“For example, the leptin hormone, produced almost exclusively by fat cells, is a central regulator of appetite and the rate of energy expenditure.”
Leptin was the first hormone identified as secreted by fat and found to have a clear function. Since then, researchers have discovered many more. The hope is that a better understanding of fat cells and how they communicate with the rest of the body might lead to improved treatments for obesity and its related conditions.
Murphy, a professor at the University of Auckland, has been involved in the Human Cell Atlas project, an international study that is mapping every cell type in the human body. Since she specialises in weight management, Murphy naturally opted to focus on fat cells and her work has contributed to a groundbreaking discovery.
“We took cells from deep inside the abdomen from people who were having surgeries,” she says. “In New Zealand, we selected people who were being kidney donors so they weren’t overweight or unwell ‒ they were as healthy as they possibly could be.”
Using innovative genetic technology, the scientists examined the samples and found, in addition to “classical” fat cells, several sub-types of a different kind of fat cell. These atypical cells each had a slightly different job and were involved with things like inflammatory processes and the immune response.
What was particularly surprising was that the newly identified fat cells didn’t develop from the classical fat cells, gaining new functions. Instead, it was the other way around: the atypical cells seemed to lose their special powers, turning into classical fat cells then getting on with the more basic job of holding on to lipids.
While some of these atypical cells were present beneath the skin in subcutaneous fat, there were a lot more of them deep inside the abdomen. It has long been known that excess fat around the middle and inside organs and blood vessels is bad for health. Called visceral fat, it has been associated with greater inflammation, but scientists didn’t know what might be going on at a cellular level to explain it.
Now, new theories are being formed. One is that if people have a high-calorie diet and therefore lots of classical fat cells, there isn’t enough space for the atypical cells to progress from their earlier phases, and they get stuck. Rather than being stored as relatively healthy, subcutaneous fat, they spill over into areas where they shouldn’t be, because the excess fat cells have to go somewhere.
“So, they may end up in the pancreas and cause diabetes, or in the neck, leading to obstructive sleep apnoea,” says Murphy.
More research is needed, but it could lead to new tools for preventing conditions such as diabetes if it turns out that the higher the proportion of unique cells a person has compared to classical fat cells, the greater their risk of metabolic complications.
“At the moment we use risk factors like family history, weight, blood pressure and cholesterol levels, but that’s not as precise as being able to say, ‘Well, actually, your fat cells are up to their health threshold so it’s a good time to be reducing your weight,’” says Murphy.
Some people have a lower capacity to store fat subcutaneously and are more likely to store it viscerally – they may appear thin but still be at risk of problems like heart disease and diabetes.
She hopes to conduct a larger study to see if the balance of classical and atypical cells changes as an individual loses or gains weight. “Then we may be able to determine where the health threshold might be and if it is different for men and women, different by age or ethnicity. There are so many variables to consider as we try to put the clues together.”
