Even if you’re not trying to lose weight, your fat may be working against you. Researchers have found that in healthy and moderately overweight people, fat cells can lose their ability to break down and release fat molecules normally.
When this process, called “triglyceride turnover,” is disrupted, triglycerides (the building block molecules of fat) can stay in the body longer — which may contribute to weight gain. While this research does not pin down exactly what causes some people to gain weight while others stay thin, identifying this process gives scientists a new potential target for weight loss research.
A team of scientists led by Peter Arner of the Karolinska Institute in Stockholm examined the age of triglycerides in abdominal fat cells taken from 41 subjects who were in the “normal” (Body Mass Index of 17-24) to “overweight” (BMI from 25-29.9) range. They found that the triglycerides in the bodies of their overweight subjects were an average of two years old, while those of their lean subjects averaged one and a half years old — an indication that triglyceride turnover drops in heavier people.
The team’s next experiment looked at what might prevent fat molecules from cycling at a normal rate. To find out, they took fat cells from 333 new subjects and exposed them to chemicals that break up triglycerides. They found that fat cells with older fat molecules, as well as fat cells from subjects whose BMIs were higher, saw reduced rates of triglyceride turnover, suggesting that heavier people may not be able to properly process fat.
These results raise an important question: which comes first? Does weight gain cause the fat cells to malfunction, or is it a problem with the fat cells that induces weight gain?
For Michael Jensen of the Mayo Clinic it’s helpful to imagine the human body as a car, with muscles as the engine and fat cells serving as the gas tank: “Overweight people don’t have bigger engines, just bigger gas tanks. That means any given triglyceride molecule…in the big gas tank is less likely to be sent to the engine than [it would be if it were] in a small gas tank.” In other words, a larger number of fat molecules are in reserve to fuel the same amount of muscle, so the fat has to wait longer before it’s used, which is why it tends to stick around. “As long as [the fat cells] don’t dysfunction, we’re fine.”
Fahumiya Samad from the Torrey Pines Institute for Molecular Studies agrees: “the current thinking in the field is that it’s not the amount of fat, but the dysfunction of the fat” that causes the health problems often associated with weight gain, she says.
The results of this study could help change the way we think about obesity, Gary Taubes argues via email. Taubes, author of Good Calories, Bad Calories and Why We Get Fat says, “it’s not that people eat too much and that makes them a little fat, and then they’re on the ‘slippery slope’ [to obesity]. It’s that the same amount of food consumed will lead to a greater or lesser degree of fat stored in the fat cells, depending on this [triglyceride breakdown] factor.” Taubes adds that this research may have identified “at least one factor that determines why some people gain fat easily and others stay lean easily.”
Dr. Samuel Bernard, one of the study’s primary authors, cautions that the origins of weight gain are still unclear. People can become overweight for a variety of reasons, he says, whether they are “genetically determined to have a slower [triglyceride] turnover, or lack exercise or have bad nutrition, but they eventually become fat, which triggers a slower [triglyceride] turnover rate” in their fat cells.
But by linking weight gain with fat cells that have problems processing fat, Bernard thinks his research has uncovered another clue that can help determine the root cause of why people get fat in the first place, and possibly develop new approaches to weight management.
A version of this article previously appeared in SCOPE on October 11, 2013