Humans Have Become the "Fat Primate"

Did you know that fat is necessary to feed our brains? Fat was useful to early humans, as it fed their growing and hungry brains, as well as protected them against starvation. Now, fat is needed because we have moved from the “fat primate” to, in many cases, the “obese primate.”

Most people don’t know that fat was necessary for the human brain development of early humans. From an evolutionary perspective, it makes sense that our bodies store more fat for our busy brains. In the past six to eight million years, since humans and chimps split, human brains have approximately tripled in size, while chimpanzee brains haven’t changed. Human brains use more energy than any other organ, which is why we need fat.

On the other hand, humans need fat because they are chubby. Scientists know that humans are chubbier than chimpanzees. Chimpanzees, being one of our closest animal relatives, have around 9 percent body fat, whereas a healthy range of body fat for humans is between 14 percent and 31 percent. Devi Swain-Lenz, a postdoctoral associate in biology at Duke University, and her colleagues have found that there are differences in how DNA is packaged inside the fat cell of humans versus that of chimpanzees. As a result, the researchers say this higher body fat percentage has decreased the human body’s ability to turn bad calorie-storing fat into good calorie-burning fat.

To get additional knowledge on how humans became the fat primate, a team led by Swain-Lenz and Duke biologist Greg Wray compared fat samples from humans, chimpanzees, and a more distantly related monkey species, Rhesus Macaques. Using a strategy called ATAC-seq, they scanned each species genome for differences in how their fat cell DNA is packaged. Normally, most DNA in a cell is condensed into coils and loops, tightly wound around proteins. This means that only certain DNA regions are loosely packed enough to be accessible to the cellular machinery that turns genes on and off.

There are two different types of fat: calorie-storing fat, which is the white fat, and the calorie-burning beige and brown fat. The white fat is what builds up around our waistline. Beige and brown fat are specialized fat cells that can burn calories rather than store them.

As a result, the researches identified nearly 780 DNA regions that can turn white fat cells to brown fat cells in chimps and macaques. The strands of DNA have become more bunched and “locked up for business” in humans, leading to fat storage, making humans fatter than chimps. The regions of the genome that help turn white fat to brown were inaccessible in humans but not chimps, which is another reason for the increase in fat storage. Swain-Lenz said, “We’ve lost some of the ability to shunt fat cells towards beige or brown fat, and we’re stuck down the white fat pathway.” It’s possible to activate the body’s limited brown fat by doing things like exposing people to cold temperatures, “But we need to work for it,” Swain-Lenz said.

In the developed world, our body’s ability to store fat has led to an obesity epidemic. Could scientists discover gene therapy for obesity? Researches are trying to figure out if boosting our body’s ability to convert white fat to beige or brown fat could make it easier to slim down. “Maybe we could figure out a group of genes that we need to turn on or off, but we’re still very far from that,” Swain-Lenz said. “I don’t think that it’s as simple as flipping a switch. If it were, we would have figured this out a long time ago.”

[Source: Science]

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