“Adipose tissue is an underappreciated and misunderstood organ.” It’s with these words that Aaron M. Cypess, MD, PhD, begins his recent review published in the New England Journal of Medicine.
As obesity rates steadily rise, “the riskiest approach to human adipose tissue is to dismiss its importance,” he adds, especially because there has been “an explosive growth” in our understanding of white and brown adipose tissue over the past 5 to 10 years.
This news organization asked Dr. Cypess, a National Institutes of Health (NIH) scientist whose research focuses on brown fat, to discuss some of the main points in his review, titled, “Reassessing Human Adipose Tissue,” and clear up some misconceptions about fat.
You write that, for people who struggle to lose weight, “fat is often a source of misery, not marvel.” Why is fat a marvel?
When I started medical school in 1992, fat was just a thing that stored calories. You had to get it out of the way when you operated on the stomach or intestines. Now we know it’s not just one cell, it’s multiple types of cells, including immune cells and some blood cells. There’s cell turnover, and cells can get bigger or smaller, so it’s a dynamic tissue. It impacts the immune system and affects insulin sensitivity.
Why use the term “adipose tissue” and not just “fat”?
People think of fat cells and that’s it. However, adipose tissue (fat) has multiple cell types, and they each matter. There are adipocytes (fat cells) – which can be white, brown, beige, or pink – as well as immune cells, fibroblasts, blood vessels, and parts of nerve cells.
The main function of white adipose tissue is to store energy in the form of triglycerides. Brown adipose tissue consumes glucose and triglycerides, generating heat. Brown fat cells within depots of white fat are termed brite cells (a portmanteau of brown and white) or beige cells. Pink fat cells have been found in breast tissue in mice.
What do we now know about white fat and brown fat? Can brown fat change to white fat or vice versa?
White adipose tissue is commonly separated into visceral fat and subcutaneous fat, which have negative and neutral or positive metabolic effects, respectively. It is capable of more than doubling in mass and then returning to baseline.
White adipocyte-derived hormones include leptin, which is low in starvation, and adiponectin, which regulates glucose and lipid metabolism. White adipose tissue is essential for the proper function of the reproductive system, including secretion of hormones and lactation.
Brown adipose tissue protects newborns from cold as they develop the ability to shiver, and in adults it is found in depots in the neck, shoulders, posterior thorax, and abdomen. The amount of brown adipose tissue varies according to sex and lowers with increasing age and increasing body mass index.
There is much more white fat in the body than brown fat. It appears that activating brown fat leads to beneficial effects on metabolism, though we don’t know yet all the steps for how that happens.
In mice, you’ve got white fat depots and brown fat depots, and some brown fat can be found in the white fat.
With humans it’s much more complicated, and I’ve seen this in the operating room myself, and on slides. Where you find brown fat cells you also find a certain proportion of white fat cells, not an exclusive brown fat depot like you see in a mouse.
It is hotly debated right now whether brown fat can change to white fat and vice versa (transdifferentiation). The beige fat cells are supposed to be the kind that can shuttle between more white-like or brown-like. They can sometimes be white or sometimes brown. It can be very contentious in [scientific] papers and meetings.