Fat is an endocrine organ. Like any other organ, it secretes hormones and other bioactive compounds that affect our physiology and determine our health.
Subcutaneous fat sits just below your skin. It's the most conspicuous and least aesthetically-pleasing fat, comprising love handles and big droopy bellies, saggy arms and flabby necks, but it's less actively harmful than many other types of body fat. Subcutaneous fat is the primary secretor of leptin, a strong regulator of appetite and metabolism, and of adiponectin, a marker for metabolic health with potentially anti-atherosclerotic effects.
Gluteofemoral fat is lower body fat, specifically the stuff that sits on your butt, hips, and thighs. In women, its presence indicates (and may even determine) good metabolic health. And it may not just be a signal for health, but an actor. Fat depots on the butt and hips actively secrete greater amounts of palmitoleic acid (PDF), a fatty acid with insulin-sensitizing effects. Gluteofemoral fat contains a greater proportion of omega-3 fatty acids, which are used to construct baby brains.
Visceral fat lies inside the abdominal cavity. It surrounds and envelops the organs. Contrasted with subcutaneous fat, visceral fat releases far less leptin and adiponectin. Instead, it secretes large amounts of IL-6, an inflammatory cytokine strongly correlated with systemic inflammation.
Intrahepatic fat is fat inside the liver. More than any other type of fat, intrahepatic fat is strongly associated with the metabolic complications of obesity.
Epicardial fat is visceral fat that surrounds the heart. If you've ever gotten a cow heart wrapped in hard yellowish fat, that's epicardial fat. Large amounts of epicardial fat are associated with obesity, diabetes, and hypertension. And while epicardial fat appears to be primarily a signal of metabolic disturbances, it also exerts direct effects on heart function and releases inflammatory molecules that affect surrounding tissues.
Intermuscular fat lies between muscles. The less you use a muscle, the more it atrophies and the more fat will replace it. You may have seen the MRI of two thigh cross sections—one from a lifelong athlete and one from an age-matched couch potato. The athlete's leg is a dense circle of bone surrounded by several inches of lean muscle in each direction followed by a small layer of fat. The sedentary leg is smaller circle of bone surrounded by a mishmash of marbled meat with several inches of thick white fat. Delicious and tender when seared, I'd imagine, but terrible for the person's health and ability to function.
Intramuscular fat lies inside the muscles. Kobe beef steaks have lots of intramuscular fat. It's the marbling, the presence of fat between muscle fibers. If you actually utilize it, intramuscular fat provides a nice source of energy for the muscles. Then again, if you were using your muscles in the first place it'd be tough to accumulate much intramuscular fat. The quickest way to get rid of intramuscular fat is with low-level aerobic activity. Staying under 75% of max heart rate will keep you burning predominately fat, and exercising while on a ketogenic diet is an even better way to do it; nutritional ketosis increases exercise-induced intramuscular fat oxidation 20-fold.
Comment: For more information on the health benefits of a Ketogenic diet, see: The Ketogenic Diet - An Overview
Brown fat isn't really fat. It is, but it isn't. Like muscle, it's highly metabolically active. We use it to generate heat (thus burning energy) in response to cold exposure. Babies have a ton of brown fat, since they can't shiver to stay warm, and until recently researchers assumed adults didn't have much at all. Now we know that's wrong. Cold plunges, swimming in cool water, and even going outside in short sleeves and shorts in cold weather can all stimulate the formation of brown fat in adults (I wouldn't advise dunking your newborn in an ice bath for the health benefits). Those jerk babies aren't the only ones enjoying that sweet, sweet brown fat after all.
Best of all, "training" for brown fat—in one study, exposing yourself to cool weather (60°F) for just 2 hours a day for six weeks while wearing light clothing—can increase energy expenditure and reduce overall body fatness. That's not even a big dose of cold. Forgetting your jacket at home is probably enough.
Comment: For more information on brown fat and cold therapy, see:
Different populations have different body fat distribution patterns. Let's look at a few:
South Asians (Indians, Pakistanis, Bangladeshis) have smaller subcutaneous "reservoirs" than whites, so a larger proportion of weight gain in this population diverts to visceral—and more dangerous—fat stores. Dr. Ron Sinha wrote about this (and still does on his blog) in the fantastic South Asian Health Solution.
Compared to Australian whites, Japanese men have more body fat for a given BMI.
Black Americans have less visceral fat and more subcutaneous fat for a given BMI than white Americans (PDF). Oddly, this doesn't translate to a lower risk of diabetes.
Australian Aborigines have more trunk fat and less limb fat for a given BMI than Australians of European descent. Aborigine women have higher waist circumferences and waist:hip ratios than Aussie European women for a given BMI.
Men and women carry fat differently. I mentioned that briefly above in the section on gluteofemoral fat, and I covered the differences extensively in a previous post. Go read that now. In short, men are more likely to store fat on the trunk and around the waist, leading to the fat-guy-with-stick-legs syndrome. Women tend to store fat in the butt, thighs, and hips. Upon reaching menopause, women stop producing as much estrogen and begin storing more fat in the waist and abdomen.
The eternal question persists: is it cause or signal? Risk factor or actor? For most of these types of fat, the answer is probably "both." Fatty liver indicates metabolic dysfunction, but it can also impair the liver's functions and lead to insulin resistance and eventually diabetes. People with lots of brown fat may be lean and healthy because they spend a lot of time being active in cold weather, but the brown fat also increases caloric expenditure. Large amounts of intramuscular and epicardial fat indicate a sedentary lifestyle, which is damaging in its own right, but the fat secretes inflammatory compounds with real biological effects.
Teasing apart which link along the chain of causality is to blame is probably impossible. That doesn't mean we can't make a few safe recommendations.
Women shouldn't stress out about a little butt, hip, and thigh fat. It's likely a good sign.
Belly fat is bad. Fat around your heart is bad. Fat in your liver is bad. Subcutaneous fat looks bad and is hardest to burn but might not be too unhealthy. Losing weight will reduce all of it.
To target belly fat, intense training works best. The resultant spike in catecholamines will preferentially target visceral fat. Just be sure to get enough sleep, rest, and recovery, as chronic stress and under-recovery tends to chronically elevate the catecholamines and make belly fat more stubborn.
To target liver fat, limit sugar, eat lots of choline (from yolks and liver), and practice high intensity interval training. When you drink alcohol, make sure you protect your liver with saturated fat (beef, cocoa, coconut fat all protect against alcohol-induced fatty liver).
To target intramuscular fat, exercise in a low-carb or ketogenic state. Nothing too intense is required. A long, reasonably intense hike (lots of hills) on an empty stomach twice a week might do the trick.
To target epicardial fat, exercise. Both intense and moderate-intensity training seem to work.
To get more brown fat, forget your jacket. Heck, burn it. Just don't stand too close to the fire.
If it sounds confusing, it shouldn't. The basics apply. Recommendations haven't changed. I just find all the different types of fat incredibly interesting. Don't you?
Thanks for reading, everyone. Take care and be sure to let me know what you think about all this.
Reader Comments
to our Newsletter