How useful is fat _really_?
January 6, 2007 2:48 PM   Subscribe

Fat: Insulation, padding, and storage. But are there any drawbacks to having low bodyfat that I might encounter? I'm lowering bodyfat - currently in the realm of fitness but perhaps going to touch on the realm of vanity. There seem to be no drawbacks currently, any exceptions? How much would the benefit be and how much fat involved? Are there optimum amounts (such as a point beyond which increased fat doesn't increase how long you could survive without food)? To what extent (any?) could fat from previous rich eating help nutritionally balance an unbalanced diet later on? To what extent could it not?

Bonus question: Evolutionarily speaking, how much fat is the optimum amount for the ideal adult human male - maximising benefits while minimising costs. (Yeah, I imagine it varies by environment, so include whatever details of that are relevant)
posted by -harlequin- to Health & Fitness (24 answers total) 3 users marked this as a favorite
I seem to recall having heard in high school health class (potential grain of salt warning) that "optimum body fat percentage" is 20% for men and 30-35% for women. I could be mistaken, as could my high school health teacher.
posted by DoctorFedora at 3:15 PM on January 6, 2007

You might be interested to learn that fat in the human body is a lot more than just insulation, padding, and storage. The cells in the body that store fat are powerful communication devices that integrate a whole bunch of metabolic signals and regulate a variety of processes in the body. There was a neat issue of one of the Nature journals in the last month or so that contained several reviews on the subject.
posted by rxrfrx at 3:27 PM on January 6, 2007

To what extent (any?) could fat from previous rich eating help nutritionally balance an unbalanced diet later on? To what extent could it not?

If you're talking about dietary fat, some nutrients benefit from some dietary fats to absorb into the body. But the rest of your post seems to focus on body fat. Your stored body fat will not provide the benefits of dietary fats as you burn off the body fat. Your body will convert it into sugars which are the only way it can be used for energy.

As far as reducing your body fat, the risks are lower for men. Women need to be careful not to go too far below the recommended minimum body fat % because they risk losing their ability to menstruate or conceive children. However in most cases, this is not permanent, and menstrual cycles would go back to normal once a normal body fat percentage is achieved.
posted by tastybrains at 3:28 PM on January 6, 2007

Also, as quoted from

Your ideal weight and fat-lean ratio varies considerably for men and women and by age, but the minimum percent bodyfat considered safe for good health is 5% for males and 12% for females. The average adult body fat is closer to 15%-18% for men and 22%-25% for women.

This makes more sense than DoctorFedora's gym teacher's estimates because 30% body fat is fairly high.
posted by tastybrains at 3:31 PM on January 6, 2007

But are there any drawbacks to having low bodyfat that I might encounter?

Hell yes. Fat is not just inert padding, it's an actively secreting tissue with multiple biological functions. This is why obesity is related to so many diseases, too much of these adipokines leads to inflammation and other negative effects. But at the same time some level of adipokines is necessary and lowering them beyond a certain point causes other problems. The most obvious example is that women become infertile once body fat drops too low (men also show disorded hormone levels once body fat reduces too much). I can't remember off hand what the all the problems, but active inflammation and disordered satiety is going to be in there.

So yes, there are most definitely optimum amounts of body fat and having it become too low is a bad thing. As far as I can recall about 20% sounds right for an adult male, but it's not as high as you might think as fat is stored away in all kinds of places.

To what extent (any?) could fat from previous rich eating help nutritionally balance an unbalanced diet later on? To what extent could it not?

I don't really understand this bit. If your body fat gets too low then it isn't going to be any better if you ate more fat in the past, your fat content is still too low. The state of your body now and your current diet are going to be the most relevant things to your health, barring any damage carried over from previous bad habits which is yet to heal.
posted by shelleycat at 3:32 PM on January 6, 2007

Hmm, actually maybe the 20% number was for women. I never took much notice of the values for men.
posted by shelleycat at 3:33 PM on January 6, 2007

According to the UMich sports medicine page, normal body fat is 10%-20% for men (6-10% for athletes) and 15%-25% for women (10-15% for athletes). This site goes into the numbers in more depth, citing numbers from the American Council on Exercise, classifying % of fat on a scale from essential to obese.

Anedoctally, I can tell you that not having a lot of body fat and therefore not having a lot of insulation or padding can be frustrating. I get cold easily and have sharp elbows and hipbones.

Medically, you can check any site discussing anorexia to tell you all the lovely things that happen to your body when you starve it. Not all of those health problems are literally caused by not having body fat. I'm not saying you're necessarily anorexic -- it's just the easiest way to find the medical ramifications of excess weight loss.
posted by desuetude at 3:42 PM on January 6, 2007

Fat is also padding. The soles of your feet will get sore and your butt will get sore, which is just not worth it. I noticed this start to kick in at around 9% (I am however female).
posted by goo at 3:54 PM on January 6, 2007

If you drink alcohol then one drawback to loosing body fat (and I'm not talking about anything crazy here, just going from out-of-shape to in-shape) is that you're tolerance goes down quite drastically. I know this through bitter experience...
posted by ob at 4:31 PM on January 6, 2007

I'll second desuetude. The padding part is helpful.

As a scrawny bloke, I have a hard time with hard seats due to my boney arse. I once had a scrawny girlfriend, and the sex wasn't great because we were both so boney. I also seem to feel the cold more than others, but I don't mind this, as I deal with heat better than others, which is more regularly an issue where I live.
posted by pompomtom at 4:39 PM on January 6, 2007

Response by poster: The cells in the body that store fat are powerful communication devices that integrate a whole bunch of metabolic signals and regulate a variety of processes in the body.

In this case, the question would be how little fat is needed to ensure the body has sufficient cells for these systems. Presumably these systems, not insulation/padding/storage, would be the bases of minimum-considered-safe values like the 5% for males and 12% for females tastybrains linked too.

(I haven't checked out the links yet - just clarifying stuff)

To what extent (any?) could fat from previous rich eating help nutritionally balance an unbalanced diet later on? To what extent could it not?

I don't really understand this bit.

For example, if you've been living the rich diet of an American, and so have decent bodyfat, but suddenly your diet changes for the worse - even though you're getting plenty of food, it's not nutritionally complete (maybe something like Katrina happens, or maybe you go to college and can choose you own food for the first time, whatever :), to what extent can the nutritional deficiencies in the diet be compensated for by your body making withdrawals from it's fat-bank? From the sound of what tastybrains said, sugars are the only nutrition your body can obtain from fat, so the answer sounds like "Not Much", but I'm not really familiar with how much can be done with sugars.
posted by -harlequin- at 4:43 PM on January 6, 2007

To what extent (any?) could fat from previous rich eating help nutritionally balance an unbalanced diet later on?

Define "nutrition." My understanding is that fat only stores energy for you - you won't be getting any protein, fiber, or vitamins from burning fat. If you're going to radically reduce your caloric intake, you should still eat well; get plenty of fiber and take a vitamin supplement if necessary.
posted by rkent at 5:10 PM on January 6, 2007

Response by poster: I should also clarify that I'm not seeking advice on fat, diet, methods, or weightloss. I'm just curious about this thing that we all learned in school is important to the human animal, but which I have never really experienced much use for myself.

Basically, what does it take for all that theoretical usefulness to become real-world noticeable etc.
posted by -harlequin- at 5:25 PM on January 6, 2007

rkent has got it right. Vitamins and minerals cannot be synthesized from fat or sugars. These things are essential to your body's functioning; you mayen't even be able to metabolize your fat stores if you're deficient in some areas.

Luckily, lots of vitamin and minerals reside in low calorie foods like vegetables and... vegetables. Fruits would be better if agriculture hadn't bred the bejeezus out of them for increased sugar and decreased fiber and skin. Most of the vitamins and minerals are going to be in the skins.

As for vitamin supplements: be careful you're not taking too much. Most are water soluble, so you'll just pee it out if you get too much. Some are fat soluble, so they'll stick around and might cause problems... but only in very high doses.

Also: if you're going to take vitamin supplements, make sure you get something that actually dissolves in your stomach. Do some research.

An analogy: fat is like gas for a car. Essential to make things go. Vitamins and minerals are like the brake fluid, oil, radiator fluid, transmission fluid, etc... if you run out of these, the car slowly loses it's ability to utilize fuel and perform other essential functions. It won't go without fuel, certainly, but it'll also mess up if it lacks enough of the other stuff.
posted by Mister Cheese at 5:33 PM on January 6, 2007

Remember this study?
posted by LoriFLA at 6:03 PM on January 6, 2007

Ah, thanks for the clarification.

I should've gone back to the post itself instead of focusing on the comments. This makes it simpler: fat stores can only compensate for a decrease in caloric intake, not a decrease in vitamins and minerals. I haven't heard of any studies that separates out calories from vitamins/minerals.

Upon further thinking: it's really complicated. If you're not getting enough glucose and you run out of it, you might not be able to utilize your fat due to the the interactions of the glucose-fatty acid cycle. Some theory points to the idea that fat needs glucose to be utilized... others say that it's the other way around.

Or you can think about this: if you're not getting enough complete proteins in your diet, and start a deficit of the essential proteins (those that cannot be made from other proteins), then you're going to start lacking the building blocks necessary for the very enzymes that help metabolize fat.

I think it'd be really difficult to determine an optimum amount of fat evolutionarily. First we'd have to determine: optimum for what? Presumably hunter gatherer societies. How do we test that? Look at modern hunter gatherer societies... oh, wait, we run into a problem there: how can we be sure that they represent what the past was like, when humanity was evolving. And what is optimum? Average lifespan was, what, 45? Do a google scholar search for "hunter gatherer body fat" and you find all sort of interesting things. But it's not applicable to a society where food is readily available because you won't get in to situations where you're not going to get enough fat.

Once again: what is optimum? It will definitely vary from individual to individual. Presumably enough extra fat to be able to comfortably exercise. Athletes typically exercise more than sedentary people and have lower body fat percentages... but they are also eating a whole lot of fat to compensate. So I guess you could go as low as athletes safely. To quote Exercise Physiology again: "successful male distance runners have almost less than 9% fat." But that's considering that you benefit from lighter mass; a football player might have 15% because they need more padding. And you need to take in as much to maintain that level of body fat anyways. For sedentary people, then, the optimum might be in the range I quoted at the beginning: enough to avoid the detriments of obesity and to provide fuel for extended physical activity when necessary.

I find it more useful to try to apply these concepts to the individual, then, since metabolism really varies a lot depending on what one is doing and what one wants to achieve in life. For example, your optimum might be, "the capability to sexually reproduce." In that case, you need the minimum amount of fat necessary for the synthesis and maintenance of your reproductive organs aaand live long enough to actually get a mate at the very least. You also may need enough fat to be visually pleasing to a female who wants to copulate with you (largely cultural). Usually males don't have the burden of needing their fat to help feed their offspring, so they can get away with less. But what, then, if you lived in a society where you needed to hunt to provide food? You'd need more fuel reserves to go on those long hunts, but you'd need to be light enough to actually be able to hunt down fast animals. Maybe a male marathon runner's percentage would be ideal, but then you've got to consider that male marathon runner's can get food after they run, and a hunter gather may not.

Errr..... I'll quite rambling, since I don't have a definitive answer.
posted by Mister Cheese at 6:29 PM on January 6, 2007

it's going to bother me that there is an apostrophe in "runner's"

I'll take this as an opportunity to ramble on some more:

Optimal fat percentage will also vary by age, as LoriFLA also points out. I'll add: notice how babies have a lot more fat? They're growing! Need lots of energy! Physiologically, you'll need just enough fat to keep functioning and not fall apart. But environmentally, you will also have to consider stuff like: is the fat level I have attractive? Do I have enough to prevent injury? At least that's what we've got to worry about in modern times.

Evolutionarily, our bodies evolved to store as many calories as possible. Our body just grabs on to the stuff and stores it because that's what our ancestors needed: we didn't evolve to accommodate an optimal amount of fat, we evolved to get as much fat as we can in anticipation of the next period of time without food. Evolutionarily, it's "as much fat as this body can store from the stuff we've caught." Which is why there are problems with obesity today. Body fat storage wasn't really selected for in the direction of minimum but maximum because that maximum was going to be used up shortly during periods of no food. The minimum, then, is as much as necessary for the functioning of the body + what is needed for day to day physical activity. That too will vary from individual.
posted by Mister Cheese at 6:40 PM on January 6, 2007

Response by poster: You'd need more fuel reserves to go on those long hunts

A long hunt before you get to eat - even one lasting days - is hardly going to make a dent in someone's bodyfat, be it 9% or 29%, let alone take it down below 5% (or whatever someone's magic number is) to where it's negatively affecting things, so (assuming an all-year-round source of prey) fat wouldn't seem to be of help to the hunter. Unless it's the case that having more fat makes it easier for the body to metabolise fat (like how lungs compensate for the very limited rate of gas exchange through the membrane by having a large surface area of membrane).

Is metabolising fat a slow process that bottlenecks the body's need for energy, such that having extra fat gives a fatter pipe to that energy? Hmmm, athletes having the highest energy needs and low bodyfat suggests against a bottleneck.
posted by -harlequin- at 6:49 PM on January 6, 2007

Hmmm... yeah, you're right about a long hunt without getting to eat won't really put a dent in body fat. You might be right about that fat metabolism is a slower process, bottlenecking the body's need for energy, but at higher levels of physical activity the body switches over to carbohydrates so... no problem there. By "having extra fat gives a fatter pipe to that energy" mean "having more fat available means the body is going to be able to utilize more of it simply because there is more to go around?" The body adapts via training/exercise to utilize more of the free fatty acids that are released into the blood stream; the pipe is more the ability to take up and use fat than supply of fat. Of course, if there isn't any fat to use, then you've got a problem, but you're right, prolonged going without food won't put too much of a dent in your supply. I guess the bare minimum would be safely above where even a tiny little dent would cause problems. And who knows what the magic number is for that! People vary. And it probably only applies to people who exercise very intensely... fat is necessary to stave off for a little while the utilization of carbohydrates (mid level activity) so that it's available for high level activity. It's also there after the high intensity exercise is finished so one isn't completely without energy at the end of it.

So I guess you don't need more fat than an athlete needs if you can keep yourself fed with enough carbohydrates. If you don't have a super high metabolism. If you don't mind getting cold or having a sore butt. If your friends don't keep nagging that you've got to skinny and you need to eat more food and they're like, "your ass isn't as round as it used to be," and you're sad because that's the only thing you had going for you.
posted by Mister Cheese at 7:23 PM on January 6, 2007

The optimal amount of fat is certainly going to vary for each person and during each eprson's life, all other biological parameters do. And the health parameters affected by body fat content can be measured for each person and tracked to ensure you're within the optimal bracket. Actual monitoring is going to be the only way you can know for sure because genralisations never entirely hold for each person, but general fat and weight brackets can be determined and pretty well supported by the evidence, so these are what is given in lieu of direct specific medical advice. The values quoted by desuetude above match the most closely the ones I've seen in medical papers or discussed in classes.

Annoyingly it's been a while and I can't remember the details of the biologiocal pathways affected by adipokines (and am on holiday so don't have my usual resources at hand). Hormone levels are important, for men as well as women, and sugar metabolism, sateity and overall inflammation are definitely involved, and I'm pretty sure immune function was in there too (although that could be linked to the inflammatory state). The idea of fat as an actively secreting tissue is fairly recent though with most work done in the last five to ten years.

Just because men don't have the obvious effect of loss of menstruation doesn't mean they're any less at risk (although it takes a lower body fat level to trigger problems in men), and keep in mind that a professional level marathon runner is not normal and their lack of fat does have health implications (the female ones usually stop menstruating, for example). The atheletic lifestyle gives other health benefits and it does ameliorate the problem, but putting the body into an active inflammatory state for an extended period of time will have a negative payoff eventually.

Overall the qurstion of what is potimal is a reasonably difficult one. But the reasons for having fat stored in your bosy are fairly straight forward overall (we're still working on the details, like of much in biology), aren't all related to energy storage and are definitely real.
posted by shelleycat at 7:31 PM on January 6, 2007

Eep. Let's just blame all the typing mistakes on me being on holiday, OK?
posted by shelleycat at 7:34 PM on January 6, 2007

anecdotally, when my guy friends' bodyfat gets below a certain amount (pretty low- like 7%) they get sick easier and stay sick longer. And if their illness is something dehydrates them or keeps them from eating, it can get serious quickly.

Some of this could be correllation and not cause- they both are pretty athletic, and maybe it's the stress of that that's causing problems. One is very careful to eat well (and sufficiently) and the other isn't.
posted by small_ruminant at 11:35 PM on January 6, 2007

New Scientist had an enlightening article a year or two ago about how body fat was more than just storage, it should be considered an organ. More recently they had articles about how being a little overweight could actually be a health benefit, as you were more likely to survive a sickness and do well in older age.
posted by tomble at 11:41 PM on January 6, 2007

About the "being overweight helps you survive a sickness or old age" comments: keep in mind that these studies are done on a general population that eats a pretty terrible diet. In that population, it is likely that it is beneficial to be a little overweight. That does not mean that this is a universal truth. It is likely that being at a normal weight, is the most benificial for people who eat a healthy diet, full of green vegetables and other nutrient rich foods. Usually, centenarians are not overweight.

Also: it baffles me, but some of the studies that prove that being a little overweight is healthier, do not take into account that people who do not feel well/are ill are more likely to be underweight. It is the same with the studies about how good alcohol is for you: they often do not take into account that people who are ill are less likely to drink alcohol.

As a response to the original question: what finally prompted me to lose weight (I lost 100 pounds) was the realization that fat was not just padding, but that especially belly fat was an active risk for cancer, amongst other things. I used to think that all the risks of being obese were because obese people tend to live an unhealthier lifestyle. It was quite shocking to learn that those fat cells themselves were causing cancer and diabetes.
posted by davar at 2:20 AM on January 7, 2007

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