Before winding up this article, I really want to drive the point home: lower body fat is not better.* Not from a performance standpoint and not from a health standpoint.** Although it’s probably as related to energy balance as body fat per se, the point is that when folks get beyond a certain point of leanness, hormone levels are disrupted in both men and women (and a great deal of this is controlled by leptin, discussed in detail in other articles on the site).
The normal menstrual cycle in women may stop (this is called ammennorrhea), indicating a problem with estrogen production. This tends to cause bone loss which is a very serious problem.
In men not using drugs to maintain their hormone levels, testosterone can approach near-castrate levels as they reach the lower limits of body fat. Complaints of zero sex drive (and not being able to get it up even if the drive were there) are common among natural bodybuilders who get extremely lean.
And that’s just the tip of the iceberg. Thyroid, growth hormone, IGF-1, metabolic rate, and the immune system are all severely depressed under situations of extremely low body fat (genetic oddities excepted). Cortisol goes through the roof at extremely low body fat levels especially when a lot of training is being done.
This is all just part of a coordinated set of adaptations that occur with both starvation and dieting (dieting is just starvation on a smaller/slower scale) to try to keep you alive.* It’s frustrating but actually makes perfect sense, at least to your body.
A body fat of 5% in a man is likely occurring because there is no food. Your body can’t tell the difference between you starving it because you’re a crazy bodybuilder or you starving because there’s no food available ; it reacts the same way.
If there is no food, the last thing your body wants is for you to get your mate pregnant. Either there isn’t enough food to keep it alive or you’ll be dead before it’s born, and unable to fulfill your fatherly duties (watching TV, drinking beer, changing the oil and killing the occasional spider). So testosterone crashes to make it impossible in the first place. If you’re starving, chances are so is your mate so that’s a double whammy.
A woman dieting to 12% or lower body fat wouldn’t be able to bring a baby to term safely in the first place, so the body prevents it by shutting down the menstrual cycle. I should note that the reasons for the shutdown are actually more complex and some dieting women will lose their period at higher body fat percentages. It’s actually more an issue of energy balance than body fat per se but that’s beyond the scope of this article.
Some women look upon the loss of their period as a benefit, just one less monthly messy (sorry) hassle. But the bone loss and estrogen issue is not a joke, and can (will) cause problems down the road. Studies of female gymnasts and ballerinas are finding low bone densities similar to post-menopausal women. Certainly not the picture of a healthy athlete. And if the bone doesn’t develop during puberty, it may never develop at all.
For those truly obsessed with body image, body fat percentages of ~8 for men and 14-15% for women should be safely sustainable year round although it will require nearly fanatical devotion to daily diet and training (or having picked the right parents to start with). As long as the person is at least eating at maintenance, hormones shouldn’t be too mucked up.
The same probably holds for athletes (most claims of insanely low body fat levels like 2-3% in athletes being gross over estimations).* Without drugs, most athletes won’t perform well trying to compete at 5% body fat.* They certainly won’t train effectively at that level, if for no other reason than the caloric restriction needed to get to that low of a body fat level will hurt training performance and adaptations.
As I noted above, bodybuilders usually don’t have a choice, they have to be walking anatomy charts when they step on stage.* But with few exceptions, they don’t maintain that body fat level for any extended periods in the first place.
