These days, being thin sure seems important: we associate thin with "sexy", "beautiful", "successful". And in sports, there is a practical reason to be lean; to be a good athlete, you need to be able to move yourself quickly and powerfully, and muscle is what enables that. But what good is fat? It seems like dead weight, it seems like something that would only hold you back. So the thinner the better, right?
Not exactly. It's true that fat doesn't have a lot of practical purposes for athletes (some notable exceptions are to improve buoyancy for swimmers and to help insulate cross-country skiers from the cold). So it is tempting to try to cut out as much fat as you can in the name of a faster time in the next race. But you can only safely do that up to a point.
We still need some fat for all of our systems to run smoothly. For example, many vitamins are fat-soluble, which means they can only be digested with the help of fats. The body also uses fat to trap diseases and toxins while it fights them off. And fat plays a vital role in the skin, hair, nervous system, and cell function. So we need some of it around, but how much?
It depends drastically on gender. Men need to be at least 2 percent body fat, and women need to be at least 10 percent. The variation comes from the disparate needs of the male and female reproductive systems. If you want to get a feel for where that line is, someone at these lower limits would be incredibly skinny. At 2 percent you would be able to see every muscle in stark relief as well as most bones (see the picture below). 10 percent looks a little softer, but still very angular and lean.
It would be near impossible to get below 2 percent voluntarily, so men don't have much to worry about in this department. We can get basically as thin as we want to be for whatever our particular goals are. But women need to be more careful, because their lower limit is much more attainable. Elite female runners will bump up against that line, and the first thing they notice is problems with their menstrual cycle. But as with everything in fitness, there are variations from person to person. There are probably some women who, naturally and without any special diet or exercise, would be below 10 percent body fat. If that is you, I wouldn't worry that you are too thin, because if your body is settling into that composition on it's own, then it is doing it for a reason and it is probably fine.
To find out your exact composition, the best way is with a Tanita bathroom scale. This is a simple but sophisticated machine that uses electrical conductivity and tells you your complete composition-fat percentage, muscle mass, bone density, and water content. Maybe this is more than you wanted to know about yourself, but if you are a serious athlete, this information can help you to fine-tune your training. I don't have one, but I've read about them and seen them, and they are really amazing.
As difficult as losing weight is for most, you would think that there wouldn't be much risk of being underweight. But last year when I was getting ready for a race, I made a concerted effort to trim down to get closer to my original race weight. And what I found was that losing weight can be very addictive. It comes from this positive feedback loop; you set a goal, you achieve it, and you feel great about your accomplishment, so you want to set another goal. And once you get in this mode, it can be hard to realize where to stop.
The phrase, "I need to be thinner..." is incomplete; it needs a purpose, it needs a reason: "I need to be thinner in order to..." There are lots of good reasons to lose weight (and some poor reasons), so as you move forward make sure you keep that target in mind so you will know when you've done enough.
Bonus pic: This is a famous photo of the duel between Mark Allen and Dave Scott during the 1989 Ironman Traithlon. Look at Dave Scott's legs!