Q: I'm wondering what's the healthy fat percentage for both guys and girls. I've measured my fat percentage using some electric device (i know you said it's not very reliable) but it measured 10.1% of fats in me and i'm a guy. I can see 4 not very defined abs but the thing that puzzles me the most is that sometimes it's more defined than other days, what is the occurrence for this?

A: These things definitely vary from person to person, but in general 10% body fat is considered the cut-off for having a six-pack. So if I had to guess I'd say that your body fat is probably slightly higher than 10%, maybe closer to 12% -- but as I said different people measure different so you may well actually be at 10%. The healthy body fat range is very wide and as far as I know there's no single guideline, but something like 8-20% for men and 15-30% for women is generally considered healthy. Body fat levels inevitably creep higher with old age as our bodies are unable to maintain the muscle mass of younger years.

As far as to why sometimes your abs are defined and sometimes not, it's very simple -- the more water weight your body carries the less defined your abs (or any other muscle group) will be.

As I mention in my http://www.mikesfitness.com/content/how-to-lose-weight article all of us always carry significant amount of water weight in our intestines and tissues. This is natural and healthy. But the actual amount of water weight we carry varies constantly, depending on how much and what and when we drink and eat and eliminate bodily wastes, as well as how much we sweat. Eat more carbs, more sodium, and drink more liquids and you'll carry a lot more water weight for the next day or so. Go on a low-calorie liquid diet, consume diuretics such as alcohol or caffeine, engage in physical activity that leads to sweating and your water weight go down. And of course in serious cases of dehydration your water weight can fall below healthy levels and even lead to death!

Usually most people have least water weight in them right after waking up and urination and bowel movement and before eating or drinking anything. I recommend this time to weigh yourself and keep track of your body fat (if that's your goal) because at this time your water weight is most consistent from day to day. Then as the day goes on, different food and liquid intakes and different levels of physical activity and sweating lead to very different (but generally higher than in the morning) water weight levels by the evening.

So your abs are likely most defined early in the morning and get less and less defined as the day goes on. There could certainly be exceptions to that, such as days when your perform heavy physical activity and sweat a lot but don't eat or drink a lot. But the bottom line is what controls your ab (and other muscle) definition from day to day is the amount of water weight you retain. Over longer periods of time the actual amount of body fat of course determines how you look but when looking at the way your definition changes during, say, 24 hours it's all about the water weight.

Just keep in mind that that you should NOT intentionally dehydrate yourself just for the sake of slightly better definition -- it is extremely unhealthy and in the long run will cause all sorts of health problems.