Sweat stinks. It's just a fact of life, and probably something you've never thought much about. Interestingly enough, though, there are entire teams of scientists who study sweat and body odor. What those scientists have learned is quite enlightening, and not always what you'd expect. Keep reading for a few interesting tidbits about what makes sweat stink.
Low-carb diets have been all the rage for decades now. The names may change (Atkins, South Beach, Paleolithic, etc.), but the general gist behind these diets is to eat more protein (and sometimes fat) and less carbohydrates.
Low-carb dieters have long complained of stinky sweat, and many have theorized that this undesired side effect might be due to ketosis (a chemical change in the body that can accompany extremely-low-carb diets). However, scientists have discovered that increased levels of ammonia in the sweat may be to blame. In one study, men who had eaten a low-carb diet for three days had higher levels of ammonia in both their blood and sweat (measured after exercising), compared to men eating normally.
Some types of bacteria stink (or, more accurately, they produce stinky smells, since the actual bacteria themselves don't usually stink). Just ask anybody who's studied or worked in a microbiology lab. As you might suspect, some of those stink-producing bacteria live on the skin, thriving in warm, moist places like the armpits. Also, the armpits have apocrine glands, which produce proteins, fats, and various other compounds that can serve to "feed" the bacteria.
One of the components of sweat that contributes to its stink is androstenone, a steroid found in urine and armpit sweat. Interestingly, some people think androstenone smells terrible, some people think it smells good, and some people can't smell it at all. Scientists have discovered genes (and mutations in genes) that explain these differences.
So, if one person thinks you smell fine but another thinks you reek, it might be genetics to blame.