Human Feces

Several factors can cause that.

1. You might simply be eating less protein. You only need about 45 grams a day, which accounts for less than 400 calories. The rest of your diet can be anything that makes you happy and healthy.

2. You might be eating a lot of processed foods. Artificial coloring, and to a lesser extent natural dyes, often sail through the digestive tract without much attrition from chemical breakdown.

3. Other components of your diet are passing through without being totally digested. Even with that incredibly long small intestine, human digestion is remarkably inefficient. Some nutritionists claim that more than half the mass of complex nutrients we eat (anything bigger than a sugar molecule, which means pretty much everything else) is not digested. Individual results may vary greatly. Something as simple as not having enough bacterial culture in your intestines can exacerbate this -- which is easily corrected by adding yogurt to your diet. Still not all differences in individual digestion are easily explained. I eat about 3,500 calories a day, my life is not particularly active, yet I don't have a weight problem. (Don't envy me, nobody gets a perfect life and you wouldn't want to trade for some of my other health problems. Trust me on this.) Clearly a whole bunch of my food is not being digested and I have no idea why.

4. I'll say the following just because it would be irresponsible not to, but please don't overreact to it. Blood in your stomach will make your stool darker, even black. This can come from a fondness for raw or rare meat, or the blood soup that is a delicacy on some Chinese menus. Or it can come from bleeding ulcers. It happened to me so I had to give up aspirin, ibuprofen, and all non-steroid anti-imflammatory drugs (NSAID's). Now I get to take Darvon, which is a nice high, so I'm not complaining. If you have really dark stools, and I mean literally black, see your doctor right now. But what you've described doesn't fit this category.