To find out if someone has a problem with how their body uses food, doctors look for special chemical clues in the body. Here are some important ones:

1. Amino Acids - High levels of these can show problems like phenylketonuria (PKU).
2. Organic Acids - Some specific organic acids, like methylmalonic acid, can be found in conditions like methylmalonic acidemia.
3. Fatty Acids - Unusual patterns of fatty acids can appear in a condition called medium-chain acyl-CoA dehydrogenase deficiency.
4. Enzymatic Activity - If certain enzymes are not working well, it can help diagnose problems like galactosemia.

Finding these clues early on is very important for getting the right treatment.