Is rapamycin found in food, or can you get it naturally through your diet?

Rapamycin is not a nutrient. None of the studies reviewed describe rapamycin as something that can be obtained through eating or drinking. Rapamycin was originally a compound isolated from a bacterium found on Easter Island and is used exclusively as a pharmaceutical drug or research substance, not as a dietary component.

What food does do is influence the signalling pathway that rapamycin acts on. That pathway is called mTORC1, short for 'mechanistic target of rapamycin complex 1'. Leucine, an amino acid that occurs naturally in protein-rich foods such as meat, fish, eggs and dairy, activates this mTORC1 pathway. That is, however, precisely the opposite of what rapamycin does: rapamycin inhibits that pathway. Leucine is therefore not a natural version of rapamycin; functionally speaking, it works against it.

Long-chain fatty acids from food, such as those found in oily fish or vegetable oils, can also activate the mTORC1 pathway via receptors on cell membranes. Again, this is an indirect interaction with the same pathway, but there is no rapamycin in those fatty acids and the effect moves in the opposite direction to what rapamycin does.

Certain plant compounds, including resveratrol (in red grapes), quercetin (in onions and apples) and berberine (in various plants used in traditional Chinese medicine), are described in one abstract as substances that can dampen the mTOR pathway, similar in direction to rapamycin. However, the evidence for these substances is weak: the study in question describes associations, not proof that they function clinically as substitutes for rapamycin. There is no such thing as 'natural rapamycin through food'.