cell
Explanation
The comic shows two men at a table. One, a bald muscular man, smugly dismisses the other's protein shakes: "Oh you like protein shakes? That's cute." When asked what he eats every day, he answers: "A single gigantic bird-cell." The caption reads: "The main advantage of the ostrich-yolk diet is talking about the ostrich-yolk diet."
The joke works on two levels. First, it is a commentary on diet culture and the one-upmanship that pervades fitness communities. People who follow unusual or extreme diets are often stereotyped as being more interested in telling everyone about their diet than in the actual nutritional benefits. The bald man's smug condescension is a perfect caricature of this behavior.
Second, there is a biology joke at the core. An egg yolk is technically a single cell -- one of the largest cells in the biological world. An ostrich egg yolk is the largest single cell produced by any living animal. So "a single gigantic bird-cell" is a technically accurate but deliberately pretentious way of saying "I eat an ostrich egg yolk." The humor comes from reframing a mundane food item in the most impressively scientific-sounding language possible, which is exactly the kind of rhetorical trick diet enthusiasts use to make their regimen sound more extreme and impressive than it actually is.