Your gut is roughly 40 percent smaller than it should be for a primate your size, and your brain is roughly three times bigger.
Those two numbers are the same.
Aiello and Wheeler laid it out in 1995 and called it the expensive tissue hypothesis. Brain tissue and gut tissue are both metabolically ruinous, costing far more per gram than muscle or bone. A body cannot run both at full size. Something has to give.
Gorillas made their choice. Enormous fermentation gut, modest brain, fourteen hours a day chewing to break even.
We made the opposite one. The gut shrank by nearly the same amount the brain grew, and the freed-up energy went north into the skull.
But a small gut only works if the food arriving in it is dense enough that you do not need a barrel to process it. You cannot shrink your digestive system and then live on leaves.
The trade only balances on fatty animal food.
You are running a brain that your digestive system can no longer feed on plants, which is a bold configuration to hand to a species and then advise it to eat more salad.