system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how come I need protein, carbs and vitamins etc, when a horse can get all they need from hay and be fitter than me? Relatively speaking.
gpt
They can digest they bulk of plant material which we can not digest.  This allows them to extract more nutrients and calories from the cells of plants.

Your body can create the most proteins it needs, with a some exceptions.  It is more effective very often, to simply get proteins from where they exist already (like nuts, legumes, animals or even insects). 

The Horse has different needs and different exceptions.  Wild horses don't only feed on hay (if ever), and domesticated horses may have supplements added or shots given to them.