India has the highest malnutrition rates on the planet (and low life expectancies), and 1 billion Indians could not win a single Gold medal in the last Olympics (worst performer on a per capita basis). Even tiny countries with populations of only a few million can win multiple Gold medals. Yet a billion Indians could not.
It is entirely possible for a human to live only on vegetables, but that's not how we were designed. In evolutionary terms, human beings are omnivores. If your only food source is vegetables you'll get malnutrition.