Your brain can run on ketones made from fat & your liver can produce all the sugar you need from protien. Exogenous sugar almost always produces elevated bloodsugar levels that elicits an emergency hormone response. Your pancreas naturally produces homones that help you produce the sugar you need & break down stored fat. But when you consume sugar your body has to work to produce & store fat rather than break it down. You can't burn fat when you are making it. If you have become acclimated to using fat for energy then the transition from burning fat you have consumed to fat you have stored is fairly seamless, so energy levels are stable & it's easy to either lose weight or remain lean.

A lot of the sugar in fruit (fructose) can't even be used by your cells & has to be converted to either glucose or fat. Fructose, like alcohol, leads to cirrhosis of the liver.

In my experience, eating steak & eggs every day has improved my health & made it far easier to stay lean & strong. And vegans or fruitarians tend to be frail & unhealthy. Steve Jobs died of pancreatic cancer because of his fruitarian diet.

The form & bioavailability of nutrients in plant products is just dramatically inferior. There are no essential carbs, only essential fats & protiens.