I have a friend who wouldn't eat anything that contained any degree of fat. After several years of denying his body the proper nutrients his body basically started to devour muscle tissue instead.
He started out looking like someone who could pose for the cover of Men's health or a muscle magazine, since he was completely ripped. He wound up losing over 50 pounds and most of his muscle mass. And, he was so thin at one point that he looked like he had either been a prisoner of war or a chemo patient.
It took months and many doctors to figure out what the hell was wrong with him. Then they linked his health issues to bad nutrition.
His wife was just as bad. She was nine months pregnant and out power walking when her water broke.
There is something called over doing it. Some fat/carbs in your diet is a good thing.
I've actually read that Almonds are a very healthy snack because they contain a healthy balance of carbs and protein plus a good amount of fiber.