Try a vitamin B comlex, should help.
I feel so weak and tired, I've been on a fruitarian diet for just one day and my body isn't coping, is there anything i can take for maximum energy that won't make me add weight?
Personally, I think diets that don't give you a full balance of protein, energy and vitamins is always going to leave you feeling abnormal.
Only eating fruits, even for a few days, isn't a great idea - and I think you're feeling the effects of that. Biologically, humans aren't adapted to living on a fruitarian diet.
You can still lose weight while eating a more varied range of food. Fresh vegetables and fruits are great for energy and vitamins. Fish, nuts and seeds can give you the protein you need...
I would avoid simple carbs (soft drinks, sugar, syrup etc...) and also reducing the amount of starch/complex-carbs by avoiding bread, potatoes, rice and pasta.
You can replace them with dairy items, eggs, chicken, fish and meat.
Also avoid frying food where possible. Baking and boiling will mean less oil in your diet.
Try cod liver oil it's healthy and gives off multiple health benefits such as vitamin a. Also I might be wrong, but some people say milk is good for energising people, unless fruitarian diet also encompasses the requirements of being a vegan... if so I'm sorry for being clueless.