Some while ago I researched this subject for a detox website. I love fruit and vegetables (and meat before you ask) so it was interesting for me plus I got paid for it too.
In general fruitarians follow a diet of fruits, vegetables, nuts and seeds but not grain and definitely no animal products. Many won’t even eat a plant but just the fruit that naturally falls from the plant, bush or tree. Others include pulses and olive oil.
Apart from possible vitamin deficiencies, B12 for example, which cannot be obtained from fruits, most adults following this regime are deemed fit and healthy.
Some followers believe that fruitarianism started with Adam & Eve. And even Idi Amin joined in when in exile/retirement in Saudi Arabia, although it didn’t do him much good as he died of multiple organ failure in 2003.