A healthy diet is one that gives the body essential nutrition.
Hopefully.  As long as you get the essential nutrition, like fluids and protein, you should see decent results no matter what.
I have quite a lot of energy, but I know some people just aren't as lucky. Is their a medical condition for being constantly tired? There is a condition called chronic fatigue syndrome in which people have long-term fatigue in a way that limits daily activity. That must be awful. I wonder what causes it. Depression perhaps? I don't think the cause is fully understood.  It could be genetic, could be biological, or it could be psychological. Well, I guess the medical profession had better get to the bottom of it. It must be destroying people's lives! I wouldn't know, but there is some evidence that cognitive behavioral therapy combined with increasing activity can help. That makes sense. If a person exercises it often brings ones energy levels up. There also hasn't been any evidence of any serious bad effects. That is good. A healthy diet, positive thinking and exercise are things we all need. Perhaps that's what's needed.
A healthy diet provides the body with essential nutrition: fluid, adequate amino acids from protein, essential fatty acids, vitamins, minerals, fibre and adequate calories.