Betta Splendens will thrive if fed on a healthy diet. Unfortunately for us however, than they be quite fussy when it comes to feeding. One thing you should remember though, is that they prefer live food over dried/frozen food; although this shouldn't be fed all of the time as they may become accustomed to live food which can be expensive and messy.
Due to Bettas fussy diet, they often fall ill to constipation - This, however, can be quickly treated by feeding it a cooked pea. This will, in theory, treat the constipation quickly and efficiently. Also reduce the amount of food you feed your Siamese fighter for a week - But don't worry, Bettas can remain healthy for a week without food, so long as it has been fed well before hand. On the other hand you could prevent your Betta from getting constipated in the first place by keeping food portions small and refraining from using dried flake foods as much as possible.
In their natural habitat, Siamese fighting fish feed mainly on meaty foods such as insects and insect larvae. In an aquarium condition, they can be fed live and frozen brine shrimp, daphnia and occasionally bloodworm. Frozen and live foods provide the same nutrition content however your Betta will probably be more thankful of live food as it means he can actually swim round and try and catch his food rather than it just sinking to the bottom of the tank and rotting.
If you would rather feed your Betta Splenden dry food, then you should only use specially formulated Betta pellets such as Hikari Betta. These pellets provide your fighting fish with all the necessary nutrients for it to remain healthy. I personally would recommend that if you insist on using dry food - that you at least feed your Betta live food every few weeks, otherwise it may become bloated.
Feeding your Betta Splenden a good diet will reward you in the long run as your fish will become colourful, strong and most importantly - healthy.