Bettas are the fish equivalent of a rugged cowboy in an Old West movie: they look like they could handle anything that comes their way with that mean-looking overbite. But hunger saps them of their energy and will to do anything but float around. Luckily, their hunger is rarely legitimate.
Bettas are tricksters when it comes to meal time. You can drop in three pellets of food, and your colorful friend will eat them all. Drop in another three, and he'll eat those too. This will go on and on, until your betta makes himself sick by eating too much. He'll act like he's always hungry and willing to eat. If you feed him food specifically meant for bettas and do so daily according to the manufacturer's suggested amount, he's not going to be in danger of becoming malnourished, as long as he eats it.
Your betta isn't trying to play a bit of reverse psychology on you with that angry appearance -- he likes tearing into meat way more than scarfing down a bit of plant material. It's one of the reasons plants are usually left alone in a betta's tank. But if it comes down to starving or eating a bit of green, your betta's going to choose the latter and start picking apart your plants. If you're feeding him betta food regularly, he shouldn't resort to eating plants, even though bettas are notoriously picky eaters.
If your betta seems a bit thinner than usual or is starting to look like a skeleton, you've got yourself a little fish who's either starving or sick. Bettas don't suddenly lose weight if they haven't eaten for a day or two. It takes a week or more of fasting before you'll notice your friend is starting to shed the pounds, or grams in a betta's case.
Lack of Energy
Bettas have a busy schedule that consists of swimming around their aquarium, checking to make sure everything's in order and looking at anyone who passes by. Floating in one place is not on the agenda, except when they're tired. If you notice your betta lethargic most of the time, and especially when you stand in front of his tank, he's either sick or hungry.
Feeding your little swimmer betta-specific food will prevent hunger from malnourishment, but he may have the munchies even if you drop in his food regularly. Bettas need to live in water that's between 75 and 86 degrees Fahrenheit. Anything colder or hotter will lead to stress, and your betta's body can start to shutdown. So if his water temperature is off, he won't eat even if the food is up for grabs. Illness can also lead to malnourishment, from visible parasites to a fungal infection of the mouth. Lots of medication for common illnesses exists, usually in the form of a liquid solution that you add to the tank.