Malnutrition doesn't have a number on a scale. People of all sizes and backgrounds can be malnourished and not even know it. Food insecurity and obesity affect a majority of the population, meaning more people may be malnourished than they realize. Malnourishment is a real issue, and it's not an effective way to live or diet. You must have balanced meals every day to fully nourish your body and keep all your organs strong. If not, you're at risk of several severe health problems.
If you're not getting enough nourishment, your brain will start to suffer. Healthy fats are necessary for good nutrition, as human brains are mostly made of fat. When people diet, they may try to avoid foods with a lot of fat in them. While cutting out bad fats might be a good move, eliminating the good fats is a major mistake.