People whose diet is badly out of balance are said to be malnourished - malnourished people can be fat or thin or unhealthy in another way
Eating too much:

excess carbohydrate or fat in the diet can lead to obesity
obesity is a common disorder in developed countries - its defined as being : 20% or more over maximum recommended body mass
hormonal problems can lead to obesity although the usual cause is a bad diet, overeating and a lack of exercise.
health problems that can arise as a result of obesity include : arthritis (inflammation of the joints), type 2 diabetes (inability to control blood sugar level), high blood pressure and heart disease, it is also a risk factor for some kinds of cancer.
too much saturated fat in your diet can increase your blood cholesterol level
eating too much salt can cause high blood pressure and heart problems