While being thin can often be healthy, being underweight can be a problem if it's the result of poor nutrition, if you're pregnant, or have other health problems. So, if you're underweight, see your doctor or dietitian for evaluation. Together, you can plan how to reach your ideal weight. Someone who is thin but not healthy could be what we call “skinny fat.” While they may not be overweight, their diet consists mainly of junk food, fast food and processed foods and lacks the nutrients of a diet rich in fruits, vegetables and healthy fats.
Your muscles are weak and gradually weaken because they don't get stronger with exercise. A “thin” person may not need to count calories or worry about the amount of food they eat. However, regardless of size, it's about the quality of the food. Cardiovascular and mental health benefits are extremely important regardless of your weight, Basbaum shares.
On the other hand, being thin alone doesn't guarantee that you're healthy. A person can be thin and still have a high risk of getting sick. These people are referred to by terms such as thin on the outside but fat on the inside (TOFI) and metabolically obese but of normal weight (MONW). They also tend to have a number of risk factors for diabetes and heart disease, and are in poor condition.
This may be because they have more fat around their waist. Taken together, this puts them at greater risk of heart disease and premature death.