We all go on a "diet" once in a while; some of us do it for short periods of time to reach a specific goal, such as fat loss, improving gut health or making weight for competition. A "diet" is usually seen as a temporary state of under-eating for people who are generally healthy.
However, for some people under-eating is a way of life, and can take any of the following forms:
• Calorie restriction
• Extreme eating restrains
• Eliminating entire food groups (like fats or carbohydrates)
• Seeing certain foods as evil or dangerous, making the person who eats them evil or dangerous too
• Eating just enough for a sedentary lifestyle, but overexercising at the same time
Under-eating—just like overeating—for long periods of time can have serious effects, as consistent research findings have proven. Regardless of your weight, you will experience the same adverse effects when you restrict food. Our caveman brain has evolved to help us survive periods of famine that our ancestors had to cope with periodically; this is a part of our brain that controls metabolism, the heart/heart rate, temperature regulation, digestion and hormones in the body. Starvation can mean many things—not only insufficient calories. Starvation can cause mineral, protein and fat deficiencies which are needed for our brain to properly function as well.
The brain's response differs from person to person (which is why personalized plans are key), but common ways in which the brain tries to conserve energy and nutrients will cause very significant feelings like chilliness, anxiety and restlessness, poor skin and nails, slowed digestion and digestive pains, slowed hear rate, hormonal imbalance and slower metabolism. This has a huge effect on our emotional health as well.
The effects of sustained under-eating can occur in people of every weight, but greatly increase in those who are underweight; Specific effects of under-eating are universal, and they'll change every aspect of personality. Your personality, thoughts and feelings are directly affected by what you eat, and under-eating is dangerous. All the terrible symptoms of under-eating—including self-hatred and feeling fat—will begin to disappear once you are able to start eating properly and regain a healthy body weight and composition.
The National Centre for Eating Disorders, UK