The Effects Of Under-Eating

Many people who want to change their weight under-eat for a while. This is commonly known as “dieting”. Under-eating for a short while in someone who is broadly a healthy weight will do little damage, but for some people under-eating becomes a way of life.

What is Under-Eating

Many people who want to change their weight under-eat for a while. This is commonly known as “dieting”.  Under-eating for a short while in someone who is broadly a healthy weight does little damage, but for some people under-eating becomes a way of life. Under-eating can take many forms;

  • Calorie restriction to keep your weight under control
  • Severe eating restraint to keep weight under strict control
  • Eliminating specific food groups such as all fat or all carbohydrate because of health fears, or fear of putting on weight
  • Clean eating programmes that make certain foods evil or dangerous. Clean eating advocates often heap moral opprobrium on people who do eat these foods which means that if you eat a forbidden food, you aren’t just eating a bad food, you are also a bad person. Forbidden foods among clean eaters might include meat, carbs, dairy food and “white flour”
  • Under-eating, as with someone with bulimia who is afraid that if they were to start eating, they would go out of control
  • Extreme under-eating in someone who has an unhealthy compulsion to reach or attain a very low body weight as in anorexia nervosa
  • Even overexercising, without eating enough for your activity

Both overeating and under-eating for a long period of time have serious effects. Knowledge about the effects of under-eating and underweight have come from many sources, such as studies of the effects of famine and concentration camp victims, as well as research studies into the effects of food restraint in animals and humans. Consistent findings have emerged. This means that whatever reasons you have for restricting food, you would experience the same adverse effects no matter what you weigh. In her book Sick Enough, The Medical Complications of Under-Eating, Dr Gaudiani describes how the cave-person brain works to help us live. This is the part of our brain that runs aspects of our physiology which is out of our control; our metabolism, heart, temperature regulation, heart rate, digestion and hormones. This part of our brain has evolved to help us survive common famine conditions that our ancestors had to cope with. Starvation can mean a lot of things, not just having insufficient calories. Starvation can include deficiencies in minerals, protein and the fats which our brain uses to function properly. Most of us get our needs met over time, we don’t have to eat a perfect diet every day because much of what sustains us is held in reserve. But the stores can run dry in just a few days. This will make the cave-brain kick into action to save us. The cave-brain response differs from one person to another. But the common ways that the brain tries to conserve energy and vital nutrients will cause very predictable physical sensations such as chilliness, restlessness, cold hands and feet, poor skin, slowed digestion and digestive pains, slowed heart rate, reduction in essential hormones and poor metabolism. This has a big effect on our emotional well-being, how we feel and how we think.  

  • The Psychological Effects: Feelings & Moods

    Depression:  Mood is badly affected by under-eating.  Many of the nutrients in food affect the emotional centres of our brain and reducing these nutrients or affecting their balance has an adverse effect on mood. Under-eaters generally feel depressed, low, and prone to be easily irritated or enraged. Panic:  Very low weight under-eaters may feel spaced out a lot of the time and not in touch with their feelings. But panic attacks are a common symptom which is often associated with panic about weight gain. Panic symptoms are not just emotional; they commonly arise if the body’s stress systems are not getting the nutrients that they need to keep us balanced and able to cope with life. Panic attacks subside only when weight becomes normal for your age and height. Obsessions and Compulsions:  One of the most prominent changes of under-eating is heightened obsessiveness.  This refers to the tendency to be inflexible and to carry out rigid routines.  One may feel compelled to walk a certain distance each day and some people may become very anxious about keeping things clean and tidy. This may interfere with the ability to be spontaneous and relaxed. The obsessiveness is often striking when it comes to eating and exercise.  Extreme under-eaters find they must eat in a particular way. Eating can become a mini ceremony in which food has to be arranged on a place or eaten in a particular order. These rituals can include cutting food into very small pieces and taking tiny bites. Some people become obsessive-compulsive to such a degree that they could be described as suffering from a starvation-induced OCD.  Hoarding objects – including bags of food – is common among people who just don’t eat enough. Withdrawal & Looking Inward: Under-eating can make you feel better in the short term especially if people draw attention to your willpower or weight loss. But a person will become prone to withdrawal and a great deal of self focus if under-eating continues and becomes a way of life. Some people with the eating disorder anorexia nervosa have a very poor social life but a very vivid inner life, which is expressed by writing stories or poetry. Such writings are usually about the emotional pain of the disorder, but sufferers are closed from the experiences of life outside. This again can be explained by the effects of undernourishment in the brain; also, the tendency to avoid the company of other people is encouraged by low mood and over focusing on all the rituals which must be done to keep hunger at bay. Loss of Sexual Feelings:  A loss of sexual appetite is expected and is due to hormonal changes. Feeling Fat:  The phenomenon of feeling fat or bigger than you are is common among under-eaters whether they have an eating disorder or not, and predictably this leads to more restraint. We do not fully understand WHY this happens and experts believe that the experience of fatness is really a sensing of all the buried feelings which have led to the need to under-eat in the first place. Fat feelings increase with weight loss if a person is already underweight, and thus eating more, rather than striving for even more weight loss is the only solution to the problems of “feeling huge”.

  • Thinking & Consciousness

    The Psychology of DietingThinking is affected by consistent under-eating. This is not surprising since the brain requires a great deal of energy to “compute”. One effect of under-eating for a while is that thinking becomes inflexible so that it can become difficult to set priorities and switch easily from one task to another. Concentration:  is always impaired, although not everyone is aware of this because they force themselves to focus on the task in hand.  Indeed, under-eaters often describe themselves as more alert and purposeful, but this cannot be sustained for long. Constant thinking about food and weight, which is a side effect of under-eating, can interfere with the ability to attend to other things. Obsession about Food and Weight:  One who under-eats for any reason starts thinking more about food and about their weight if they are also sensitive about what they weigh. This may lead someone to become fanatically interested in cooking, cooking for other people, recipes, books about nutrition, and they even dream about food and eating. Sometimes they give up old interests and other hobbies. Depth of Thinking:  Among people who under-eat, consciousness may contract. The best way to explain this is that very little seems to matter, in some cases all you might think about is how to get or avoid the next meal, how to control your weight. Or how to ensure that your food is “right” for you. This makes it very hard for someone to give attention to other things.  A narrow range of consciousness will lead in many cases to social inadequacies such as a lack of empathy and difficulty in sharing experiences of connection and joy. Hearing a Voice:  One comment effect of under-eating is that many people start to hear a Voice which tells them to eat even less. This Voice may also tell them that they are greedy if they even think of eating more or eating a food which could cause weight gain. Sadly, if there is extreme under-eating, the Voice threatens to punish the person who thinks it might be a good idea to try to eat more. Irrational Thinking:  The effects of poor nourishment on the brain may lead to difficulty in making good sense of day to day impressions and experiences and this in turn will result in unhelpful interpretations of everyday events. If someone is critical, for example, this may be interpreted as that they “have it in for you” whereas the ability to think rationally may lead you to the conclusion that they are simply having a bad day. Do you have a list of good foods and bad foods or safe foods and unsafe foods? Perhaps you feel instantly fat and worthless even if you even think of eating a forbidden food, or horrible if you have eaten a small amount of chocolate?  These are irrational thoughts cause the brain to play tricks on your experience and keep you trapped in an unhappy relationship with food.

  • Effects on Behaviour

    National Centre For Eating Disorders - helping with eating disorder treatment since 1985Changes in feeling and thinking because of under-eating is going to have a great impact on personal behaviour and relationships in ways described above.  People who have under-eaten for a long time, come to think that this is part of their personality and they are not aware that how they think, feel and behave is only the effect of their eating habits. In other words, the real self is being “masked.” Cravings: The strongest effect of under-eating on behaviour is caused by powerful cravings, usually for foods rich in sugar and fat. Some people respond to these cravings by doing all sorts of things to keep their mouth and appetite distracted, such as exercising excessively, smoking, drinking alcohol or even sucking stones. For other people, their control of cravings is undermined by binge eating, which in turn will lead to more efforts at restraint and in some cases harmful acts such as purging, and more cravings down the line. A great deal of research exists to prove that food cravings are in fact biologically driven because of under-eating/low weight and will not resolve unless the person is able to eat a diet which is nutritionally dense, sufficient in calories. Cravings will also persist until someone is a healthy weight. Restlessness: Under-eating makes people restless and this accounts for the busyness and high activity levels we can see in a person with anorexia. We know that starving rats run on on an activity wheel much more than those who are well fed.  

  • Physical Effects

    Under-eating at any weight has a marked effect on physical health. There is a common mis-perception that under-eating prolongs life, and this is reinforced by evidence that consistent overeating does indeed reduce life span. The exact effects of under-eating depend on the nature and the extent of the diet and the degree of weight loss.  In no order of importance; Heart, Circulation and Temperature: The heart is a muscle which can be eroded by extreme under-eating and thus becomes weaker. Blood pressure may fall to dangerous levels and pulse rate slows up. Sluggish circulation can lead to ulcers on the legs and feelings of extreme cold. But the most worrying effect of a weakened heart would be if arrhythmia occurred (irregular beating). Sex Hormones and Infertility: To protect more important life processes, sex hormone production is interrupted. Sexual feelings decrease, and the signs of puberty in females and males disappear, such as menstruation and nocturnal emissions respectively.  Normal functioning may be delayed even after a return to normal eating and weight. Bones: Hormonal and nutritional changes have a profoundly disturbing effect on bone growth and density. The years of puberty are the time when bones grow and strengthen. Should they fail to do so at this time they will never recover. The result in later life will be osteoporosis, stooping and a high risk of fracture. Digestion: The digestive tract in under-eaters slows right down and as a result food moves slowly through it and feels uncomfortable. This explains the heightened sensitivity to feelings of fullness and bloating, which is misleading.  In extreme conditions the stomach and gut begin to hurt which leads some people to think that they have food allergies, which is usually untrue. Skin and Hair: The effects of under-eating are variable from one person to the other. Skin can become dry and show signs of early ageing. It may turn orange in the very low weight or may be covered in very fine fuzz.  Some people find that their hair becomes thin. Sleep and Rest:  Under-eaters find it hard to sleep and may wake early with a sense of restlessness which drives them to go out and exercise. Low Blood sugar (hypoglycaemia): Blood glucose is the fuel which drives most metabolic processes including the activity of our muscles together with the day to day activity of the brain. Because blood glucose is so vital to life, our cave-brain does much to keep blood sugar levels stable. If blood sugar falls too low, this balance can be achieved by releasing glucose stores from the liver, plus breaking down muscle tissue and even (if sugar stores are very depleted) by cannibalising vital organs.   Hypoglycaemia can occur in people who are very low weight, not eating enough calories and who have also depleted their glucose stores in their muscles and liver. If they cannot breakdown muscle fast enough to restore glucose in their blood, or if they don’t have enough muscle left their heart may stop beating.  Low blood sugar is the most common precursor to death in anorexia and may occur during the night when blood sugar naturally falls.


The effects above are the result of sustained under-eating and can occur in someone of any weight, but if weight is also very low the effects of under-eating are greatly increased. Under-eating has specific effects which are universal, and which change every aspect of personality.  Some distressing symptoms when a person tries to eat more are directly caused by under-eating. Example: being bloated is caused by under-eating and is not usually a sign that you have eaten too much. Your personality, thoughts and feelings are directly affected by what you eat. The effects of under-eating are dangerous. Under-eating may make you feel special and stronger, but this is the eating disorder telling you that this is so, and this eating disorder also makes you unhappy and tired. Under-eating will keep someone locked in to the eating problem by making you more obsessed and preoccupied by food and more irrational about how you really look, more inflexible and more easily bloated. All these horrible symptoms – including self-hatred and feeling fat, will begin to recede if you are able to start eating and regain a healthier body weight. If weight is already low, you may need some expert support to help you manage this change and keep you safe.   Adapted from Fairburn, CG, Cognitive Behaviour Therapy & Eating Disorders Guildford Press 2008 and Sick Enough, A Guide to the Medical Complications of Eating Disorders. J. Gaudiani. Routledge 2018