There are numerous reasons for being underweight, many of which include underlying medical conditions. Otherwise, genetics, disease, malabsorption, disturbance of appetite, or a psychological issues can cause a low body weight - which is a significant health concern.

Some people report that they find it very difficult to gain weight.  There may be a host of reasons why they are underweight, and the most common factor linking overweight and underweight people is a lack of self-esteem and frustration with their body image.

Genetics and physiology

Some people are genetically predisposed to being thin, and are born with fewer fat cells.  Others grow rapidly.  They are unable to consume the amount of food required to maintain body processes, keep active and put on weight.

People who have a greater proportion of muscle tissue, or who are genetically lean, utilise more energy than others because their bodies have a higher metabolism.  This makes weight gain difficult as the body burns off energy before it can be stored.

Psychological

Stress, depression and simply not caring about food can lead to a reduced food intake.  The increased adrenaline levels present in the body whilst under stress leads to a greater amount of stored energy being used, and decreased fat synthesis.  This will lead to weight loss.  Stress caused by a hectic lifestyle may also be a cause, as the amount of time available to eat a balanced diet is reduced.

Anorexia nervosa is a psychological eating disorder in which there is a preoccupation with body weight leading to self-starvation.  Common amongst younger people, the disorder is diagnosed when BMI is below 20 kg/m2, with an extreme fear of gaining weight and an unhealthy body image.  In women, there will also be an absence of menstrual cycles. 

Although the condition is not unheard of in older people, there are often more understandable reasons for being underweight.

Depression, bereavement and other emotional situations can cause loss of appetite and motivation to look after oneself. 

Disease

There are certain diseases that can cause sudden unexplained weight loss.  Hyperthyroidism is a condition where the thyroid over-produces the hormones that regulate our metabolism.  This has an effect on the tissues in the body, and causes weight loss and tiredness among other symptoms.  Other conditions such as cancer, Crohns disease and allergies can also lead to weight loss.

People intuitively know when all is not well, and sudden weight loss should be seen as a reason to seek medical advice.

The effect of age

A diminished energy intake, resulting in extreme thinness can occur in older age due to a variety of reasons such as malabsorption and disturbance of appetite.  This results in under nutrition (more common in the over 65s), whereby it is not only calories that are deficient, but also essential nutrients such as vitamins and minerals.

Health risks

There is greater risk of nutritional deficiency as we age due to interrelated problems such as physical and mental illness, social isolation and poverty.  Such deficiencies are one of the prime health risks if a low body weight is the result of an inadequate diet.

The health concerns associated with a low body weight can be just as debilitating as if overweight.  Low body weight is an established risk factor for osteoporosis.  Thin women with a lack of body fat have a decreased production of oestrogen.  This hormone protects against bone density.  Men can also suffer from this disease  especially when underweight.

Extreme thinness can be an indicator that protein is being broken down to provide energy.  Loss of protein in this way affects the bodys ability to repair tissues.  Increased frailty and associated injuries can also be attributed to low body weight.

Nutrient deficiencies

When low body weight occurs due to inadequate food intake, nutrient deficiencies are relatively common, leading to a variety of problems.  A diminished immune response, increased vulnerability to infection and disease, and delayed wound healing can be linked to nutrient deficiencies.

There is a difference between nutrients and calories, and this needs to be understood in order to prevent the reduction of essential nutrients in our diets.  People who embark on slimming diet by reducing calorie intake often reduce nutrient intake without realising. 

An example of this is inadequate intake of calcium and iron due to the reduction of dairy products and meat.  A mixed diet, containing all the nutrients that the body requires is an essential part of healthy eating.

Under nutrition is caused by: decreased food intake decreased absorption and usage of nutrients increased metabolism.

 

Why does food intake decrease? Illness Impaired appetite/disinterest in food  depression, bereavement, stress Shortage of money  can only afford cheap, non nutritious food Oral and swallowing difficulties  poor dentition, reducing ability to eat Failing vision  difficulties in preparing meals Psychological disorders  anorexia nervosa and bulimia Lack of mobility  difficulties in shopping for food Abuse Late life paranoia Dementia

 

What causes a decrease in nutrient absorption? Medication Alcoholism Malabsorption Stones Chronic disease states Ageing

 

Why does metabolism increase? Hyperthyroidism

Weight gain

Weight gain can be much harder to achieve than weight loss.  The process often requires medical supervision.

As a guide, when low body weight is due to lack of food intake (not to disease or eating disorder), the aim is to try and enjoy food and gradually increase calorie content and amount consumed.

The principals of healthy eating should be followed, as well as making food enjoyable and interesting.  Three meals per day are recommended.  Snacks and increased portion sizes should also be considered.  Emphasis should be placed on energy dense foods, adding fat where possible.  Fat in moderate amounts, such as olive oil, is part of a healthy balanced diet. 

As long as plenty of fruit, vegetables, complex carbohydrates such as bread and pasta, and nutrient dense foods such as milk are included, weight gain and healthy eating is possible.

Exercise is a vital part of being healthy, whether under or overweight.  Resistance training such as weight training is ideal.  Exercise increases metabolism, stimulating the body to use energy effectively, and increases appetite. 

Exercise encourages muscle mass, and balances fat gain.  The relationship between these two factors is essential to a persons health and fitness.  To learn more about exercise, read the Activity & health topic.

20/05/2009