Chapter 24 WEIGHT LOSS
The major influences on body weight are caloric intake, intestinal absorption and utilisation. The purpose of this chapter is to outline an approach to the diagnosis and management of weight loss, particularly in adults.
In research studies, weight loss may be defined as a weight loss of >5% of usual body weight over a period of 6–12 months. Weight loss >10% is usually associated with protein–energy malnutrition while weight loss >20% implies severe protein–energy malnutrition and significant organ dysfunction. One quantitative measure of nutritional status is that of body mass index (BMI). This is defined as body weight in kg divided by height in metres squared (kg/m2). A BMI lower than 17 is consistent with undernutrition. Other anthropometric measures include arm circumference and triceps skin fold thickness, but these are affected, at least to some extent, by skin changes associated with ageing.
The weight loss that occurs with ageing is largely caused by a reduction in food intake. This has been attributed to a variety of factors including age-related losses in taste and smell, age-related decreases in opioid receptors and enhanced sensitivity to the satiating effects of cholecystokinin. Other factors may include changes in the secretion of leptin and sex hormones and age-related changes in intestinal absorption, sometimes mediated by changes in gastrointestinal motility. In the setting of acute and chronic disease, anorexia and weight loss can be caused by a variety of factors including ill health, weakness and side effects from medication. Some of these effects may be mediated by elevated levels of cytokines such as interleukins and tumour necrosis factor and by elevated levels of corticotrophin-releasing factor. At a practical level, at least 20% of inpatients in acute hospitals have a nutrient intake of <50% of their calculated maintenance energy requirements.
Potential causes of weight loss are strongly influenced by the age at presentation. For example, in young adults, unintentional weight loss is often due to psychiatric disorders such as depression, anorexia nervosa, bulimia, psychoses and substance abuse. In contrast, weight loss in the elderly is often due to multiple factors including social isolation, bereavement, dementia, depression, poor dentition and side effects from drugs. In middle age, there may be a greater contribution from cancer, alcoholism, endocrine disorders and chronic inflammatory disorders. Cancer is diagnosed in up to 25% of patients with weight loss over the age of 50 years. Some of the factors that contribute to weight loss are outlined below and are summarised in Figure 24.1.