Voluntary and Involuntary Weight Loss

An intake of protein calories and/or any of the 39 essential nutrients that is less than the minimal required threshold leads to disruption of the balance between energy expenditure and nutrient requirements, resulting in various undernutrition syndromes and weight loss. Still, weight loss may be an ambiguous finding, as the presence of oedema may mask depletions in fat or lean body mass, or, conversely, a gain in fat and lean body mass may not be apparent in the presence of massive diuresis. Undernutrition may be primary, due to an inadequate dietary supply of essential nutrients, or secondary, in which nutrient intake is adequate but disease or excessive utilisation prevents adequate absorption or metabolism. Although primary and secondary mechanisms often reinforce each other, the respective pathways to weight loss are different. The secondary form takes place as a consequence of a hypercatabolic state, leading to progressive 'self-cannibalism' [2, 3]. When examining a patient with weight loss, a physician should ascertain whether it is voluntary or unintentional.

In the majority of individuals, approximately 50% of the energy introduced by the consumption of food is utilised for maintenance of basal metabolism, such as body temperature; 40% is utilised for physical activity (50% in the extremely vigorous activity performed by athletes); and 10% is used for dietary thermogenesis, i.e. the energy required for digestion, absorption, and metabolism of food.

Weight loss may be a result of decreased food intake, malabsorption, loss of calories, or increasing energy expenditure. A deficit of 3500 Kcal correlates with a loss of 0.45 kg of body fat, but water gained or lost must also be taken into account. During the first few days of restricted food intake, most weight loss is attributable to water loss. In a historical experiment, Brozek [4] studied the kinetics of weight loss in young healthy volunteers on a hypocaloric diet (1010 Kcal daily) over a 24-day period (Table 1). On days 1-3 , 70% of the weight lost by the volunteers consisted of water, and 1 kg lost correlated with a deficit of 2596 Kcal. On days 11-13, weight losses were due to a reduction of fat tissue, and 1 kg lost correlated with a deficit of 7043 Kcal. On days 22-24, weight losses were due almost exclusively to reductions of fat and muscle tissue, and 1 kg lost correlated with a deficit of 8700 Kcal.

Involuntary weight loss (IWL) is defined as a clinically significant and progressive loss of at least 4.5 kg or > 5% of the usual body weight over a period of 24-48 weeks [5]. A weight loss > 10-20% represents the condition known as protein-energy malnutrition (PEM). A more correct reference than usual body weight to measure

Table 1. Weight loss of young volunteers consuming a diet of 1010 Kcal/day during a period of 24 days


Days 1-3

Days 11-13

Days 22-24

Losses kg/day 0.80

Caloric deficit (Kcal/kg loss) 2596

0.23 19 69 12 7043

85 15 8700

normality or a gain or loss of weight is ideal body weight (IBW). IBW is defined as the weight that corresponds to the maximal expectancy of life. IWL results acutely from impaired absorption and utilisation of energy and protein, or gradually from a prolonged deprivation of nutrients (starvation). A rapid and frequently dramatic weight loss, including loss of total body water, fat, lean body and skeletal mass is associated with traumatic injury, infections, burns, major surgery, pulmonary diseases, and cancer [6, 7]. IWL and PEM may develop together, increasing morbidity and the mortality rates of underlying diseases.

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