Modelo de Valor Agregado Lineal mixto de dos etapas

Another approach to the question o f the desirability o f weight control has been to focus upon whether it is those individuals that would most benefit from weight control that are involved in it, and whether it is affecting individuals to whom weight loss may be harmful.

Certainly weight is a strong predictor o f weight control attempts such that weight control is most prevalent in the overweight. A study o f a representative sample of British adults reported that 61% o f overweight and 66% of obese women were trying to control their weight compared to 22% of normal weight women. (Wardle and Johnson in press). Amongst adolescents too, overweight girls are much more likely to be trying to control their weight than those who are underweight. The Health survey for England 1995-97: The Health o f Young People, reported that 71% of 13-15 year old girls whose BMI put them in the top weight for height quintile were trying to lose weight at the time of the survey, compared with just 2% of the lowest quintile, and 33% o f the middle quintile (Prescott-Clarke and Primatesta 1999).

Chapter 1 : The case for and against weight control

It has been argued that weight loss attempts in women who are not overweight cannot be justified for health reasons, and that they are therefore undesirable and can in some cases be dangerous for both physical and mental health. Amongst adolescents, whilst weight control and weight concern is most common amongst the overweight, many of those wishing to lose weight nonetheless fall into the normal weight category. Data from the National Health and Nutrition Examination Survey in the United States (Strauss 1999) showed that 52% o f 12 to 16-year-old girls who identified themselves as overweight were actually of normal weight. Ryan et al (1998) found that two out o f three o f the Dublin teenagers who reported wanting to be lighter were o f normal weight or underweight. Whilst some o f these girls perceived themselves to be overweight, others (27% of those wishing they were lighter) wanted to lose weight despite recognising that their weight was normal or low. This supports the idea that weight concern has probably penetrated too far into the adolescent sub-culture, affecting those whose current weight does not pose a risk to health. Nonetheless, the assumption that weight concern leads only to negative consequences in girls who are not overweight may not be justified. For many girls weight control efforts may lead to positive, healthy, dietary changes. The average Western diet is far removed from that recommended by nutritionists, to the extent that any individual adhering to the recommendations might^onsidered by many to have an unusual preoccupation with healthy eating, and be assumed to be ‘dieting’.

It is also possible that for some normal-weight girls, dietary restriction plays a part in preventing weight increase to unhealthy levels. The relationship between BMI and weight control practices is made particularly difficult to interpret due to the dynamic nature of the relationship between the two. Whilst it may be intuitive that girls o f higher BMI will engage in more weight control practices, to the extent that such practices are successful they act to reduce BMI. If it is the case that weight control practices can successfully play a role in maintaining a healthy body weight or reducing age-related weight gain, then it would be expected that weight control practices would be found in some women of normal weight, who were successfully maintaining a healthy weight. Where normal

Chapter 1 : The case for and against weight control

weight girls are using recommended weight control techniques to maintain their weight this may be acceptable. A US study of weight control practices in normal weight women (Biener and Heaton 1995) found that whilst 45% of white, and 25% o f black, female dieters were o f normal weight, the majority o f these (62%) used the recommended method o f healthy dietary modification combined with exercise, and a similar proportion reported that the main reason for their desire to lose weight was to improve their health and fitness. Although weight control in normal weight individuals may be undesirable if it leads to unhealthy weight reduction, to the extent that it maintains normal body weight and improves diet and exercise levels it can be argued that it confers benefits.

1.3.3.1 Dieting and Nutrition

One possible ill consequence of dieting in individuals who are not overweight is the danger of under-nutrition. Severely restricting dietary intake in childhood and adolescence can have serious effects on health, resulting in reductions in growth velocity, amenorrhea, diseases of micronutrient inadequacy, protein catabolism, and metabolic effects (Mallick 1983; Kirkley and Burge 1989). There is little evidence, however, either supporting or refuting the suggestion that these problems present a real risk for most adolescent dieters. In non-clinical populations, dieting is predominantly associated with overweight, suggesting that dieters are not experiencing extreme or prolonged caloric restriction (Wardle and Marsland 1990; Rosenblum and Lewis 1999; Neumark-Sztainer et al 1999). One study which examined dieting frequency and caloric intake in US adolescent girls (Field et al 1993) found no association between dieting and caloric intake o f pre and early adolescent girls, although a weak negative association was found in older adolescents. This research suggests that young dieters are not likely to be experiencing inappropriately low caloric levels in the diet, but also raises the question o f whether their attempts to lose weight are likely to have any positive effects on the weight o f those who would benefit from weight loss.

Chapter 1 : The case for and against weight control

With regard to micronutrients, young female dieters have been found to be at risk of failing to meet recommended daily intakes of calcium and iron; a deficit that is also common amongst non-dieting girls (Kirkley and Burge 1989). A study of the nutrient intake o f North American dieters (Neumark-Sztainer et al 2000) found that dieting adult women had generally healthier patterns o f nutrient consumption than non-dieters, consuming less fat and fewer calories, but higher calcium, vitamins A and C, fibre and finit and vegetables. Amongst adolescent girls similar results were seen with regard to fat and calories, but there were no significant differences between dieters and non-dieters in terms of micronutrients, although dieting girls did eat more vegetables. This seems to indicate that younger dieters are engaged in a form of dieting that does not provide as many health benefits as that practiced by adult dieters. Whilst some young dieters may be experiencing low levels o f specific micro-nutrients, in the absence of more research the balance of evidence does not support the view that the majority are under-nourished.