Education Resource from the Society for Endocrinology
Dr Susan Jebb
MRC Human Nutrition Research, Cambridge
Summer School 11-14 July 2006
The Møller Centre, Storeys Way, Cambridge, UK
Dietary modification is the cornerstone for effective weight loss, whether used alone, alongside other lifestyle changes including increases in physical activity or as an adjunct to medical or surgical interventions. Successful treatment of obesity requires a sustained cut in energy intake to drive weight loss, followed by a life-long period of weight control, when energy intake matches energy needs. In a society where food is plentiful, attractive, available 24/7 and at relatively lower cost than ever before, this is a challenge for the whole population. However the difficulties are likely to be particularly acute in those with an established weight problem who have already demonstrated a genetic, metabolic or behavioural susceptibility to obesity.
The main aims for the dietary management of obesity are firstly, to induce an energy deficit that can be sustained for sufficient period to allow at least a 10% weight loss. A 500 to 1000 kcal daily deficit will lead to weight loss of 0.5 to 1 kg/week. Secondly, to achieve a dietary composition which is associated with good health, independent of body weight. Concern about obesity is strongly linked to the increased risk of chronic diseases such as cardiovascular disease, type 2 diabetes and some cancers. The WHO paper on Diet, Nutrition and Chronic Disease demonstrates the many synergies between dietary strategies to prevent each of these diseases (1). Sound nutritional management of obesity should emphasise the benefits of decreasing saturated fat and salt and increasing the proportion of unrefined carbohydrate and fruit and vegetables. For children it is also essential to ensure the diet continues to provide sufficient nutrients for growth and development.
There are three clear strategies to decrease energy intake. Firstly to decrease the energy density for the diet, secondly to decrease portion sizes of energy dense foods and thirdly to develop a structured meal pattern.
Energy density: In controlled experimental studies a high dietary energy density has been strongly linked to increased energy intake, a phenomena known as passive over-consumption (2). Recently a prospective study has shown an association between the habitual energy density of the diet and the development of overweight in children (3). A similar study in adults found no association in the population as a whole but a significant link in subjects who were overweight (4). In practical terms reducing the energy density of the diet involves reductions in dietary fat, since this contains more than twice as many calories gram for gram compared to protein or carbohydrate. Decreasing added sugars and increasing the proportion of fruit and vegetables can also contribute to decreases in overall dietary energy density.
There is one important exception to the “energy density rule”, which relates to drinks. Liquid calories have low satiating properties relative to foods of similar energy content but higher viscosity (5). As a result liquid calories, whether as sugar-rich cordials or carbonates, or alcohol, tend to supplement rather than substitute for other items (6). A large prospective epidemiological study has shown significantly greater weight gain among individuals consuming the largest volume of soft drinks, but this finding is not wholly consistent (7). Nonetheless the growing body of evidence suggests that sugar-rich drinks are a specific risk factor for obesity and their high energy content and frequently low content of micronutrients makes them an obvious focus for strategies to maximise the nutritional density of the diet. In practical terms, patients should be advised to choose water or artificially sweetened beverages. Fruit juice with its naturally high sugar content should also be limited and preferably consumed well diluted.
Portion size: By definition, larger portions are associated with greater energy intake and in recent years portion size has also emerged as a critical risk factor for obesity. Larger portions are associated with increased consumption but no additional satiation and minimal compensation at subsequent eating episodes (8) (9). In today’s supersize culture, larger portion sizes are strongly habituated and establishing new portion size habits for energy dense foods, more closely tuned to energy needs, is critical.
Obese patients need careful education to adjust habitual portion sizes. Satiety can be maintained, despite reductions in energy dense foods, by parallel increases in low energy dense foods, such as vegetables or fibre-rich carbohydrates (10). Behaviour modification techniques can be very useful in establishing these new eating habits.
Structured meals: The relationship between eating frequency (or snacking) and the development of obesity is complex and poorly understood (11). However, there is good evidence that a structured eating program can increase weight loss (12). Currently there is a shift away from generalised dietary guidelines to more formal eating plans which help patients to learn a new way of shopping, cooking and eating.
A greater degree of external control can be achieved using commercial meal replacement products. Studies in subjects that choose to use these products show significantly improved weight control (13) and randomised control trials have also shown clear benefits (14). However despite the increasing range of products available only a small proportion of patients will choose to use them long-term. Very low calorie diets (VLCDs) represent the most extreme form of structured nutritional management. These diets provide around 600 calories per day and contain the full daily recommended intake of micronutrients according to strict guidelines (15). This is a much greater energy deficit than can be achieved by conventional foodstuffs and weight losses are correspondingly greater (16). Similar results can be obtained with a diet based on three pints of semi-skimmed milk daily with additional vitamin and mineral supplementation (17). However within the period of acute weight loss there is little opportunity for the re-education of dietary habits. A control phase involving the gradual reintroduction of food is recommended to avoid rapid weight regain. However with on-going support in well motivated subjects, this can be a very effective form of weight loss.
‘Quick-fixes’: A wide variety of other diets are continually promoted to the public focusing on one or other food group, identifying a new ‘super food’, or a dieting ‘trick’ to stimulate weight loss. All are based around diets providing about 1000-1500 kcal/d and none have been shown to offer any specific advantages over and above the degree of energy restriction. The commercial success of recent bestsellers such as high-protein or low glycaemic index (GI) diets rests on the potential for these foods to be satiating and thus curb energy intake. Although there is some evidence for this in short-term experimental studies, the impact on the longer-term control of energy intake is less clear. Hunger and satiety are only part of the overall drive to eat and lack of hunger does not always translate into an absence of eating. These diets tend to be associated with initially greater weight losses, either due to satiation or the placebo effect of a novel strategy, but after 6-12 months, weight loss is similar to conventional dieting techniques (18) (19) (20).
Ultimately the success of any nutritional intervention to treat obesity depends on adherence to the low calorie regimen. This can be enhanced by regular ongoing support and this probably explains the relative success of the commercial weight loss sector, with an established network of community based peer support (21). Unsurprisingly different approaches may be more or less successful for different people (22). The art of weight management is to match the diet to the characteristics to the individual patient, but this has been poorly studied. Instead more research is focusing on general behavioural strategies which motivate and support people to make changes to their lifestyle (23). Levels of awareness of the need to cut calories to lose weight is high but the greatest barrier appears to lay in the implementation of this knowledge. The value of cognitive-behavioural therapy alongside conventional dietary management is increasing recognised (24).
Weight loss is difficult but long-term weight-loss maintenance is frequently the most challenging aspect of the nutritional management of obesity (25). Many people have the motivation to make short-term changes, but developing long-lasting new food habits is very difficult. Unfortunately there is negligible clinical support for obese patients at this time, despite evidence to show improved weight-loss maintenance with on-going professional support. There is some hope that the universal prevention strategy being developed to tackle obesity in relation to both diet and physical activity will provide some assistance. This includes reformulation initiatives to cut saturated fat, sugar and salt, innovation to offer healthier options, restrictions on the promotion of foods, especially to children and improvements in nutritional labelling. . However, the reality is that the impact of these measures may be insufficient for recent weight losers who are at high risk of weight regain. Dietary habits of “successful slimmers” are poorly understood, but one long-term study in the USA has highlighted a number of dietary behaviours adopted by this group which may enhance weight loss maintenance (26). These include low fat foods, proportionally more carbohydrates, smaller portion sizes, restrictions on the intake of energy dense foods, regular meals (especially breakfast) and increases in fruit and vegetable intake. These strategies broadly reflect the principals outlined for weight loss; reductions in energy density, portion control and regular eating habits.
The opinions expressed in this paper are those of the speaker and do not necessarily reflect the views of the Society
Revised:
23-Aug-2006