Blinded by science: The National Heart Foundation of Australia's position statement on dietary fat and overweight/obesity.

(see pages 174-176)

There can be no doubt that the problem of overweight and obesity has now become one of the most important public health issues for Australia. Figures from the 1999-2000 AusDiab study indicate that the prevalence of obesity throughout Australia has risen further so that 19.1% of men and 20.1% of women are now obese (1). In addition, over half of all adult females and 60% of all adult males are now classified as overweight. More alarming is the fact that in 1995 around one in five schoolchildren in Australia were overweight or obese (2). This situation is even more disturbing within the indigenous population where a 1994 survey found that around 25% of the Aboriginal population and almost half of the people from the Torres Strait Islands were obese (3).

This rapid increase in the levels of obesity has the potential to profoundly affect the current and future health of all Australians. Overweight and obesity have been linked to a number of serious chronic health problems such as diabetes, hypertension, cardiovascular disease and cancers (4). In addition, excessive weight is a major contributor to reduced quality of life through its impact on back pain, osteoarthritis, breathing difficulties, sleep apnoea and increased psychosocial problems (4). Cardiovascular disease rates in Australia have been falling consistently since the mid-1970s but it remains one of the major causes of premature death and morbidity in Australia (5). Better control of risk factors such as high blood cholesterol and hypertension, as well as lifestyle change and a reduction in smoking rates, have been identified as key contributors to this reduction in cardiovascular disease rates (6). However, overweight and obesity are also major risk factors for the development of cardiovascular disease and there is a real concern among heart disease experts around the globe that the continued rise in rates of overweight and obesity has the potential to undermine their efforts to control this disease (7). In line with this concern the National Heart Foundation of Australia decided to undertake a review of the scientific literature to assess the relationship between dietary fat intake and overweight and obesity and produced a statement that is published in this issue (8). The statement focusses on the relationship between dietary fat and overweight/obesity, as the Heart Foundation's current nutrition guidelines, which recommend a reduction in saturated fat but not total fat, did not include the potential for dietary fat to influence cardiovascular risk through promotion of obesity.

Establishing associations between diet and disease and assessing the true impact of dietary interventions is fraught with a number of complications (9). In particular, assessing the dietary behaviour and patterns of free living individuals and populations and attempting to define associations in relation to weight status is subject to an enormous array of confounding factors. In an attempt to isolate and study the impact of a single factor, researchers are often required to conduct their experiments under totally artificial situations and deliberately manipulate the composition of foods which creates a situation where the physical and food environment bears little resemblance to the real world. For example, it is possible to manipulate the fat content of food products while keeping the energy density and palatability constant (10). Short-term feeding trials that adopt this approach to identify the influence of one factor, independent of all others, area useful scientific tool but this process rarely reflects real life. The findings of such research, therefore, need to be interpreted within that context and as part of the wider body of scientific research. Science provides the foundations for appropriate nutrition policy development but policy makers must take care that they are not blinded by science to the realities of the prevailing food environment. Unfortunately, after assessing the National Heart Foundation of Australia's position statement on dietary fat and overweight/obesity it is hard to escape the conclusion that this is precisely what happened.

Research has indicated a number of potential mechanisms by which dietary fat could influence weight gain and the development of obesity. Dietary fat increases the energy density of food (together with water it accounts for almost 99% of the variance in energy density of foods) (11). Reducing the total fat in the diet is a useful method of reducing the energy density and has been associated with reduced energy intake, modest weight loss and prevention of weight regain (12). In comparison to carbohydrate and protein, fat has a poor ability to suppress hunger (satiety) and bring episodes of eating to an end (satiation) (13). The capacity for fat storage in the body is virtually unlimited but the ability of fat to stimulate its own oxidation in response to increases in fat intake is very poor when compared against other macronutrients (14). Most of these effects of dietary fat were identified in the Heart Foundation's review of the literature but their relevance to the formulation of the position statement was rejected because the objectives of the review stated that the effect of fat needed to be independent of energy intake. The rationale for this objective is perplexing as, although a wide array of potential factors has been implicated in the development of obesity, they all must exert their influence on energy balance through either energy intake or energy expenditure. There is some evidence that dietary fat may exert some small effect on energy expenditure (through efficiencies in storage for example) but its major influence on energy balance is always going to be mediated through energy intake. From a policy perspective it is not important whether the effects of dietary fat on weight gain are mediated through energy density or other mechanisms. The more important question is whether dietary fat intake contributes to weight gain and whether reducing total dietary fat assists weight loss and the prevention of weight gain. Applying the same principles it should not be expected that under controlled situations that reducing dietary fat would result in a greater weight loss than reducing energy intake as this is the mechanism by which a low fat diet impacts on energy balance.

Removing the limitations imposed by the original objective of the review, it is difficult to see how a role for total dietary fat in the prevention of weight gain and treatment of overweight and obesity can be rejected on the evidence presented by the Heart Foundation's own review. It is therefore disappointing to see the final policy statement make a definite recommendation to avoid a focus on reducing total dietary fat based on tenuous logic. The evidence presented to support a return to a level of dietary fat intake equivalent to 35% of total dietary energy is sparse and the assertion that polyunsaturated fatty acid intakes are decreasing and saturated fat intakes are increasing in the Australian diet is not supported by the reference cited (15).

If this statement had been prepared and released by any other organisation then it may soon be forgotten. However, the National Heart Foundation of Australia has tremendous respect and leverage within community and political dimensions. This current position statement on dietary fat and overweight/obesity will be perceived as being at odds with the recommendations of a wide range of national and international organisations including the National Health and Medical Research Council's guidelines on prevention of diabetes and the treatment of obesity and the Dietary Guidelines for Australian Adults(16-18). The inevitable controversy that this position statement will generate is a further illustration of the often long and torturous passage between science and policy and the problems that may beset those who attempt to shortcut this process.

References

(1.) Cameron AJ, Welborn TA, Zimmet PZ, Dunstan DW, Owen N, Salmon J, et al. Overweight and obesity in Australia: the 1999-2000 Australian Diabetes, Obesity and Lifestyle Study (AusDiab). Med J Aust 2003;178:427-32.

(2.) Magarey AM, Daniels LA, Boulton TJ. Prevalence of overweight and obesity in Australian children and adolescents: reassessment of 1985 and 1995 data against new standard international definitions. Med J Aust 2001;174:561-4.

(3.) Cunningham J, Mackerras D. Occasional Paper: Overweight and Obesity; Indigenous Australians 1994. ABS Catalogue No 4702.0. Canberra: Australian Bureau of Statistics; 1998.

(4.) World Health Organization. Obesity: Preventing and managing the global epidemic. Report of a WHO Consultation: WHO Technical Report Series 894. Geneva: World Health Organization; 2000.

(5.) Thompson PL, Hobbs MST, Martin CA. The rise and fall of ischaemic heart disease in Australia. Aust N Z J Med 1988;18:327-37.

(6.) Lloyd BL. Declining cardiovascular disease incidence and environmental components. Aust N Z Med J 1994;24:124-32.

(7.) World Heart Federation. Obesity Threatens the Heart. Press release for World Heart Day, 30 September 2001. http://www.worldheart.org/heartnews/press_release_finall.pdf Accessed August 2003.

(8.) National Heart Foundation of Australia. Position statement on dietary fat and overweight/obesity. Nutr Diet 2003;60:174-6.

(9.) Marshall JR, Chen Z. Diet and health risk: risk patterns and disease-specific associations. Am J Clin Nutr 1999;69:1351S-6S.

(10.) Stubbs J, Ferres S, Horgan G. Energy density of foods: Effects on energy intake. Crit Rev Food Sci Nutr 2000;40:481-515.

(11.) Yao M, Roberts SB. Dietary energy density and weight regulation. Nutr Rev 2001:59;247-58.

(12.) Hill JO, Melanson EL, Wyatt HR. Dietary fat intake and regulation of energy balance: implications for obesity. J Nutr 2000;284S-8S.

(13.) Blundell JE, Macdiarmid JI. Fat as a risk factor for overnutrition: satiation, satiety and patterns of eating. J Am Diet Assoc 1997;97:63S-9S.

(14.) Schutz Y, Flatt JP, Jecquier E. Failure of dietary fat intake to promote fat oxidation: a factor favouring the development of obesity. Am Clin Nutr 1989;50:307-14.

(15.) Cook P, Rutishauser IHE, Seelig M. Comparable data on food and nutrient intake and physical measurements from the 1983, 1985 and 1995 national nutrition surveys. Brisbane: Australian Food and Nutrition Monitoring Unit; 2001.

(16.) National Health and Medical Research Council. Evidence based guidelines for the management of Type 2 Diabetes: Primary prevention, case detection and diagnosis. Canberra: NHMRC, Commonwealth of Australia; 2001.

(17.) National Health and Medical Research Council. Draft clinical guidelines for weight control and obesity management in adults. Canberra: NHMRC, Commonwealth of Australia; 2002.

(18.) National Health and Medical Research Council. Dietary guidelines for Australian adults. Canberra: NHMRC, Commonwealth of Australia; 2003.

Tim Gill

NSW Centre for Public Health Nutrition

University of Sydney