Foods and nutrients provide important insights into optimal eating patterns.

Understanding the dietary intake of any population requires the investigation not only of the intake of individual nutrients but also of the intake of foods and relevant grouping of foods. With the successes of intervention trials to prevent chronic disease where overall patterns of food intake have been modified (1) and the failure of nutrient-based intervention trials, (2) there is an increasing focus on the foods we eat. Worldwide there has been a shift to the use of food-based dietary guidelines. Development of food-based dietary guidelines requires an understanding of the existing food intake patterns, taking into account sociocultural and economic factors. (3) By understanding the existing food intake patterns within a population, we can identify appropriate targets for improving dietary quality.

Investigation of food intakes allows us to assess a range of other nutrition issues and aspects of the diet that would not be reflected solely in nutrient intakes. For example, dietary variety, meal patterns, specific dietary behaviours such as breakfast eating, and overall dietary patterns. In addition, we can also attempt to understand the determinants of food behaviour, and understand why people eat the way they do. (4) Considering how foods are prepared and eaten can also highlight important biological consequences. For example, it has been shown that lycopene from tomatoes is significantly more bioavailable from tomatoes that have been cooked with olive oil than tomatoes cooked without oil and raw tomatoes. (5) A focus on food intakes also allows the application of innovative methods of dietary assessment such as the use of short questions to monitor fruit and vegetables intakes. (6)

Investigation of food intakes alongside nutrient intakes can highlight the impact of changing food patterns on nutrient intake. For example, research in the UK on food and nutrient intakes among children found that although median vitamin C intakes had not changed substantially between the 1950s and 1990s, the main sources had changed dramatically from fruits and vegetables to soft drinks and cordials. (7) Analysis of food intakes in conjunction with nutrient intakes will become increasingly important with the increase in fortified food products available with nutrients being provided by unconventional sources (e.g. calcium-fortified orange juice).

The paper by Mann and colleagues highlights some of the methodological issues surrounding analysis of food intakes. (8) The authors compared a food grouping system used in the analysis of the 1995 National Nutrition Survey (NNS) (9) with a classification system developed by CSIRO (10) and their own classification, both of which involved more detailed groupings for the meat and meat dishes. In their study, the main outcome of choice was the contribution of each food group, particularly red meat, to fat intakes. It is possible that more detailed groupings of other foods, such as the dairy group, would also affect the rankings for fat intake, and similarly the contribution of foods to other nutrients would be substantially affected by more detailed classifications, for example fibre intakes and cereal foods. Their work highlights the need for consistent and objective food grouping systems so that data are systematically presented, as comparisons across studies are difficult when different food grouping systems are used.

The food grouping system used in the NNS is a hierarchical classification system based on the unique food code identifier. (9) For example, the 'meat, poultry and game products and dishes' group can be further divided into categories that distinguish muscle meats, poultry, processed meats and mixed dishes containing meat, and there are further subclassifications within each of these groups. By using standard classification systems, other researchers can easily compare their work, and can easily understand the contributors to each food group, for example in the paper by Webb et al. (11)

Decisions about the groupings of foods are often based on nutrient profile but other factors such as cultural or social factors may also influence classification. Categories such as 'fast food/takeaway food' are likely to pose difficulties in comparing trends over time as the range of foods available changes over time, with the potential for large differences in nutrient profiles (e.g. sushi).

Mixed dishes represent a substantial challenge in any food grouping system. With an increasingly complex food supply, changing dietary patterns and an ethnically diverse population, this issue will remain. The United States Department of Agriculture has developed a database to overcome some of these issues, which allocates all foods consumed into one of 30 major foods groups aligned to its food selection guide. (12) This uses a recipe file to disaggregate mixed dishes allowing ingredients to be attributed to their appropriate food groups. A similar approach has been used in previous analysis of red meat intake from the 1995 NNS where the yield of meat from mixed dishes was estimated by recipe calculations based on either information provided by respondents, commercial data or standardised recipes. (10)

As recognised by the Dietary Guidelines for Australian Adults, lean meat, fish, poultry and alternatives such as eggs, legumes, nuts and seeds, are important contributors of nutrients to the Australian diet including protein, iron, zinc, vitamin [B.sub.12] and n-3 fatty acids. (13) Importantly, the paper by Mann et al. shows that only 4.4% of saturated fat intakes in the diet was provided by red meat and highlights other substantial sources of saturated fat; (8) information highly relevant for the development of dietary advice for the prevention of cardiovascular disease.

Much has been written in the media about red meat with recent debate focusing largely on the link between red meat and cancer. (14) Research has suggested that factors other than the fat content of red meat, such as the production of N-nitroso compounds through interactions with haem iron, may be implicated in the development of some types of cancer. (15,16) The World Cancer Research Fund (WCRF) recommends that red meat intakes be limited to less than 80 g per day. (17) These recommendations are broadly consistent with the Australian Guide to Healthy Eating, which suggests that adult men and women consume 1-2 serves and 1-1.5 serves per day, respectively (depending on physical activity levels) from the lean meat, poultry, fish and alternatives group, with advice to consume lean red meat three to four times per week. (18) Although the WCRF recommendations are currently being updated and due for release in 2007, Australian research published in 2004 continues to support the WCRF recommendations. (19) By using evidence based on research into both foods and nutrients, we can determine the optimal balance of dietary intakes to meet nutrient requirements and promote long-term health.

Sarah McNaughton

NHMRC Research Fellow

Centre for Physical Activity and Nutrition Research

REFERENCES

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13 National Health and Medical Research Council. Dietary Guidelines for Australian Adults. Canberra: Commonwealth Department of Health and Ageing, 2003.

14 Anonymous. Nutritionist concerned about CSIRO diet. The Age 7 January 2006. (Cited 8 Mar 2006.) Available from URL: http://www.theage.com.au/news/national/csiro-diet-referred-to-pm/2006/01/08/1136655079825.html

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17 World Cancer Research Fund. Food, Nutrition and the Prevention of Cancer: A Global Perspective. Washington, DC: American Institute for Cancer Research, 1997.

18 Kellett E, Smith AS, Schmerlaib Y. Australian Guide to Healthy Eating. Canberra: Commonwealth Department of Health and Family Services, 1998.

19 English DR, MacInnis RJ, Hodge AM, Hopper JL, Haydon AM, Giles GG. Red meat, chicken, and fish consumption and risk of colorectal cancer. Cancer Epidemiol Biomarkers Prev 2004; 13: 1509-14.

(See paper by Mann et al., pages 69-78)