Marion Nestle and Chris Bunting open a four-page special on food and the future with a look at the power of the US and UK food industries.

In my 25 years as a nutrition educator in the United States, I have found that food-industry practices are discussed only rarely. The reasons for this omission are not difficult to understand. Most of us believe that we choose foods for reasons of personal taste, convenience and cost. We deny that we can be manipulated by advertising or other marketing practices.

Nutrition scientists and practitioners typically believe that food companies are genuinely interested in improving health. They think it makes sense to work with the industry to help people improve their diets, and most are outraged by suggestions that food-industry sponsorship of research or programmes might influence what they do or say. Most food company officials argue that any food product can be included in a balanced, varied and moderate diet. They say that their companies help to promote good health when they fund the activities of nutrition professionals. Most officials of US federal agriculture and health agencies understand that their units are headed by political appointees whose concerns reflect those of the political party in power and whose actions must be acceptable to Congress. Members of Congress, in turn, must be sensitive to the concerns of corporations that help fund their campaigns.

In this political system, the actions of food companies are normal, legal and thoroughly analogous to the workings of any other major industry - tobacco, for example - in influencing health experts, federal agencies and Congress. Promoting food raises more complicated issues than promoting tobacco, however, in that food is required for life and causes problems only when consumed inappropriately. But the primary mission of food companies, like that of tobacco companies, is to sell products. Food companies are not health or social service agencies, and nutrition becomes a factor in corporate thinking only when it can help sell food. The ethical choices involved in such thinking are considered bad too rarely.

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Early in the 20th century, when the principal causes of death and disability among Americans were infectious diseases related in part to inadequate intake of calories and nutrients, the goals of health officials, nutritionists and the food industry were identical - to encourage people to eat more of all kinds of food. Throughout that century, economic improvements affected the way Americans ate in important ways: we got access to foods of greater variety, our diets improved and nutrient deficiencies gradually declined. The principal nutritional problems among Americans shifted to those of overnutrition - eating too much food or too much of certain kinds of food. Overeating causes its own set of health problems; it deranges metabolism, makes people overweight and increases the likelihood of "chronic" diseases - coronary heart disease, some cancers, diabetes, hypertension, stroke and others - that now are leading causes of illness and death in any overfed population.

People may believe that the effects of diet on chronic disease are less important than those of cigarette smoking, but each contributes to about one-fifth of annual deaths in the United States. Addressing cigarette smoking requires only a single change in behaviour: stop smoking. But because people must eat to survive, advice about dietary improvements is much more complicated: eat this food instead of that food, or eat less. The "eat less" message is at the root of much of the controversy over nutrition advice. It conflicts directly with food-industry demands that people eat more of their products. Thus food companies work hard to oppose and undermine "eat less" messages.

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I first became aware of the food industry as an influence on government nutrition policies and on the opinions of nutrition experts when I moved to Washington DC in 1986 to work for the Public Health Service. My job was to manage the editorial production of the first - and as yet only - surgeon general's report on nutrition and health, an ambitious government effort to summarise the entire body of research linking dietary factors such as fat, saturated fat, cholesterol, salt, sugar and alcohol to leading chronic diseases.

My first day on the job, I was given the rules: no matter what the research indicated, the report could not recommend "eat less meat" as a way to reduce intake of saturated fat, nor could it suggest restrictions on intake of any other category of food. In the industry-friendly climate of the Reagan administration, the producers of foods that might be affected by such advice would complain to their beneficiaries in Congress, and the report would never be published. This scenario was no paranoid fantasy. Federal health officials had endured a decade of almost constant congressional interference with their dietary recommendations. Agency officials had learned to avoid such interference by resorting to euphemisms, focusing recommendations on nutrients rather than on the foods that contain them, and giving a positive spin to any restrictive advice about food. Whereas "eat less beef" called the industry to arms, "eat less saturated fat" did not. "Eat less sugar" sent sugar producers right to Congress, but that industry could live with "choose a diet moderate in sugar". When released in 1988, the surgeon general's report recommended "choose lean meats" and suggested limitations on sugar intake only for people particularly vulnerable to dental cavities.

Since 1988, I have become increasingly convinced that many of the nutritional problems of Americans - not least of them obesity - can be traced to the food industry's imperative to encourage people to eat more to generate sales and increase income in a highly competitive market.

Ambiguous and confusing dietary advice is only one result of this imperative. The industry also devotes enormous financial and other resources to lobbying Congress and federal agencies, forming partnerships and alliances with professional nutritional organisations, funding research on food and nutrition, publicising the results of selected research studies favourable to industry, sponsoring professional journals and conferences, and making sure that influential groups - federal officials, researchers, doctors, nurses, teachers and journalists - are aware of the benefits of their products.

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Overweight on its own constitutes ample evidence that many Americans consume more calories than they burn. The calories provided by the US food supply increased from 3,300 per capita in 1970 to 3,800 in the late 1990s, an increase of 500 per day and nearly twice the amount needed to meet the energy requirements of most women and a third more than that needed by most men. These supply figures tend to overestimate amounts of food consumed because they do not account for wastage, but they do give some indication of trends.

In addition to revealing how much people are eating, food supply and dietary intake surveys indicate changes in food habits over time. The increase in calories reflects an increase in consumption of all major food groups: more vegetables and more fruit (desirable), but also more meat and dairy foods, and more foods high in fat and sugar (less desirable).

Despite the introduction of artificial sweeteners, the supply of calorie-laden sweeteners - sugars, corn sweeteners and honey - has gone up. Because of the inconsistencies in data, the trend in fat intake is harder to discern. Fat in the food supply increased by 25 per cent from 1970 to the late 1990s, but dietary intake surveys do not find people to be eating more of it. Although government nutritionists conclude that Americans are eating less fat, they also observe that people are eating more food outside the home, where foods are higher in fat and calories, a trend that has long-term health implications.

From such observations, we can conclude that the increased calories in American diets come from eating more food in general, but especially more of foods high in fat (meat, dairy, fried foods, grain dishes with added fat), in sugar (soft drinks, juice drinks, desserts) and in salt (snack foods). It can hardly be a coincidence that these are just the foods that are most profitable to the food industry and also the foods that this industry most vigorously promotes, through increases in the sizes of food packages and restaurant portions as well as billboards and television commercials.

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Food companies are in business to make money; that is their job. From the perspective of stockholders, it is irresponsible for companies to make decisions that will not lead to more profits. If companies offer foods of minimal nutritional value and people buy them, companies will continue to make and market them. If fat, sugar and salt help to sell products, companies will market top-of-the-pyramid products in the name of freedom of choice. If "nutrition" - added vitamins, reduced fat - helps to sell products, the companies will use nutrition as a marketing tool. If the market is not expanding, food companies will increase the range of their marketing targets to children, urban minorities and people in developing countries - whether or not the products displace more nutritious foods in the diet or add unnecessary calories. These actions parallel the tactics of tobacco companies. And in the same way that cigarette companies' promotion of smoking raises ethical issues, so does the food industry's promotion of minimally nutritious products and of overeating in general.

Adopting actions such as refusing to buy processed foods, buying organic or not taking our children to fast-food restaurants are just a few ways to apply ethical principles, but the higher cost and inconvenience of doing such things are certain to preclude those choices for many, if not most, people. Unless we are willing to pay more for food, relinquish out-of-season produce and rarely buy anything that comes in a package or is advertised on television, we support the current food system every time we eat a meal. That is why we must support the consumer lobbying groups and ensure that this kind of "voting with our forks" extends beyond the food choices of individuals to larger political arenas.

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Marion Nestle is professor and chair of the department of nutrition and food studies at New York University. Her new book, Food Politics : How the Food Industry Influences Nutrition and Health , will be published next month by the University of California Press, ?19.95.