"Everyone" agrees that Americans must change their lifestyles in this country if they are going to improve their eating habits, according to the results of a survey presented at a conference on "Nutrition and the American Food System" last week. But the same people do not speak with one voice when it comes to deciding what should be done about it.

The conference was sponsored by Community Nutrition Institute, a public interest group which monitors federal programs in food and nutrition. It was underwritten by Food Marketing Institute, the trade association for the supermarket industry and Family Circle Magazine, one of the largest circulation magazines, which is sold in supermarkets.

The survey, commissioned by Family Circle, was conducted by Marketing Science Institute of Cambridge, Mass. Almost all the participants in this latest assessment of nutrition in America are in occupations connected either directly or indirectly with food: nutritionists, dietitians, employees of the food industry, nurses, consumer activists, government program administrators, nutrition science academics. The attitudes of the ordinary consumer were not a part of this study, which may account for the incredible unanimity of opinion on certain subjects.

Almost all the respondents (98 per cent) believe that the greatest barrier to improved nutrition is the tendency of people to overeat and not exercise much; 94 per cent think the lack of knowledge is an additional factor.

There is a great deal of agreement that one way to change people's health habits is to educate the public. Nutrition education literature, a nutrition advertising campaign on public broadcasting and funds for nutrition research were popular choices as methods to achieve that goal.

But when the survey gets down to specifics on how to make people change their bad habits, unanimity disappears. While a very large percentage of consumer activists, nutritionists and dietitians in and out of government think it would be useful to have food manufacturers provide percentage-of-ingredient information, only 55 per cent of the farmers and 47 per cent of the food manufacturers agree.

There is even less agreement on the concept of having the food retailer use the nutritional value of a product as a criterion for stocking his shelves. Only 46 per cent of the food retailers and farmers and 36 per cent of the food manufacturers think well of that idea, while 92 per cent of the consumer activists think it would be worth doing.

The idea that nutrition label information in advertising might be useful brings a strong "no" from food retailers (62 per cent) and manufacturers (78 per cent). But 95 per cent of the consumer activists and 88 per cent of the government nutritionists approve of the idea.

When it comes to deciding how people ought to change their eating habits there is almost unanimous agreement about several suggestions. Betwen 92 and 100 per cent of all the respondents believe people should eat more fruits, vegetables and whole grain products. A significant majority believe people shoud decrease their consumption of sugar and salt. Yet when asked if it would be a good idea to substitute nonfat milk for whole milk, only 23 per cent of the farmers and 29 per cent of the food industry nutritionists think that idea has merit, while 61 to 80 per cent of the other groupings of respondents approve.

There is no consensus on reducing the amount of butterfat, eggs and other high cholesterol foods, or decreasing the amount of red meat and increasing the amount of fish and poultry in the diet.

Such changes are recommended in the Senate Nutrition Committee's Dietary Goals for the United States, a report that was released last winter.

The cattle industry has protested the red meat recommendation. The egg industry has complained about the recommendation on cholesterol. The sugar industry doesn't like the recommendation to reduce sugar consumption. The canning industry opposes the recommendation to eat fresh and frozen fruits and vegetables instead of canned. These special interest groups have voiced no objections to the parts of the report that do not affect the way they make a living.

The survey seems to by saying the same time: It's okay to tell people they must improve their eating habits, and it would be a good idea to educate them on how to go about this. But when it comes to the hard questions about what to tell them, viewpoints differ along lines of self-interest.

This inability to agree on how to go about improving the American diet cropped up againg and again during the two-day meeting.

Family Circle publisher, Robert F. Young, told the conference: "The greatest challenge is to find the common ground on which we can move ahead while we define and resolve those areas on which we disagree, or on which more research is needed before progress can be made."

If the survey says anything it is that common ground will be pretty hard to come by.