FIRST THE good news.

Americans are eating fewer calories now than they did 15 years ago.

Now the bad news.

Americans are fatter now than they were when they were eating more.

So far the only explanation for this anomaly, according to the man who is charged with making use of the information, is that people are leading even more sedentary lives than they did in 1965. Dr. Mark Hegsted, administrator of the Agriculture Department's Human Nutrition Center, has said that unless further analysis of the data offers a different explanation, "we probably have to conclude that there have been rather large decreases in physical activity."

The data on which Hegsted bases his analysis were gathered in a nationwide food consumption survey made during 1977 and 1978, but the statistics were only made public last fall. The delay confounds and annoys Hegsted. This latest survey is based on the kinds and quantity of food used in 15,000 households over a seven-day period. It includes a comparison with the results of a similar survey taken in 1965.

It must come as a surprise to many people that Americans were less active two years ago than they were 15 years ago. What about the national sports "craze," the tennis players, the joggers?

Hegstd professed puzzlement. "I don't understand it. You wonder what the error in the data is. There are some people around, he said, "who say these figures for consumption are too low." Hegsted himself believes the "figrues for alcohol consumption are understated."

In addition to an overall decline in the consumption of calories (the average is down 10 percent, from 3,210 to 2,900 per day), fat consumption had dropped 9 percent (from 154 to 140 grams per day), but men were four pounds heavier than they were 10 year previously. Women were only slightly better off.

Another good news-bad news statistic that surfaced in the study is that poor people are eating more like rich people. That should be good news, except that the American diet is too high in fat, sugar and salt, according to Hegsted. At the same time, Hegsted reported that 3 percent of the households in the survey, representing about 7 million people, said they didn't have enough to eat. "It's ridiculous in a country that has more food than it knows what to do with that we can't deal with people who are poor and have a problem getting food," Hegsted said.

The survey also shows increased consumption of six of the seven vitamins and minerals that were examined: vitamin A; the B vitamins, thiamin, niacin, roboflavin; vitamin C and iron. Consumption of calcium has decreased. The USDA thinks the decrease in calcium consumption may be related to the fact that a smaller proportion of the population is under 18. Children and teen-agers are the largest consumers of milk, which is a major source of calcium.

The overall increase in nutrient consumption means that Americans are eating food with higher nutrient density. There are more nutrients per calorie, or fewer "empty" calories. What analysis of the survey does not reveal yet is whether these nutrients came from natural sources or from enriched foods. Since enriched foods often are lacking in the trace nutrients their natural counterparts contain and since the survey did not examine the consumption of trace nutrients, there are no data on these essential elements in the diet.

So nutritionist ask whether Americans are getting enough trace nutrients like zinc, manganese, potassium and folic acid. If the findings about the source of vitamin C in the diet are an example, there is something to worry about. While consumption of citrus fruits, an excellent source of vitamin C, (and several trace nutrients) was up 100 percent, consumption of vitamin C-enriched soft drinks (without trace nutrients), punches and preprepared desserts was up 207 percent.

Robert Risek, who supervised the survey, said that such a breakdown will be available sometime in the next "two to 2 1/2 years." That means all the final results will not be available until almost five years after the survey was completed, a frustrating problem for Hegsted.

But the preliminary data cast aside any doubt about the ability of advertising and widely accepted myths to influence diet. While vegetable consumption was up overall, the consumption of potatoes, which have yet to dispel the notion that they are "fattening," was way down. So was the consumption of other "fattening" foods such as bread products and dried beans.

Like so many other quirks in the American diet, people reduce their intake of sweet products such as syrup, jelly and candy, along with sugar they add to food themselves. But that decrease was far outweighted by the increase of sugar-sweetened drinks and desserts. In addition, the consumption of alcohol rose dramatically. It was up 186 percent.

Overall the consumption of carbohydrate-rich foods was down even more than the consumption of fat. The fact that carbohydrates provide only as many calories as protein, four per gram, while fat provides nine, has yet to gain widespread credence among the public. And most people have not yet learned the distinction between complex carbohydrates (whole grains, fruits and vegetables) which offer nutrients and fiber, and simple carbohydrates (refined sugars) which offer calories and almost nothing else.

The consumption of eggs was also way down, due, no doubt, to theories linking cholesterol to heart disease, which only now is being given a second look. The total amount of protein products consumes was unchanged, but within that group there were some differences. Beef, poultry, fish and nut consumption increased while the amount of pork and luncheon meats declined along with eggs and dry beans.

Hegsted is disappointed in the findings because, he said, Americans "aren't getting any better" at adjusting their energy intake to their energy use. "The question is, are we getting any worse?"