SECRETARY of Agriculture John Block and his family apparently had enough to eat during their recent week on USDA's $58 thrifty food plan. However, if the secretary, and not his wife, Sue, had been required to prepare the food, he probably would have had to come home early from work.

There are no quick dinners, no frozen pizzas or pot pies. One of the reasons the plan is both economical and healthy is that it doesn't use any convenience foods. The suggested menus call for baking "from scratch" and for planning ahead--roasting pork for one night and putting aside part of it for another dish later in the week. All the foods on the weekly $58 shopping list are basic commodities and staples.

The Blocks probably never ate a set of more scientifically-planned meals in their lives. The food plans meet recommended dietary allowances (RDAs) developed by the National Academy of Sciences for 11 nutrients and 80 percent of the RDAs for three additional nutrients.

If you followed the thrifty food plan, 35 percent of your calories would come from fat and 12 percent would come from sugar and other sweeteners. You'd also get 350 milligrams daily of cholesterol, and 1,600 milligrams per 1,000 calories of sodium.

Betty Peterkin, deputy director of the consumer nutrition division in USDA's Human Nutrition Information Service, whose career has been spent formulating food plans, calls these levels "moderate." They are lower than people ordinarily consume but higher than some authoritative groups recommend. For example, she points out, the American Heart Association recommends a diet with 30 percent of calories from fat, while consumption figures show that 42 percent of the calories in the average American's diet come from fat.

USDA's philosophy in formulating the plan was that it would be more acceptable to families if it disrupted their usual food practices as little as possible. Nutritionists figured out what people were eating by studying the results of the l977-78 nationwide food consumption survey. That government survey looked at food consumption patterns of 11 groups divided by age and sex in 4,400 households eligible for the food stamp program. Quantities and types of food consumed were gleaned from the survey. Because very few people eat a model diet, consumption patterns had to be changed in order to meet dietary goals. So USDA nutritionists, with the aid of a computer, developed meal plans that would keep changes to a minimum, while at the same time staying low in cost.

It's not surprising that certain foods have to be eliminated in order to obtain consistently cheap, nutrient-packed meals. For a four-person family, the sample meal plan developed by USDA calls for a 78 percent cut in the amount of soft drinks consumed by the average low-income family. It also means a 41 percent drop in cheese consumption, a 37 percent decrease in meat, poultry and fish and a 22 percent cut in eggs and sweets.

Frozen entrees, potato chips, ice cream--all relatively expensive sources of nutrients--are also cut from the diet. On the other hand, dried beans and nuts, grain products, and, to a lesser extent, fruits and vegetables, are increased above the average levels consumed by the low-income households--in order to meet nutrient requirements and to keep costs low.

If you are looking for any frivolous or gourmet-type foods in the thrifty food plan menus, you won't find them. The weekly shopping list, for example, calls for low-cost meats like liver, bologna, tuna fish and chicken, along with a bone-in beef chuck roast that will be used for several meals. Meat alternatives like kidney and navy beans are also on the list, as well as eggs and peanut butter. And the quantities in the meat and meat alternative group are small. One breakfast calls for three eggs for four people. One of the weekly meal plans uses 2.87 pounds of turkey drumsticks for two meals plus soup.

The sample plans are heavy in starch--in fact most of the low-income households that tested the meal plan said it called for more breads and starchy foods than they usually consumed. Peterkin says the plans have to meet caloric needs--to fill people up--and legumes and grains are better nutrient sources than vegetables, which are more expensive and don't provide as many calories. Some form of bread, including home-baked muffins and biscuits, is included in nearly every meal, often alongside other starchy foods like potatoes, macaroni and rice.

The thrifty food plan was revised and updated recently from a l975 plan, and Peterkin says the latest one is more appetizing. It includes more meat, fish and vegetables. But costs, taking inflation into account, have stayed the same as in l975. Peterkin says she wasn't surprised that a healthy diet plan could be calculated without an increase in cost, even though requirements for several more nutrients--including zinc, folacin and vitamin E--had to be met in the new plan. The changes that had to be made for nutritional purposes, such as reduced fat levels and increased carbohydrates, generally are not high-cost changes, Peterkin notes, and similar adjustments had to be made in the higher cost food plans developed by USDA. However, including zinc, a mineral that is relatively scarce in the food supply and found mainly in red meats, meant an increase in the amount of meat in the l983 plan from eight years earlier.

But there are problems with the thrifty food plan. Lynn Parker of the Food Research and Action Center, a Washington-based group that lobbies on behalf of poor people, says most of them can't afford even the thrifty food plan--72 percent of food stamp recipients don't get the full $58 food stamp allotment and most have other bills that compete with food dollars. In addition, she says, the plan is based on a hypothetical family of two adults and two elementary school children. If there is a pregnant woman in the family, an adolescent, or an adult who does heavy physical labor, higher caloric and nutrient levels are needed.

Parker also complained about the small margin allowed for waste--there's only a 5 percent waste figure computed into the plan. If food spoils or a child drops something on the floor, it's not taken into account in the plan, says Parker. The plan also assumes that people will have working stoves and refrigerators and a cupboard stocked with basic staples like cornstarch and vanilla extract.

Parker calls some of the portion sizes unrealistic, considering the way a typical family eats. Some of the menu plans, for example, call for a serving of half a banana or half an apple. "This can put a family into the situation of feeling pretty deprived," she noted.

Even John Block admitted to missing certain foods. "If you had a hard day, you might want to have a glass of beer or a soft drink," he told reporters after his week on the diet. The diet makes no provision for such extras--not even an ice cream cone at the shopping mall, something Block said he missed the most over the weekend.

There are ways families can splurge once in a while, says Peterkin. For example, they can save money for treats if they use only nonfat dry milk. (The plan calls for nonfat dry milk for cooking and regular milk for drinking.) The purpose of the plan, she emphasized, is to show that people can get a healthy diet on a small budget.

Sue Block said she found the plan takes time and good management skills, and highly recommended USDA's booklet, "Making Food Dollars Count," which contains recipes for two weeks of sample menus. The secretary says he has instructed the Food and Nutrition Service to make the guide available in all food stamp centers and in Extension Service offices. The guide is available as Home and Garden Bulletin No. 240 from the U.S. Government Printing Office.

"I'm not saying it's easy," Peterkin said about the plan. "It's never easy except if you can walk into the store and buy whatever you want and forget about the cost."