People have been eating cereals for a long time. In fact, you might say civilization began with cereals. The cultivation of grains 10,000 years ago gave early man a dependable food supply and freed him to develop other skills and talents.

Cereals remain the most important cultivated food plants, providing as much as 85 percent of the day's calories in some developing countries. In times past (and still in certain societies), they were eaten as a paste or soaked to make gruel. But people have learned to grind and process different parts of the grain kernel into a host of products like flour, starch, oil, bran, syrup, sugar -- and breakfast cereals.

As anyone who visits a supermarket can tell, no food has been manipulated in more ways than basic grain. The ready-to-eat or dry category has seen the most expansion, with marketing experts and food technologists combining their ingenuity to pull, puff and twist grains into innumerable shapes and tastes.

The lineup of hot cereals has also grown. New packaging, the addition of nuts, fruits and flavorings, and a minor avalanche of instant and quick-cooking varieties have padded the repertoire.

Hot cereals can be very good foods. Served with low-fat milk and a glass of vitamin C-rich juice, they make a superior and low-cost breakfast. Compared to the ready-to-eat varieties, many are much closer to the original grain. As such, they are what nutritionists call "nutrient dense," meaning they contain a high concentration of nutrients relative to their caloric content.

As harvested, the main cereal grains in our diet are nutritionally similar. Wheat, corn, oats, barley, rye and rice (as well as sorghum and millet, used in this country mainly as animal foods) -- all are the seeds of cultivated grasses. They are approximately 10 to 15 percent water and 70 to 80 percent complex carbohydrate (starch). The protein content varies between 9 and 13 percent, and the protein quality also varies, but this is of no practical importance in the United States. They contain little fat and no cholesterol.

Grains also share a common structure. They have three basic parts: an outer protective bran layer; an embryonic segment called the germ; and the endosperm, a large starchy section.

Products like Wheatena and others that specify 100 percent whole-grain cereal are processed only minimally, thus retaining more of the nutrients of the orginal grain. They are simply broken into smaller pieces for faster cooking.

Other cereals undergo more complicated processing. Oats are thrown from a rotating disk against a rubber ring to split off the hull. The inside of the kernel, called the groat, is then steamed and passed between rollers to produce rolled oats. Alternatively, the groats may be cut into pieces one-third the original size, then steam-heated to make quick-cooking rolled oats.

Highly milled cereals, such as Cream of Wheat and other farinas, are made from the endosperm or carbohydrate part of the grain, and therefore contain virtually no fiber. They are enriched with thiamin, riboflavin, niacin and iron, which were lost in the milling process. Some may also have nutrients added like Vitamins A, B-12 and C, which are not usually found in cereals. This type of fortification defies nutritional logic and is highly questionable. And not all the nutrients destroyed in milling are added back, so cereal made from refined grains is likely to lack some nutrients present in the original grain.

Most hot cereals cook in one to five minutes, but pretreatment with enzymes and the addition of salt can allow cereals to absorb water even faster. Many instant cereals need only a dose of hot water to soften them. This may help you get out the door more quickly in the morning, but there is a trade-off.

While cereals naturally contain little sodium (less than 10 milligrams per serving), instant cereals have 100 to 350 milligrams per serving. Keep in mind, too, that extra sugar and other sweeteners can boost calorie counts by 50 percent or more.

Processing and added "frills" may not only alter nutrient quality but also hike up the price. Old-time favorites like rolled oats cost between 3 cents and 6 cents a serving, as compared with two or three times as much for their instant counterparts. How can you, the consumer, make the right choice? Two of the necessary elements, the ingredients list and nutrition label, are printed on the package. And if you are lucky enough to live in a state that has adopted unit pricing, you can readily see how much convenience costs. With the facts in place, the choice is up to you.