Restaurants are businesses, and that means they respond to consumer demand. What consumers have demanded for years are rich foods and big portions, which means that the restaurant concept of a portion or of a healthy alternative is seriously out of whack with what the nutrition experts recommend. Does that mean you should stop eating out? Heck, no! But it does mean you need to use caution when navigating a menu.

Restaurants don’t make friends by serving up teensy little portions. In fact, tiny servings probably sank nouvelle cuisine, the 1980s fad that put one string bean, three garden peas, half an artichoke heart, and one sliced cherry tomato on a lettuce leaf and called it the salad course.

Reality dictates that the portions on restaurant plates rarely come within hailing distance of the official serving sizes issued by the U.S. Department of Agriculture. To protect yourself from humongous servings, you need to store real-life versions of the recommended portions in your memory banks.

To do that, use an 8-ounce measuring cup and a kitchen scale to run through some basic practice drills at home:

• Broil a small steak or roast a chicken breast. Use a kitchen scale to weigh a 3-ounce portion. Does the steak look like a deck of cards? How about a small calculator? That’s one serving.

• Boil some rice. After the rice is done, fill the measuring cup to the halfway mark. Take out the rice and roll it into a tennis ball or a billiard ball. Whatever. That’s one serving.

• Shred some greens. Fill the measuring cup to the 8-ounce mark. Turn the greens out onto a salad plate. That’s one serving.

• Open one can of beets or fruit cocktail. Fill the measuring cup to the halfway mark. Spoon the beets or fruit onto a plate. That’s one serving.

• Open a can of soda. Pour it into the measuring cup, right up to the 8-ounce mark. Pour that into a glass. Add some ice. It’s probably more than you get in an upscale restaurant, less than you get at the burger barn. No matter: It’s still one certified USDA serving.

Now that you have a picture of a serving in your mind, you can slice away the extra from your restaurant plate — and take it home for lunch or dinner the next day. That’s what doggie bags are for.

When the menu says, “Eat me! I’m healthy,” ask for proof. The people who make and market processed foods are required by law to provide detailed ingredient labels on their packages. Restaurants ordinarily are exempt. They don’t have to tell you exactly what’s in the beef Stroganoff or vegetable stirfry.

The exception is a dish for which the restaurant makes a health claim. The restaurant may write “low-fat” or “heart-healthy” next to the item on the menu or mark the entry with a little red heart to signify the same thing. When a restaurant does this, the Nutrition Education and Labeling Act says the restaurant has to back up that claim.

The law is flexible; it doesn’t require an ingredient listing on the menu, but it says that the restaurant can comply by making a notebook available that accomplishes at least one of the following tasks:

• The notebook can list the nutrient content of each labeled dish or show that the dish was made according to a recipe from an authoritative professional association or dietary group, such as the American Heart Association.

• The notebook can show that the nutritional values for the dish are based on a reliable nutrition guide, such as USDA’s voluminous Agriculture Handbook No. 8, which is made up of several volumes with perhaps a thousand pages of nutritional analysis for all kinds of food. As with the new, improved labels on food packages, this policy is designed to make sure that any food that claims to be healthy actually is.