In the classic comedy Sleeper, Woody Allen's character wakes up to find himself in a future where doctors consider steak, cream pies and hot fudge to be health foods. Regrettably, research has yet to show that hot fudge sundaes will help you live to 100. But you may occasionally feel just as disoriented and befuddled by changing medical opinion as Allen's character did in the movie.

How much to exercise, what to eat, when to get screening tests and which immunizations you need—health advice seems to change as often as shoe styles and hot stock picks. It's frustrating to discover that last year's recommendations are passé, especially if you took pains to follow them. But there are sound reasons why guidelines evolve.

"They change over time because new evidence emerges that challenges what we thought was true," says Mark Monteforte, M.D., senior clinician reviewer at the ECRI Institute, a scientific organization dedicated to improving patient care. Studies that are larger, better-designed or conducted in different populations often bring to light new information. Medical organizations carefully sift through this new research to see if their advice should be revamped. The result is guidelines that are objective and based on scientific evidence, says Monteforte. And that makes them worth paying attention to.

Here are five recent changes in health guidelines that you (and possibly even your doctor) may have missed.

Have trouble fitting exercise into your schedule? You now have a new time-saving option. Just 20 minutes of aerobic exercise three times a week is enough to boost your health, according to the latest joint guidelines from the American College of Sports Medicine and the American Heart Association (AHA). But there's a catch: The workout has to be "vigorously intense." In other words, you should be too winded to carry on a normal conversation while you're exercising.

For those who like to sweat a little less, 30 minutes of "moderately intense" aerobic activity five times a week will still do the trick. At this level you're too winded to sing but can talk comfortably. Even better news: It's OK to accumulate your daily 30 minutes in short bouts, says Jennifer Mieres, M.D., an associate professor of medicine at New York University School of Medicine and AHA spokesperson. "The prior thinking was that you needed 30 minutes of continuous aerobic exercise for weight loss and heart health," she says. "We've had enough time now to study thousands of people. Even 10 minutes [at a time] gets you sustained benefits."

For the first time the guidelines also give an official thumbs-up to activities that strengthen muscles, such as working out with weights. "We've now recognized there are benefits from strength training," says Mieres. "It helps maintain weight loss and control blood pressure. It's a complement to aerobic exercise." It's considered especially important as you age to help prevent the loss of muscle mass and bone.

Healthy adults under age 65 should strength train twice a week. Aim for eight to 10 strengthening exercises with eight to 12 repetitions each.

Adults age 65 and up, as well as those between 50 and 64 who have chronic medical conditions such as arthritis, should strength train two or three times a week. Aim for eight to 10 strengthening exercises with 10 to 15 repetitions each.

Heart disease isn't just a man's disease, and the AHA emphasized this fact in 2007 when it released new heart-health guidelines for women. Their updated advice recommends using aspirin as a preventive measure.

Most high-risk women¿for example, those who already have cardiovascular disease, kidney disease or diabetes¿should take 75 mg¿325 mg of aspirin daily. However, aspirin can have side effects, including allergic reactions and an increased risk of internal bleeding, so long-term use should be started only under the guidance of a physician.

Healthy and low-risk women age 65 and up should consider taking 81 mg¿100 mg of aspirin every other day if the benefits are likely to outweigh the risks for them.

More than 20 million Americans have type 2 diabetes, but nearly one in three doesn't know it, according to the American Diabetes Association (ADA). That's why the ADA now recommends that everyone over 45 get a blood test for diabetes every three years. It's especially important to get checked if you're a Native American, African-American, Asian-American, Latino or Pacific Islander because diabetes occurs more often in these groups. The ADA also suggests:

• More frequent testing in those over 45 if warranted by initial test results or other risk factors
• Earlier testing in adults under 45 who are overweight and have other diabetes risk factors
• Testing every two years starting at age 10 for children who are overweight and have other diabetes risk factors

Influenza season is right around the corner, which brings us to a topic we'd love to ignore: flu shots. It's not just a matter of avoiding a miserable week in bed. The flu can morph into an ear or sinus infection, pneumonia or other potentially serious problems. About 200,000 people are admitted to the hospital each year with complications, according to the Centers for Disease Control and Prevention (CDC).

Your best insurance against this misery is a flu vaccine. It comes in two varieties: a shot made from killed virus and a nasal spray made from live, weakened virus. (Note: The nasal spray isn't recommended for children younger than 2 because of an increased risk of hospitalization and wheezing seen in clinical trials. It also isn't recommended for adults older than 49, because effectiveness hasn't been demonstrated in the age 50-64 group and those above 65 had an increased risk of sore throats.) Both vaccines start to protect you in about two weeks, by which point your body is producing an army of flu-fighting antibodies. The ideal time to get vaccinated is in October or November, before flu season usually peaks, but you can still get one later. And yes, you do need a shot every year since the vaccine changes annually based on which strains of flu virus are predicted to be circulating. Check for the flu shot schedule in the fall at your neighborhood Publix pharmacy.

In addition to recommending shots for adults starting at an earlier age, the CDC says individuals in the following groups should also be vaccinated:

• Young children from age 6 months to 5 years
• People with chronic medical conditions, including asthma and diabetes
• Residents of nursing homes
• Pregnant women
• People who live with or care for someone in a high-risk group

All dietary fats are not created equal, and research shows that saturated and trans fats are the ones to watch out for. Saturated fat raises blood cholesterol, which increases your risk of having a heart attack or stroke. Trans fats not only raise the bad kind of cholesterol (LDL) but also lower the good kind (HDL). In addition to making heart disease more likely, they're associated with a higher risk of diabetes. That's why the AHA recently lowered the targets. Here's what you need to know about limiting bad fats.

Saturated fat occurs naturally in foods from animal sources as well as a few vegetable oils (notably palm, palm kernel and coconut oils). To cut back, limit your consumption of fatty cuts of meat, poultry with skin, cream, butter and whole-fat milk and cheese. Many baked goods and fried foods also are high in saturated fat.

Trans fats are created in an artificial process that adds hydrogen to liquid vegetable oils to make them more solid. Look for the term "partially hydrogenated oils" on food labels. These fats are often found in stick margarine, fried foods (such as french fries and doughnuts) and baked goods (such as cookies, crackers, pastries and pizza crust).

Health guidelines change, and so should your behavior. It pays to pay attention. You might just wake up one day to the news you've been waiting for about that hot fudge.