Various vitamins and supplements. (

Medical advice is often subject to change. For instance, a new trial that’s more thorough than previous studies might cast doubt on a current practice. Recent research has found that:

●Raising HDL (good) cholesterol with drugs does nothing to protect people against heart attacks, strokes and early death.

●Routine screening for prostate cancer is less likely to save lives and more likely to lead to substantial harm from subsequent treatment.

●People who receive a brain stent to prevent a second stroke are actually more likely to have another stroke or die sooner than those who take medication and make fairly rigorous lifestyle changes.

Here are some other familiar practices and assumptions that have recently been revised because of additional research.

Take multivitamins to improve your health

About 40 percent of American adults take multivitamins, even though there’s no proof that they prevent chronic diseases or premature death. Major health organizations that focus on cancer, diabetes and heart disease advise people to avoid supplements in favor of a diet rich in fruit, vegetables, whole grains and legumes.

Feed a cold, starve a fever?

Or is it starve a cold, feed a fever? Either way, there’s no scientific basis. For the flu, the American Lung Association emphasizes good nutrition and adequate liquids to speed recovery and prevent dehydration. For colds, the ALA recommends drinking plenty of water and/or juice to keep the lining of the nose and throat from drying out.

Always stretch before and after exercise

Countering a tradition among exercisers, a study presented at a meeting of the American Academy of Orthopaedic Surgeons in 2011 found that stretching before running didn’t prevent injuries. Two systematic reviews also concluded that stretching before or after exercising didn’t reduce muscle soreness or injury. Instead, do a five-minute warm-up at a fairly easy intensity to increase range of motion and make exercise easier.

Body mass index is best for diagnosing weight issues

Obesity is defined as an excess of body fat, which increases the risk of Type 2 diabetes, heart disease and stroke. And BMI, calculated using a person’s height and weight, is considered the standard for gauging it. (To determine your BMI, multiply your weight in pounds by 703, then divide by your height, in inches, squared. Or use the calculator at www.

But the index doesn’t account for differences in body-fat composition, fitness, sex, race or age, and recent research underscores its limitations. In a 2010 meta-analysis involving nearly 32,000 people and published in the International Journal of Obesity, the BMI for obesity (30 and above) failed to identify half of the people with excess body fat.

Experts now suggest that you consider both your BMI and your waist circumference in assessing your fitness. A waist measurement greater than 40 inches for men or 35 inches for women indicates an increased risk for obesity-related conditions.

Everyone needs annual dental X-rays

If your teeth are X-rayed as soon as you settle into the dental chair, your dentist isn’t in step with current American Dental Association guidelines. To minimize radiation exposure, the ADA advises dentists to examine patients first and then order X-rays if and where they’re needed. You can get further protection by requesting a lead apron and thyroid collar, asking your dentist to use the fastest film (E- or F-speed) and having previous X-rays forwarded to your new dentist if you switch providers.

Drink at least eight glasses of water a day

This advice persists despite an absence of scientific support. And while you might drink extra water as part of a weight-loss regimen, it’s not clear that doing so will help you lose weight. The Institute of Medicine reports that most healthy people get the water they need by letting their thirst guide them and by consuming it through various sources, including other liquids and food.

Copyright 2012. Consumers Union of United States Inc.