Is the health advice you’re following outdated? Check this.

COLD AND FLU

OLD ADVICE: Take vitamin C to help avoid getting sick.

NEW ADVICE: Take vitamin D, too.

A cure for the common cold has been the Holy Grail of medicine forever. Old wives’ tales of treatment and prevention abound, from the benefits of chicken soup to the fussy counsel to bundle up in cold weather.

A study published in November and funded by the National Institute on Aging found that high doses of vitamin D (100,000 international units monthly for a year) reduced the incidence of acute respiratory infection in nursing home residents older than 60. Other research suggests the supplement may decrease the severity of colds or flu by lowering viral load and increasing immune function. Other studies show little or no effect on incidence or severity, however, so more work is needed.

Niamh Carroll, a primary care physician at Tufts Medical Center, says it’s not a bad idea to add vitamin D to your regimen, at least through the winter. The recommended dose is 600 IUs daily, or 800 for people older than 70. Carroll takes 1,000 IUs daily. “It’s hard to get [that much] through diet,” she says, “and most people in the Northeast are deficient in the winter months.”

BOTTOM LINE: Consider added vitamin D until sunny days are here again.

BACK PAIN

OLD ADVICE: Avoid activity.

NEW ADVICE: Keep moving.

“Just because you threw out your back and it hurts doesn’t mean you should rest it for an extended period,” says Ashley Rogerson, an orthopedic spine surgeon at Tufts Medical Center. Sit out that 5k, delegate the snow shoveling, and pop a few aspirin if it helps, but don’t immobilize yourself in bed. “Resume normal activities,” Rogerson says, like taking walks and even lifting light weights. “Most back pain is self-limiting; it will go away on its own.”

While this may not sound like news to some doctors — research supporting the idea goes back a decade or more — the message might not have gotten out to all of us. About 13 million US adults see their doctors because of back pain every year, but studies suggest that up to 95 percent of them will recover on their own within six months regardless of treatment. Unless the pain is accompanied by fever, numbness, tingling or weakness or occurs as the result of a serious injury, it’s safe to wait at least six weeks before seeing your primary care physician. “You can’t do it any damage,” says Rogerson. “In fact, recovery will be impeded if you rest it too much.”

BOTTOM LINE: Take it easy, but not for long.

ALZHEIMER’S DISEASE

OLD ADVICE: Keep your mind engaged by doing crossword puzzles.

NEW ADVICE: Smoke pot?

Well, don’t break out the bong just yet, but a study published in the June issue of Aging and the Mechanisms of Disease suggests that THC, the active ingredient in marijuana, seems to prevent both damaging inflammation and the accumulation of amyloid-b proteins, which are associated with Alzheimer’s. There is a caveat, however: While the new research bolsters the results of some earlier research, it was conducted on a tissue-culture model, not in patients.

Another recent paper detailed a 10-year study of 2,785 elders that found a targeted cognitive training program cut the risk of dementia almost in half. The University of South Florida’s School of Aging Studies and Byrd Alzheimer’s Institute conducted the research using material that was different from that found in popular programs like Lumosity (which earlier this year settled a lawsuit charging its own research did not back up its claim of protecting against cognitive decline). “Evidence for simply being cognitively active has receded,” says John Gabrieli, an investigator at the McGovern Institute for Brain Research at MIT. “But things where you push yourself for novel experiences by effort — new social activities and intellectual endeavors that are different and outside of your comfort level — there is some cumulative evidence that some people do benefit from that.”

BOTTOM LINE: THC might help, but novel and challenging activities are especially important.

OBESITY

OLD ADVICE: Have the willpower to exercise and watch your diet.

NEW ADVICE: Personalized medicine can help.

Two-thirds of American adults are now considered overweight, and half of that group is obese. “There’s minimal understanding among the general population and much of the medical community on how weight regulation works,” says Fatima Cody Stanford, an obesity specialist at Massachusetts General Hospital. “Until that understanding improves, diet and exercise alone will continue to fail again and again.”

Stanford says there are “100 potential reasons why you struggle with your weight.” Among them are economic factors; psychological issues; developmental aspects such as whether your parents were overweight when you were conceived; and biological and medical reasons, including medication interactions, gut microflora, and out-of-whack hormones, including cortisol, the stress regulator.

The best way to find the ones that affect you, Stanford says, is to see an obesity doctor. “Understanding the complexity helps people understand it’s not their fault,” she says. “Taking a more holistic view can make a big difference.”

BOTTOM LINE: Don’t try to go it alone.

HEART DISEASE

OLD ADVICE: Cholesterol, high blood pressure, and high blood sugar contribute to heart disease.

NEW ADVICE: They don’t really matter that much.

NEWEST ADVICE: Actually, yes, they do.

Recommendations over the years for preventing heart disease provide a classic example of why health information can be so confusing. “Two or three years ago we thought you didn’t have to be so intensive in lowering these measures,” says Christopher Cannon, a physician in the cardiovascular department of Brigham and Women’s Hospital. “That led to massive confusion among doctors and patients. Now research is coming out saying, ‘Oh, wait a minute, we had it right all along.’ We should be pushing lower on diabetes, blood pressure, and cholesterol.”

Goals to set: The three-month hemoglobin A1C test that measures blood sugar should register less than 7 percent, blood pressure should be less than 120 over 80, and LDL — or “bad” cholesterol — should be 70 mg/dL or lower. “Don’t think about the ‘good’ cholesterol, because it doesn’t do anything,” Cannon says. “We used to think you didn’t have to worry, because ‘high’ HDL cancels out LDL, but the answer is: ‘Wrong.’ It’s all about lowering the level of the bad. The ratio doesn’t matter.”

BOTTOM LINE: Get your numbers as low as possible.

Elizabeth Gehrman is a frequent contributor to the Globe Magazine. Send comments to magazine@globe.com.