Recently, I was delighted to report on a study that suggested overweight seniors may actually live longer. Wouldn't you know it, a few days later I received a health newsletter that threw water on this idea of "benign overweight."

Then, a few days later, I got the April 2014 issue of the Nutrition Action Health Newsletter. Its cover story questioned why new health studies seem to flip-flop so often.

Naturally, when a new study overturns everything we've ever heard (e.g., overweight seniors live longer), it garners more press than studies that simply repeat the common wisdom. Headlines tout "earthshaking findings" and downplay humdrum findings. We see this pattern everywhere in the media.

The newsletter cover story gives other examples that explain this flip-flopping.

Cause and effect might be reversed

For instance, NBC's Today Show featured this headline on its website: "Drinking diet sodas makes you eat more." To support the claim, the study reported that overweight or obese diet soda drinkers consumed as many calories as drinkers of sugary sodas.

But that fact doesn't mean diet sodas cause people to eat more or gain weight. Overweight people tend to consume more diet beverages because they are trying to lose weight.

Well-designed, double-blind trials with long-term follow-up have found that people who drank regular, sugary sodas gained more weight and fat.

A link doesn't prove cause and effect

Andrew Weil is one of many popular doctor-hucksters. He suggests adults should take four antioxidants a day—vitamins C and E, selenium, and mixed carotenoids (including beta-carotene).

Maybe. But a new study suggests that high-dose antioxidant supplements may cause tumors to grow by reducing p53, a protein that suppresses cancerous growths. I'll add my own caveat here: This was an animal—not human—study.

This new study is just one of many trials—testing thousands of people—that concluded antioxidant supplements either had no effect or, in a few cases, actually increased cancer risk.

Researchers launched antioxidant trials because of studies that linked antioxidant consumption with reduced cancer risk. But correlation doesn't prove causation; something else could be at work.

For example, higher blood levels of beta-carotene and vitamin C may indicate higher consumption of fruits and vegetables. But people who eat more fruits and vegetables may be more health conscious, may exercise more and may follow healthier diets.

Placebo power

Scientists try to adjust for variables like these, but they can't adjust for them all, particularly ones they don't even suspect. Here's an eye-opening example: People in a trial are more likely to have lower risk of heart disease, cancer, and early mortality if they are faithful about taking all of their pills, even if they are placebos.

Among 13,000 people assigned to take a placebo in the Women's Health Initiative, those who took the pills faithfully had 50 percent lower risk of hip fracture, 30 percent less risk of heart attack, 40 percent less risk of dying from cancer and 35 percent lower risk of dying from any cause—compared to those women who took their placebos less than 80 percent of the time.

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Not ready for prime time

In 2012, ABC News reported on a new Harvard University study which found that the quantity of calories consumed was not as important as their quality.

The study put 21 overweight or obese adults on one of three diets: low fat, low carb or low glycemic index. The study ran for one month and all diets provided the same number of calories. The authors concluded that participants on the low-carb diet burned 300 more calories each day than their counterparts on the low-fat diet, and 150 calories more than the low-glycemic dieters.

One author told a reporter that a 300-calories difference was "roughly equivalent to an hour of moderate physical activity without lifting a finger."

What's the catch?

For starters, the diets didn't make a difference in pounds gained or lost. If people burned more calories on the low-carb diet—but didn't lose more weight after a month—more study is clearly needed.

Even if a low-carb diet led to more weight loss, it's not clear people would stick with the diet. The researchers served the study participants all their meals. The authors conceded that "we did not design the diets for long term practicality."

Another analysis—the "Pounds Lost" study—was the largest (811 people) and longest (two years) to test whether people lose more weight on low-fat, low-carb, or high-protein diets. No one diet stood out by producing significant results. Participants lost an average of 13 pounds after six months and kept off an average of nine pounds after two years, regardless of which diet they followed. The lead author said, "Ignore all the hype about diets that make pounds melt away. Losing weight comes down to how much food you put in your mouth."

This study has been criticized because it couldn't guarantee that participants maintained pure low-fat, low-carb or high-protein diets over the course of the two-year study. The researchers said, "If participants in a study can't stick with a diet, it's even harder for people to do it on their own."