"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."
-- Mark Twain*
One area where that quote clearly applies is nutrition.
It seems like every week a new study comes along that turns what we thought we knew upside down. I'm not talking about all the diet books pointing people in all different directions. I'm speaking of credible scientific research that reaches conflicting conclusions.
In the past week, I read about two such studies -- one making a case for high-fat dairy products and another saying our bodies might do well with a higher salt intake.
USA Today reported a major European study that found eating full-fat versions of cheese, milk and yogurt does not increase the risk of heart attacks, cardiovascular disease or strokes, challenging the view that these foods can cause health problems.
The findings come from a meta-analysis of 29 previous studies involving more than 900,000 people around the world.
"There's quite a widespread but mistaken belief among the public that dairy products in general can be bad for you, but that's misconception," said Ian Givens, a nutrition professor at England's Reading University and one of the researchers.
"There's been a lot of publicity over the last five to 10 years about how saturated fats increase the risk of cardiovascular disease, and a belief has grown up that they must increase the risk, but they don't."
In fact, shunning high-fat dairy products could deprive people of calcium and interfere with bone development, he said.
Not surprisingly, the study drew some sharp rebukes.
The findings were criticized as "unhelpful and misleading" by Mike Knapton, associate medical director at the British Heart Institute.
"Decades of research have proved that a diet rich in saturated fat increases 'bad' (LDL) cholesterol in your blood, which puts you at greater risk of a heart attack or stroke," he said.
The research on salt (sodium chloride) reported in The New York Times was no less confounding.
We all know that eating something salty like a bag of potato chips makes us thirsty. That's the body's way of getting us to drink more fluids to maintain the proper sodium level in our blood, right?
Wrong, says extensive research led by Dr. Jens Titze, a kidney specialist at Vanderbilt University Medical Center.
His studies found that eating more salt actually made people less thirsty but more hungry. Subsequent experiments with mice showed the same. When the mice were given more salt, they consumed less water but ate 25 percent more just to maintain their weight.
The research suggests that more salt leads the body to produce glucocorticoid hormones that break down fat and muscle tissue to free up water for the body to use to dilute the blood's sodium level.
The studies call into question salt's role in gaining or losing weight. While scientists have thought a higher salt intake increases fluid intake and in turn increase weight, this research indicates that more salt leads the body to burn more calories to break down tissue.
The researchers, however, don't want to encourage people to eat more salt to lose weight for three reasons: (1) The increased hunger caused by added salt can lead to overeating, (2) more salt often causes higher blood pressure, and (3) the higher hormone levels have been linked to health problems including osteoporosis and diabetes.
"The work suggests that we really do not understand the effect of sodium chloride on the body," said Dr. Melanie Hoenig, an assistant professor at Harvard Medical School.
Why are nutrition studies so often at odds?
The gold standard for the research is a randomized, double-blind, controlled study, and such work is difficult. People need to strictly follow a particular diet, not cheat and list everything they eat. That's not easy, and not everyone can remember everything they ate the day before.
Another complication is genetic differences that cause different people to have different responses to some foods. One other issue is subtle bias in some studies that comes from sponsorship by corporations seeking to achieve positive results for their products.
Nutrition isn't the only area where we need to be careful about being certain we know something.
* Take famous quotes, for example. The quotation at the top is widely attributed to Mark Twain. But when I checked it online, I found three sites that say those words rightfully belong to a different 19th century writer named Josh Billings.
That's another example of why it can be best not to be too sure of what you're sure about.
Reach Steve Wilson at email@example.com.