Sunday, March 1, 2015
New Dietary Guidelines, Better Science-Low Fat-Low Salt Diets Don't Prove Up
For decades, many dietary recommendations have revolved around consuming a low percentage of your daily calories from fat. It has been widely thought that doing so would reduce your chance of having coronary heart disease. Most of the evidence for that recommendation has come from epidemiologic studies, which can be flawed.
But last week, the government started to address that problem, proposing new guidelines that in some cases are more in line with evidence from randomized controlled trials, a more rigorous form of scientific research.
Just recently, a study was published in the journal Open Heart in which researchers performed a systematic review and meta-analysis of the randomized controlled trials that were available when those guidelines were announced. They wanted to explore what evidence those creating the guidelines might have been able to consider at the time. The study did show that cholesterol levels went down more in the groups that ate low-fat diets. Some have used this as justification for a low-fat diet. But the difference between them was small. And these groups didn’t have different clinical outcomes, and that’s what we really care about.
Small changes in cholesterol levels from dietary changes also aren’t surprising to those who follow the research. About 70 percent of people are thought to be “hyporesponders” to dietary cholesterol. This means that after consuming three eggs a day for 30 days, they would see no increase in their plasma cholesterol ratios. Their cholesterol levels have almost no relationship to what they eat.
In 2013, researchers published a systematic review of all studies from 2003 or after. In most studies, all people didn’t respond. In the rest, only a minority of patients responded to changes in dietary cholesterol. A government committee urged repeal of the guideline that Americans limit their cholesterol intake to 300 milligrams a day, saying, “Cholesterol is not a nutrient of concern for overconsumption.
Then there is the issue of salt. A systematic review of randomized controlled trials of salt intake was published last year. Eight trials involving more than 7,200 participants looked at whether advising patients to cut down on salt, or reducing sodium intake, affected outcomes. None of the trials, including ones involving people with both normal and high blood pressure, showed a reduction in all-cause mortality.
Since pretty much all calories come from fat, protein or carbohydrates, reducing your consumption of one means that you have to increase your consumption of another. So, as the guidelines have recommended cutting down on meat, especially red meat, this meant that many people began to increase their consumption of carbohydrates. Many now believe that excessive carbohydrate consumption may be contributing to the obesity and diabetes epidemics. A Cochrane Review of all randomized controlled trials of reduced or modified dietary fat interventions found that replacing fat with carbohydrates does not protect even against cardiovascular problems, let alone death.
Read about this in the New York Times on February 23, 2015 in an article written by Dr. Aaron Carroll. Aaron E. Carroll is a professor of pediatrics at Indiana University School of Medicine. He blogs on health research and policy at The Incidental Economist, and you can follow him on Twitter at @aaronecarroll.
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