Wednesday, March 24, 2010
To Eat Saturates or Not to Eat Saturates?
As we discussed in the previous post, saturated fats are not fats that are saturated with calories or saturated with cholesterol. "Saturated" is a chemical term that can be thought of as a measure of the stability of a particular fat in the presence of heat, light and oxygen. Saturated fats are more stable than monounsaturated fats and much more stable than polyunsaturated fats. Saturated fats are generally solid at room temperature. The richest sources are from animals (lard, butter, tallow) or from tropical plants (coconut oil, palm oil).
When heart disease began to become prevalent in the 20th century, scientists looked for a cause and decided that saturated fats were a good candidate. By the end of the 20th century most people thought that the science was settled--eating saturated fats causes heart disease. But studies are starting to accumulate that suggest that this isn't necessarily so.
Two articles (abstracts here and here) by Ronald Krauss and colleagues were recently published in the American Journal of Clinical Nutrition. They did an analysis of twenty one prospective cohort studies (i.e., these were actual clinical studies, not just correlational number crunching) and examined the relationship between intake of saturated fat and the risk of coronary heart disease, stroke and cardiovascular disease. This type of analysis depends on the integrity of the authors in selecting the studies to be analyzed, especially if the authors have a bias toward a particular outcome. With that caveat, it is noteworthy that the authors concluded that "there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD [coronary heart disease] or CVD [coronary vascular disease]." (There was also no significant association with stroke, and the authors stated that results were not affected by age, sex or the quality of a particular study.)
The authors noted that when studies replaced saturated fat with a higher carbohydrate intake, this resulted in increased triglycerides, smaller LDL particles and reduced HDL cholesterol. In the studies that replaced saturated fat with mono- or polyunsaturated fat, patients saw a reduction in their LDL cholesterol, but they also reduced their "good" HDL cholesterol.
But these were the only studies that showed no relationship between saturated fat and cardiovascular disease, right? Not exactly.
In his blog, Dr. Michael Eades describes a group of 264 men who entered a study after experiencing their first heart attack. From 1957 to 1963 the treatment group ate a diet with about 13.5% saturated fat. The control group continued to eat their normal diet, which probably contained about 25% saturated fat according to Dr. Eades' ballpark estimate. After six years, both groups had the same heart attack relapse rate and the same death rate.
Eades also describes a study in which patients with ischemic heart disease ate either a high-saturated fat diet, a diet in which most of the fat was olive oil or a diet in which most of the fat was corn oil. After two years, 75% of the high-saturated fat group was alive and free from a second heart attack. Fifty seven percent of the olive oil group was alive and heart attack free at the completion of the study. And the corn oil group had only 52% alive and heart attack free at the end of two years.
Finally, a 2004 study by Mozaffarian et al. examined the progression of coronary atherosclerosis in postmenopausal women with established coronary heart disease. Comparing the intake of saturated fatty acid among these women over three years, they found that the intake of saturated fat was inversely related to the rate of progression of coronary atherosclerosis, and was unrelated both to unstable angina and to death from myocardial infarction. Unexpectedly, investigators saw a positive association between polyunsaturated fat intake and the rate of narrowing of the coronary arteries. This was particularly true in women with diabetes, lower HDL cholesterol, a lower protein intake and a higher carbohydrate intake. (It is important to note that this was a correlational study and the outcome could be affected by confounding factors that were not identified.)
To eat or not to eat saturated fat? It appears that it is up to the individual. Some studies have shown a slightly increased risk of heart disease with consumption of saturated fats, but many studies show no correlation at all. On the other hand, the increased consumption of carbohydrates, particularly refined carbohydrates, is associated with increased triglycerides, decreased LDL particle size, and decreased HDL cholesterol, all of which are associated with increased risk of cardiovascular disease. If we have to choose between fat and carbohydrate for heart health, it appears that fat may be the better choice.