In past years, much attention has been focused on reducing dietary fat to achieve reductions in serum cholesterol and lower the risk for the development of coronary heart disease. In 1993, the National Institutes of Health modified their earlier recommendations concerning modification of dietary fat and cholesterol in order to achieve appropriate reductions in serum cholesterol. These dietary recommendations consist of a step 1 diet and a more stringent step 2 diet with further reductions in dietary cholesterol and saturated fat. These recommendations are as follows:
Nutrient Recommended Intake
Step 1 Diet Step 2 Diet Total Fat 30% or less of total calories Saturated Fat 8-10% of total calories Less than 7% of total calories Polyunsaturated Fat Up to 10% of total calories Monounsaturated Fat Up to 15% of total calories Carbohydrates 55% or more of total calories Protein 15% of total calories Cholesterol Less than 300 mg/day Less than 200 mg/day Total calories To achieve and maintain desirable weight
Since fat has been labeled as a culprit, several preventive health advocates have taken the approach that extreme dietary fat reductions to 10% or less of total caloric intake might be better.
Two recent studies, however, contradict this philosophy.
Knopp et al (JAMA-November 12, 1997) published a study looking at 444 hypercholesterolemic men. He found that a reduction in fat intake below 26-28% total caloric intake not only failed to lower cholesterol further, but was actually less effective in cholesterol reduction than the moderate fat diet. He also found that in many cases the more restrictive low fat diets resulted in a lowering of the good or HDL cholesterol and a rise in triglycerides. Both low HDL levels and high triglycerides have been linked to an increased incidence of coronary heart disease.
Gillman et al (JAMA-December 24, 1997) published a study looking at 832 middle aged men and found that reductions in total fat and, in particular, reductions in saturated and monounsaturated fat were associated with an increased incidence of ischemic strokes. Reduction in polyunsaturated fats did not appear to play a role. These findings are consistent with studies on Japanese men who consume diets with little dietary fat (~10%). These men have low rates of coronary heart disease but a much higher rate of stroke. Examining the data presented by Gillman it appears that the risk of stroke increases significantly when dietary fat intake is reduced below ~32% total caloric intake.
Much attention has also been given to the type of dietary fat and the role that they may play in human physiology.
Saturated fats found in animals, dairy products and tropical oils have long been know to raise the bad or LDL cholesterol and to increase the risk of coronary heart disease. It appears they are protective in the development of ischemic stroke. This is not surprising because the physiology of coronary heart disease involves large vessel atherosclerosis whereas ischemic stroke involves the integrity of the small vessels within the brain. Entirely different mechanisms are probably involved.
Polyunsaturated fats are found in plants such as sunflowers, soybeans, corn and safflower. They effectively lower LDL cholesterol when substituted for saturated fat, but lower HDL as well. They do not appear to protect against the development of ischemic stroke.
Monounsaturated fats found in olive oil, canola oil and certain kinds of sunflower and safflower oils lower LDL cholesterol as well as polyunsaturated fat, but do not lower HDL cholesterol. They do appear to protect against ischemic stroke.
In this regard, Hu et al (NEJM-November 20, 1997) concluded, in a trial looking at 80,082 women, that replacement of 5% of energy from saturated fat with unsaturated fats would result in a 42% risk reduction for coronary heart disease. Replacement of just 2% of energy from trans unsaturated fats with unsaturated fats would result in a 53% risk reduction for coronary heart disease.
In conclusion, these recent studies continue to support the recommendations made by the National Institutes of Health. Fat reduction to ~ 30% of total calories, avoidance of both saturated and hydrogenated fats by substitution with both monounsaturated and polyunsaturated fats lead to both health and longevity. Extremely low fat diets are probably dangerous.
