A Presidential Advisory Unmasks a Definitive Answer
On June 15, 2017 the American Heart Association published a meta-analysis of studies in the form of a Presidential Advisory concerning dietary fats and their relationship to cardiovascular disease (CVD) (1). A Presidential Advisory is initiated by the president of the American Heart Association when action is needed to address a topic of current importance. The topic of saturated fat and heart health has been making headlines recently leaving in its wake deep confusion among medical practitioners and the public alike. This advisory is a systematic literature review and meta-analysis that explores study results, interpretations and methods to reach a definitive determination about the relationship between dietary saturated fat and the risk of CVD. Its conclusions are based on the weight of evidence coming from a wide range of trials and studies.
The problem with recent meta-analyses of the effect of saturated fat on the risk of CVD is twofold. Firstly, the macronutrient (fat, protein or carbohydrate) used to replace the saturated fat in some studies is not specified. All macronutrients have ramifications of their own, be they beneficial or harmful. To neglect to take the replacement food into account is to ignore a very important part of the answer to the question being explored. This omission alone is a major cause of the apparent discrepancies among studies. Secondly, many studies do not conform to high standards of design and method. Some studies have no control group for comparison or any means to check that the study subjects are actually adhering to their diet. Many studies are of too short a duration. Changes in tissue levels of macronutrients such as fatty acids occur very slowly. In fact it can take about two years to reach even 60% to 70% of the full effect of a change. The Presidential Advisory that we are focusing on here considers only studies of high standards which have one defined replacement food, that being substitution of saturated fat with polyunsaturated and/or monounsaturated fat.
Results from the main studies analyzed
From the vast pool of studies and meta-analyses that have been completed on the subject of saturated fat and CVD, four randomized controlled studies were chosen to examine in depth based on their excellent design and methodology as discussed above. These are the Wadsworth Hospital and Veterans Administration Center study from Los Angeles (2), the Oslo Diet-Heart Study (3), the British Medical Research Council study (4) and the Finnish Mental Hospital Study (5,6,7). All four of these studies compared high saturated fat intake with high polyunsaturated fat intake and looked at the resulting changes in cholesterol and rates of cardiovascular disease events.
The results of these studies showed remarkable agreement in outcomes. Replacing saturated fat with polyunsaturated fat resulted in;
Lowered blood cholesterol levels by 13% to 16%
Lowered total CVD events by 31%
Lowered rate of myocardial infarction, ischemic stroke or sudden death by 34%
Lowered rate of angina pectoris by 66%
Lowered coronary heart disease death rate by 41%
Results from other studies
The four studies mentioned above that were intensively analyzed are all randomized controlled studies. Prospective observational studies are another type of study that are carried out by a different method. They look at large populations of people over time and correlate their diet and lifestyle behaviours with their ongoing health and the occurrence of disease. The advantage of this type of trial over randomized controlled trials is the ability to study much larger and more representative populations for longer durations. Adherence has no relevance because participants choose their own food intake. These studies are also much less expensive. The weakness of prospective trials is so called “confounding factors” such as the fact that participants who are alike in one eating pattern might also be similar in other related habits. With meticulous collection of diet and health information and robust statistical methods, confounding factors can be reduced or even eliminated in order to isolate the effect of one nutrient. Prospective observational studies and randomized controlled trials are actually very complementary research approaches and, when their results are similar, causation is strengthened. For example, prospective studies show that replacing 5% of energy intake from saturated fats with the equivalent energy intake from polyunsaturated fats, monounsaturated fats or carbohydrates from whole grains is highly significantly associated with reductions in coronary heart disease risk – 25% reduction for polyunsaturated fats, 15% risk reduction for monounsaturated fats and 9% risk reduction for whole grains. Replacing saturated fats with carbohydrates from refined starch and added sugar showed a 1% higher incidence of coronary heart disease (8).
Nonhuman primate species are evolutionarily very similar to human beings and can be studied to elucidate the effects of diet on atherosclerosis. Experiments have compared monkeys on diets of lard/palm oil, monounsaturated high-oleic acid safflower oil or polyunsaturated high-linoleic acid safflower oil (9,10,11). The saturated fatty acid diet promoted higher LDL cholesterol and more coronary artery atherosclerosis. The polyunsaturated acid diet lowered LDL cholesterol and decreased coronary artery atherosclerosis. In contrast, though the monounsaturated acid diet lowered LDL cholesterol by the same amount as that elicited by the polyunsaturated diet, the amount of coronary artery atherosclerosis was much higher, closer to that of the saturated fat group.
Scientific evidence reveals that LDL cholesterol blood levels are very strongly linked to saturated fats in the diet (15,16,17.18,19,20,21,22). This evidence has been rated as “Level A – Strong” by three independent guidelines committees (12,13,14).
Coconut oil is believed to be a “healthy food” by many people. This can be attributed to the marketing of coconut oil in the popular press. The fatty acid profile of coconut oil is 82% saturated fat. A carefully controlled experiment compared the effects of coconut oil, butter and safflower oil on LDL. Both butter and coconut oil raised LDL compared to safflower oil (23). Another excellent study found that coconut oil significantly increased LDL cholesterol compared to olive oil (24). Additionally, a meta-analysis of seven controlled trials uncovered the fact that there is no difference in the amount of increase in LDL cholesterol caused by coconut oil or other foods high in saturated fats such as butter, beef fat or palm oil (25).
Dairy fats are composed of about 51% saturated fatty acids. Besides some monounsaturated and polyunsaturated fatty acids they also contain about 4% trans-fat which is produced by bacteria in the guts of cows. Many controlled trials show that dairy fat increases LDL cholesterol compared to monounsaturated and polyunsaturated fats. Prospective observational studies have found that substituting 5% of total daily calories from dairy with polyunsaturated fat resulted in a 24% to 25% lower risk of coronary heart disease and stroke (26). Substituting refined carbohydrates for dairy fat showed no reduced risk of CVD, however substituting carbohydrates from whole grains for dairy fat showed a 34% lower incidence of coronary heart disease and a 16% lower incidence of stroke (26). A nationwide health project performed in Finland with the goal of lowering the population’s extremely high CVD rate substituted high-fat milk and butter with increased fruit and vegetables, decreased sugar and sodium, increased fish, and a change to lean meats. The result was a decrease in death by coronary heart disease by 55% in men and 68% in women (27).
To sum it all up, the American Heart Association’s Presidential Advisory states;
Replacing saturated fat from dairy and meat with polyunsaturated fat from vegetable oils (but not monounsaturated fat) can prevent atherosclerosis and can reverse atherosclerosis in non-human primates.
Replacing saturated fat with polyunsaturated and/or monounsaturated fat lowers rates of CVD and all-cause mortality and, in addition, lowers triglyceride levels (an independent biomarker of CVD risk).
Saturated fat intake increases LDL cholesterol, a major cause of atherosclerosis and CVD. Replacement with polyunsaturated fat or monounsaturated fat decreases LDL cholesterol.
Polyunsaturated fat from vegetable oils (oils containing mainly omega-3 and omega-6 fatty acids) reduces CVD somewhat more than monounsaturated fat (oils containing mainly oleic acid such as olive oil).
Refined carbohydrates and added sugars are detrimental by about the same degree as are saturated fats.
Plant-based sources of saturated fat (cocoa and coconut) are perhaps less harmful than animal-based sources but are not beneficial. Unsaturated fat from nuts and seeds, whole olives and avocados are advantageous.
Unsaturated fats from fish and seafood offer sources of good fat except for the fact that harmful toxins such as pollutants and heavy metals as well as the detrimental effects of animal protein itself are ingested right along with them (28,29,30).
Vegetables, fruits, beans, lentils and whole grains are desirable foods for health.
One final thought
This advisory highlights the fact that eating meat, butter, cheese and other sources of saturated fats are simply not good for us. It applies the overwhelming weight of evidence to elucidate better dietary choices for protection against cardiovascular disease. We must remember however that numerous other health factors need to be considered before we conclude that replacement of saturated fats with other fats is the ultimate answer for health.
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