We've been told for decades to lower our intake of saturated fat to guard against heart disease. The rationale: Saturated fat can raise LDL (bad) cholesterol in the blood, a risk factor for heart attack and stroke.
But according to a new report that will be published in the March issue of the American Journal of Clinical Nutrition, there's no evidence that saturated fat is linked with a higher risk of heart disease.
Saturated fat is found mainly in animal foods such as meat, poultry, eggs and dairy products. The fattier the food - for example, rib-eye compared with sirloin steak or cream compared with milk - the higher the saturated fat content.
While it's true that lab animals fed diets high in saturated fat develop fatty plaques in their arteries, not all studies suggest this is the case in humans.
In the current analysis, researchers combined the results of 21 previous studies that followed 347,747 adults, aged 30 to 89, for five to 23 years. During that time, 11,000 participants developed heart disease or suffered a stroke.
Over all, there was no difference in the risks of heart disease or stroke between people with the highest and lowest intakes of saturated fat.
These findings may seem odd if you've followed a low saturated fat diet and successfully reduced your LDL blood cholesterol. Shouldn't a lower cholesterol level translate into less heart disease?
LDL cholesterol particles differ in shape and size, and some LDL particles are thought to carry a greater risk of heart disease than others. Small, dense LDL particles have been implicated as being more strongly involved in heart disease than larger, fluffy ones.
Small LDL particles are able to slip through the lining of artery walls. They're also more easily oxidized, or damaged, by harmful free radicals that roam the body. Oxidized LDL cholesterol is more likely to form cholesterol-rich plaques in artery walls. Research indicates that people with small LDL particles have a threefold higher risk of heart disease compared to those with large particles.
It turns out that the reduction in LDL cholesterol that's known to occur by curbing saturated fat appears to be specific to large, fluffy LDL particles.
Not all types of saturated fat raise cholesterol to the same degree. The saturated fat in dairy products (called lauric acid) elevates LDL blood cholesterol, whereas that in meat and chocolate (called stearic acid) does not. However, lauric acid also raises HDL (good) cholesterol, offsetting some of its negative effect.
Given our current scientific knowledge, it's no longer possible to focus on a single nutrient when it comes to diet and heart health. Growing evidence suggests your overall diet matters more than just your saturated fat intake.
Saturated v. polyunsaturated fats
The studies that have shown a beneficial effect on heart disease risk by lowering saturated fat are ones that have replaced saturated fat with polyunsaturated fat. The main sources of polyunsaturated fat include cooking oils (e.g. sunflower, safflower, soybean, corn, grape seed, hemp, flaxseed and walnut oil) and oily fish.
In people with high blood cholesterol, replacement of saturated fat with polyunsaturated fat has been reported to reduce the liver's production of LDL cholesterol and speed its removal from the bloodstream. Lowering saturated fat and increasing polyunsaturated fat has also been shown to decrease hardening of the arteries in animals.
The amount of polyunsaturated fat you consume relative to saturated fat appears to be more significantly related to heart disease than saturated fat alone. In other words, cutting back on saturated fat doesn't seem to lower the risk for heart disease unless you also increase your polyunsaturated fat intake. In the current analysis of 21 studies, only one investigated the ratio of these two types of fat consumed by participants.
The carb connection
When it comes to preventing heart disease, dietary carbohydrate also must be addressed. Substituting saturated fat with a high intake of carbohydrate-rich foods such as bread, rice, pasta and low-fat baked goods has not been shown to protect against heart disease. In fact, doing so may increase the risk in certain people.
Replacing saturated fat with carbohydrate - especially refined - in conjunction with obesity can create a metabolic environment that favours heart disease by boosting blood triglycerides (fat), lowering HDL cholesterol and increasing small, dense LDL particles.
Evidence suggests that limiting carbohydrate intake can reduce elevated blood triglycerides and dense LDL cholesterol, and increase HDL cholesterol.
The type of carbohydrate you eat, measured by the glycemic index (GI), may also play a role in heart disease. The GI ranks carbohydrate-rich foods by how quickly they raise blood sugar (glucose).
High-glycemic foods such as white bread, white rice, refined breakfast cereals, sweets and sugary drinks are digested quickly and cause faster spikes in blood glucose. Over time, that can lead to insulin resistance, a condition in which the body cannot effectively remove glucose from the bloodstream. Insulin resistance, in turn, increases the risk for developing Type 2 diabetes and metabolic syndrome, two potent risk factors for heart disease.
Low-glycemic foods are more gradually converted to blood glucose and include grainy breads, brown rice and pasta, sweet potatoes, bran cereals, steel cut or large flake oatmeal, legumes, yogurt, milk and most types of fruit.
Some studies, but not all, have linked a high-glycemic diet from refined carbohydrates with a greater risk of heart disease.
The thinking on diet and heart disease is moving away from focusing on saturated fat. Limiting refined carbohydrates (e.g. sugar and white flour products), losing excess weight and emphasizing polyunsaturated fats from vegetables oils and fish appear to do more to lower your risk of heart disease than simply giving up cheese.
Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com.