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Quandary About Fats in the Diet

The mantra about reducing the risk of heart disease has been to replace saturated fats with unsaturated ones in the diet. If only it were that simple.

The human body is the most complex machine on the face of the Earth. Like any machine, it requires an input of fuel to function along with a variety of chemicals for proper maintenance. In this case, those requirements are met by food. But food is incredibly complex, even a simple meal is composed of hundreds of compounds. It is folly to think that simple predictions can be made about what may happen when something as complex as food is introduced into an organism as complex as a living body, especially given that the living bodies concerned can differ in genetics, gender, age and health status. Yet simple nutritional advice is routinely given!

For example, to reduce the risk of heart disease, we are told to replace saturated fats, as found in red meat, dairy products and tropical oils such as coconut with unsaturated ones found in seed oils. Unfortunately, the story is much more complicated. (A quick chemistry lesson here: saturated fats contain no carbon-carbon double bonds in their molecular structure, while unsaturated fats contain one or more double bonds.)

This discussion was prompted by a study I came across in which researchers in Nigeria determined the fat composition of insects, specifically bees, winged termites, soldier termites and mopane worms. Since such insects are eaten in some parts of Africa, their nutritional content is of interest. What caught my attention in this report was the following statement: “The higher the polyunsaturated fatty acid (PUFA) to saturated fatty acid (SFA) ratio, the more the nutritional usefulness of a dietary oil and the PUFA/SFA ratios in the present study are good enough to discourage atherosclerotic tendency.” Atherosclerosis is the buildup of plaque in arteries that can lead to cardiovascular disease. The implied message in this study is that eating insects is “heart healthy.”

This study really has no practical relevance for us since insects are not an option in the western diet. But portraying polyunsaturated fats as angelic and saturated fats as demons is way too simplistic. For example, a large study across nine European countries found no strong association between dietary saturated fats and heart disease. Neither was the incidence of heart disease decreased by substituting unsaturated fats for saturated ones. However, the situation turned out to be different when sources of the saturated fats were considered. Saturated fat in red meat or butter increased heart disease risk, but saturated fat in yogurt or cheese actually decreased it.

The standard advice for years has been to replace saturated fats with unsaturated vegetable fats. That seems to be justified when we consider the “heart healthy” Mediterranean diet in which unsaturated fats come mostly from olives, nuts and fish. But the story is different when saturated fats are replaced by seed oils such as soy, sunflower, cottonseed or corn, as is common in the western diet. The prominent fatty acid in these oils is linoleic acid (a so-called omega-6 fatty acid due to the presence of a double bond on the sixth carbon from the end of the molecule).

Uncomfortably, studies have shown that replacing saturated fats with vegetable oils that contain linoleic acid actually increases heart disease risk. This despite total cholesterol and LDL cholesterol being decreased! That is surprising since reducing blood cholesterol is generally thought to be desirable. Indeed, numerous studies have shown that decreasing elevated cholesterol with the use of drugs such as the statins reduces the risk of heart disease, but studies that have focused on reducing cholesterol via diet have been inconclusive. This may be because statins also have an antioxidant effect that can mitigate the risk of heart disease.

The problem with linoleic acid is its susceptibility to attack by oxygen, in other words to be oxidized. When linoleic acid gets incorporated into LDL particles, as happens with fatty acids, it undergoes reaction with oxygen and breaks down to form “oxidation products” such as malondialdehyde and 4-hydroxynonenal. When LDL containing these oxidation products deposits in the walls of arteries to form plaque, it causes inflammation that in turn makes the plaque prone to rupture. When that happens, a blood clot can form, precipitating a heart attack or stroke. By contrast, omega-3 fats, as found in fish, are also polyunsaturated fats but are not prone to oxidation and replacing saturated fats with these appears to be protective.

Now back to the insects. Their fat content is mostly linoleic acid. So, the suggestion that eating insects may reduce the risk of heart disease because of a “favourable PUFA/SFA profile” is wrong. If the polyunsaturated fatty acids are mostly linoleic acid, the profile is not favourable.

Obviously, the link between diet and heart disease is frustratingly complex, but labeling polyunsaturated seed oils, increasingly found in processed foods, as “healthy,” is misguided. So is labelling all saturated fats as “unhealthy.” While in general they do raise cholesterol, the extent to which they do this depends on their molecular structure. The ones that contain 14 or 16 carbon atoms, namely myristic and palmitic acids, raise cholesterol, while lauric and stearic acids with their 12 and 18 carbon chains are relatively innocuous.

To complicate the picture further, saturated fats also raise HDL cholesterol, the so-called “good cholesterol.” Actually, the best measure of risk is not LDL, but the ratio of total cholesterol to HDL. To insert some practical numbers here, total cholesterol should be less than 5.2 mmol/L and HDL should be above 1.5mmol/L meaning that the ratio should ideally be less than 3.5. However, a diet in which saturated fats are substituted by seed oils high in linoleic acid can decrease this ratio but still increase the risk of heart disease!

If that isn’t complicated enough, determining the number of LDL particles in the blood is a better measure of risk than just measuring LDL cholesterol levels. Apolipoprotein B, or ApoB, is a protein found in LDL particles and the extent of its presence is indicative of the number of LDL particles. Levels above 1.2 grams/L are deemed to be elevated.

Now let’s try to uncomplicate things. Talking about total intake of saturated and unsaturated fats is not meaningful because fatty acids differ in their effects, and the effects are also dependent on the matrix of the food in which they are present. Therefore, it is much more meaningful to speak in terms of food rather than specific nutrients. Here goes. Limit red meat, white flour, sugar, salt, soft drinks and be wary of anything that comes in a box. Use oils such as canola, olive, or avocado that are low in linoleic acid and in saturated fats. Eat 5-7 servings of fruits and vegetables a day with at least one of berries. Cheese and yogurt are fine. Emphasize fish and whole grains. Exercise 30 minutes at least five times a week. Not so complicated. And oh yes, select your parents carefully.


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