The positive effects of flaxseed on cardiovascular health

We often hear that all plants are created equal in terms of positive impact on health, and that the important thing is simply to eat them as often as possible, without worrying about the nature of the fruits, vegetables or seeds consumed. In other words, essentially it would be the quantity that counts, and there would be no difference between eating iceberg lettuce or broccoli, or blueberries rather than a banana. This reductionist view is somewhat outdated, because we now know that there are enormous differences in the biochemical composition of plants, and some of them are in a class of their own for their content in molecules known to have positive effects on health. The inclusion of these foods in dietary habits could thus enhance the benefits associated with a diet rich in plants, especially in terms of prevention of cardiovascular disease.

Flaxseed is a good example of a food that is different from other plants because of certain unique characteristics. On the one hand, these seeds contain very high amounts of polyunsaturated fatty acids (72% of all fats), with three quarters of these fats being the omega-3 type. On the other hand, flaxseed is an exceptional source of lignans, a group of polyphenols with antioxidant and anti-inflammatory properties. In recent years, several studies have suggested that these properties may have several positive effects on cardiovascular health.

Linolenic acid: A fat like no other
Flax seeds are one of only two foods of plant origin (the other being chia seeds) that contain more omega-3 fatty acids (linolenic acid) than omega-6 (linoleic acid). For example, while the omega-3/omega-6 ratio is less than 1 for all commonly consumed vegetable oils, this ratio is 4 for flaxseed, up to 500 times higher than some vegetable sources (sunflower, for example) (Table 1).

Table 1. Proportion (%) of the different types of fatty acids present in various plant sources. Adapted from Dubois (2007). 

This predominance of linolenic acid in flaxseed is very interesting because this omega-3 fatty acid has positive effects on several cardiovascular disease risk factors (lowering of LDL cholesterol, lowering of blood pressure), and also has anti-inflammatory and anti-arrhythmic properties. All of these properties may contribute to the decreased risk of cardiovascular events associated with linolenic acid consumption observed in several studies (see Table 2).

Table 2. Examples of studies reporting a positive effect of linolenic acid on cardiovascular health. From Rodriguez-Leyva (2010). 

Linolenic acid can also be converted into DHA and EPA, two long-chain omega-3 fatty acids that have repeatedly been associated with a reduced risk of cardiovascular mortality (this conversion to DHA and EPA is quite low, in the vicinity of 5%, but is generally higher in women). Therefore, while it is now known that simply replacing saturated fats (from animal products) in our diet with unsaturated vegetable fats significantly reduces the risk of developing cardiovascular disease, this protection could be even more important when these unsaturated fats are omega-3.

The Lyon Diet Heart Study is one of the most important demonstrations of the potential of these plant-based omega-3 fatty acids in preventing cardiovascular disease. In this study, 605 myocardial infarction survivors were randomly separated into two groups, one placed on a low-fat diet as recommended by the American Heart Association, and the other on a Mediterranean diet including margarine enriched in linolenic acid (1.1 g/day). After a two-year follow-up, the incidence of cardiovascular disease, including cardiac mortality, decreased dramatically (73%) in the intervention group, raising the interesting possibility that the inclusion of linolenic acid in the diet can significantly improve cardiovascular health.

In primary prevention, most subsequent epidemiological studies have shown that people who have a high intake of linolenic acid are less likely to be affected by cardiovascular disease, a protective effect that has been observed as much in the United States (see here and here), as in Europe (Holland) and Central America (Costa Rica). A meta-analysis of 13 prospective studies indicates that an increased linolenic acid intake of 1 g per day is associated with a 10% reduction in the risk of cardiovascular disease, a protection confirmed by data analysis from 8 American and European studies comprising a total of 148,675 women and 80,368 men. The risk reduction offered by linolenic acid could even be much higher (60%) for people whose intake of long-chain omega-3 (DHA and EPA, found mainly in oily fish) is low (<0.1 g/day). In sum, most of the data collected to date suggests that increased linolenic acid intake is associated with reduced risk of cardiovascular events.

Lignans: Protective phytoestrogens
Another unique feature of flaxseed is its exceptional content of lignans, a group of complex phenolic compounds that have antioxidant and anti-inflammatory properties. While the majority of plants contain relatively low levels of lignans, these molecules are present in much higher quantities in flaxseed (300 mg/100 g), over 1,000 times more than in some commonly consumed foods (Table 3).

Table 3. Main dietary sources of lignans. From Peterson, 2010.

These lignans, mainly secoisolariciresinol and matairesinol, are metabolized by the intestinal microbiota to enterolactone and enterodiol, two molecules that have a weak estrogenic action (phytoestrogens). Since oestrogens exert a cardioprotective action (and would be responsible for the lower incidence of cardiovascular disease in women compared to men), it has been suggested that the estrogenic properties of lignans could help reduce the risk of cardiovascular events. Some epidemiological studies that focused on this issue found that this is indeed the case, i.e., that a higher intake of lignans or an increase in the blood level of enterolactone (produced by the metabolism of lignans) is associated with a reduced risk of cardiovascular events.

Flaxseed and cardiovascular disease
One of the main limitations of studies on the cardioprotective role of linolenic acid and lignans is the low content of these molecules in the traditional western diet. For example, salad dressings are the main source of linolenic acid in several epidemiological studies, while the restricted distribution of lignans in plants means that their intake may be below the threshold required to generate important cardiovascular effects. Therefore, the simultaneous presence of significant amounts of linolenic acid and lignans in flaxseed suggests that the addition of these seeds to dietary habits represents a simple (and economical) way to overcome these deficiencies and improve the beneficial impact of these two classes of molecules on cardiovascular health.

So far, the best-documented effect of flaxseed supplementation is on lowering blood pressure. For example, a randomized, double-blind, placebo-controlled study in Puerto Rico (FLAX-PAD) found that adding 30 grams of ground flaxseed to the diet resulted in a significant decrease in systolic (10 mmHg) and diastolic (7 mmHg) blood pressure. This reduction is even more pronounced in people who were hypertensive at the start of the study (>140 mmHg), with a 15 mmHg reduction in systolic pressure, a decrease even more pronounced than that obtained with the help of some antihypertensive drugs. A meta-analysis of 11 studies on the impact of flaxseed supplementation on blood pressure, however, suggests a more modest antihypertensive effect, with a decrease of approximately 2 mmHg for systolic and 1.2 mmHg for diastolic pressure. This may not seem like much, but studies show that a reduction in blood pressure of this order could decrease stroke mortality by 10% and coronary heart disease by 7%.

It also appears that flaxseed supplementation may lower LDL cholesterol levels, another important risk factor for cardiovascular disease. A meta-analysis of 28 studies showed that flaxseed caused an average decrease of 0.10 mmol/L and 0.08 mmol/L in total cholesterol and LDL cholesterol, respectively, this effect being particularly pronounced in women (- 0.24 mmol/L) and in people who had high cholesterol levels at the beginning of the procedure. A 15% reduction in LDL cholesterol was also observed in patients with lower extremity osteoarthritis, this decrease being added to that caused by statin.

In recent years, much emphasis has been placed on the importance of regularly consuming long-chain omega-3 fatty acids (DHA and EPA), mainly found in oily fish such as salmon, to reduce the risk of cardiovascular diseases. It should not be forgotten, however, that omega-3 of plant origin also have a protective role and that a high intake of foods rich in linolenic acid, such as flax seeds, can also help reduce the risk of cardiovascular events. In fact, a number of studies (here and here, for example) have reported that both types of omega-3 have complementary roles, and a combined increase in linolenic and long-chain omega-3 intake may be desirable for maximum protective effect.

In practical terms, an average daily intake of 2.2 g of linolenic acid is recommended, which corresponds to one tablespoon (15 ml) of flaxseed. It is essential to grind the seeds to increase the absorption of omega-3 fatty acids and allow the transformation of lignans into active phytoestrogens by intestinal bacteria. However, omega-3 fatty acids being very fragile and sensitive to degradation, one should buy whole seeds that can be ground when needed in a simple coffee grinder, and store the ground seeds for a maximum of two weeks in the refrigerator in an airtight container. The ground seeds have a slightly nutty flavour that goes well with cereals, yogurts, smoothies, and can even be used as a salad topping.