Omega-3s Part 2—Research: UPDATE


Last updated October 2018

More Information on Omega-3s

Introduction to the Omega-3 Fatty Acids

For our purposes, there are three important omega-3 fatty acids:

  • alpha-linolenic acid (ALA) – short chain (18 carbon) omega-3 fatty acid. It is found in small amounts in animal flesh, in very small amounts in a variety of plant products, and in relatively large amounts in soy, walnuts, canola oil, flaxseeds and their oil, hempseed oil, camelina oil, and chia seed oil. The human body cannot make its own ALA—it must be obtained through the diet.
  • eicosapentaenoic acid (EPA) – long chain (20 carbon) omega-3 fatty acid. It is found mostly in fatty fish, in small amounts in eggs, and in very small amounts in seaweed that can be concentrated into supplements. Some EPA is converted into series 3 eicosanoids which can reduce blood clotting, inflammation, blood pressure, and cholesterol. The human body can produce EPA from ALA and, to a lesser extent, from DHA.
  • docosahexaenoic acid (DHA) – long chain (22 carbon) omega-3 fatty acid. It’s found mostly in fatty fish, in small amounts in eggs, and in very small amounts in seaweed that can be concentrated into supplements. DHA is a major component of the gray matter of the brain, and also found in the retina, testis, sperm, and cell membranes. The body can convert EPA into DHA.

All three of these omega-3 fatty acids might prevent heart arrhythmias, though ALA has been studied the least in clinical trials.

A chart showing the conversion pathways for the omega-3 fatty acids can be found in The Fatty Acids.

Essential Fatty Acids

The Institute of Medicine considers there to be a dietary requirement for two fatty acids for humans over age one:

  • linoleic acid (LA) – the short-chain (18-carbon) omega-6, which is prevalent in most vegan diets due to being abundant in vegetable oils.
  • ALA – the short-chain omega-3 (described above) which can be scarce in vegan diets.

Because they’re essential fatty acids, there’s a dietary reference intake (DRI) for both LA and ALA:

  • LA – 17 g (men age 19-50), 12 (women age 19-50)
  • ALA – 1.6 g (males age 14+), 1.1 g (females age 14+)

Heart Disease

As of May 2012, the American Heart Association was still basing its omega-3 fatty acid recommendations on its 2002 position paper, Fish consumption, fish oil, omega-3 fatty acids, and cardiovascular disease (31) which recommends that adults “Eat a variety of (preferably oily) fish at least twice a week. Include oils and foods rich in alpha-linolenic acid (flaxseed, canola, and soybean oils; flaxseed and walnuts).”

Whether fish or fish oil substantially reduces the risk of cardiovascular disease is somewhat controversial and a topic beyond the scope of this article. The Linus Pauling Institute at Oregon State University provides a good review of the literature in their article, Essential Fatty Acids. Vegetarians already have about a 24% lower risk of heart disease (5), and it is not clear that more EPA or DHA could further benefit them to any significant degree with respect to heart disease. Rather, the main concern for vegetarians is with the possibility of cognitive problems due to DHA deficiency.

A meta-analysis of five prospective studies of ALA intake published in 2004 (7) suggested that high ALA intake is also associated with reduced risk of fatal heart disease (relative risk 0.79, .60-1.04). This finding wasn’t quite statistically significant. The average highest level of intake was 2.0 g per day versus the lowest of .8 g per day.

Depression

A 2007 meta-analysis from Taiwan (8) and a 2006 review from Canada (9) found that supplementation with EPA and DHA improved depression, but a 2006 review from the UK (10) found that it didn’t. There’s some evidence that EPA may be more effective than DHA in treating depression (42).

Cognition

A 2010 USA study (51) found that adding 900 mg DHA to a 100 mg DHA/day baseline diet improved some cognitive tests in adults with age-related cognitive decline but without Alzheimer’s Disease. This is the most convincing evidence that omega-3s are needed for optimal cognition, but given that 1,000 mg per day is more of a pharmacological dose of DHA than a dietary intake, it’s not clear what relevance this has to omega-3 recommendations for the general population.

One cross-sectional study found that higher omega-3 status was associated with some aspects of better cognition. Low omega-3 status was associated with lower brain mass equivalent to approximately two years of brain aging (40).

Another cross-sectional study on women 65 years and older found no difference in cognition between those in the upper one-third compared to those in the lowest one-third of EPA-plus-DHA percentage of fatty acids in red blood cells (43). However, the lowest one-third had an average EPA plus DHA percentage of 3.8 which is quite a bit higher than average vegan percentages tend to be, so it’s not clear if this finding would apply to vegans who aren’t making an effort to increase their omega-3 intake.

A 2005 paper raises concerns about low DHA and Alzheimer’s Disease:

Docosahexaenoic acid (DHA), a dietary essential omega-3 fatty acid concentrated in membrane phospholipids at synapses and in retinal photoreceptors is decreased in [the Alzheimer disease] brain. This deficiency may be due to enhanced free radical–mediated lipid peroxidation, decreased dietary intake, and/or impaired liver DHA shuttling to the brain. Decreased DHA serum content correlates with cognitive impairment. Moreover, epidemiologic studies suggest neuroprotective consequences of diets enriched in omega-3 fatty acids (37).

Omega-3 Intakes and Blood Levels of Vegetarians

According to the USDA nutrient database, a medium egg contains about 2 mg of EPA and 16 mg of DHA. That provides lacto-ovo vegetarians with very small amounts of dietary EPA and DHA. Vegans who are not supplementing with EPA or DHA have an intake of essentially zero.

The table below shows studies that have measured the ALA intakes if vegetarians—who were presumably not purposefully adding rich sources of ALA to their diets.

It appears that vegetarians meet about 50–60% of the daily ALA recommendations without special diet planning.

ALA Intake of Vegetarians
Study Population Intake (g/day)
Australia, 1999 (15) 17 vegetarian men, ~26-42 yrs 1.9
UK, 1984 (25) 10 vegan men 1.8
UK, 1984 (25) 10 vegan women 1.2
UK, 2010 (28) 5 vegan men 1.0
UK, 2010 (28) 5 vegan women .9
USA, 2014 (39) 87 vegan women, 80 vegan men 3.4

Vegans and vegetarians have been shown in many studies to have lower blood levels of EPA and DHA than meat eaters. The table below shows the results of some of these studies. The general trend is that lacto-ovo vegetarians and vegans have lower levels of EPA and DHA in their blood. In the 2005 UK study (19), time as vegan wasn’t associated with lower EPA or DHA levels, suggesting that blood levels stabilize after an initial drop.

One exception is the 2010 UK study (28) in which vegan women had, on average, higher DHA levels than even the fish-eaters. There were only 5 vegan women making this finding unlikely to be statistically significant; the average DHA level was 286 µmol/l with a standard deviation of 211 µmol/l. That means that one or two of the vegan women had very high levels of DHA but some had very low. The researchers didn’t assess how long the vegans had been following their diet.

The 2014 USA study (39) was interesting for two reasons. One, vegans who were older (27 people 70 years or older, up to 85 years) had higher omega-3 levels than younger vegans. It’s usually thought that people have a harder time converting ALA to DPA and DHA as they age. Second, vegan men had similar levels to a segment of military males who didn’t eat fish (vegan women weren’t compared). In this study, vegan ALA intakes averaged 3.4 g per day which is relatively high—vegans usually have average intakes closer to 1.5 g per day.

Lower blood levels of EPA and, especially, DHA in vegetarians doesn’t necessarily mean that they have lower levels of EPA or DHA in other tissues, but it is something to be careful about until more is known and is the reason why we have a second set of more prudent recommendations beyond the dietary reference intake for ALA (see Daily Needs).

EPA and DHA Levels in Vegans
Number EPA DHA
1981 UK17 %PCPG %PCPG
Vegans 4 0.3 1.3
Non-Veg 5 1.4 4.1
%PG %PG
Vegans 4 0.3 0.9
Non-Veg 5 0.8 2.8
1992 UK16 %PTPG %PTPG
Vegans 20 0.2 0.8
Non-Veg 20 0.9 2.1
1999 Chile18 %PFA %PFA
Vegetarians 26 0.35 1.56
Non-Veg 26 0.79 2.58
2002 Australia33 %PPL %PPL
Vegan 18 1.5 0.9
Lacto-Ovo 43 1.9 1.2
Moderate Meat 60 1.9 1.6
High Meat 18 1.9 1.5
2005 UK19 mg/l mg/l
Vegan 232 .34 .70
Lacto-Ovo 231 .52 1.16
Non-Veg 196 .72 1.69
2010 UK28 µmol/l µmol/l
Men Vegan 5 65 195
Lacto-Ovo 25 56 222
Meat Eaters (no fish) 359 47 215
Fish Eaters 2,257 58 240
Women Vegan 5 50 286
Lacto-Ovo 51 55 224
Meat Eaters (no fish) 309 57 241
Fish Eaters 1,891 65 271
2014 USA39 %RBC %RBC
Vegan 40 .56 2.28
Meat Eaters
(low fish)
78 .40 2.61
2017 UK41 %PFA %PFA
Vegan 23 .47 .91
Meat Eaters 24 1.03 2.23
%RBC %RBC
Vegan 23 .67 2.07
Meat Eaters 24 1.26 4.19
%PCPG—percentage of plasma choline phosphoglycerides • %PG—percentage of plasma phosphoglycerides • %PFA—percentage of plasma fatty acids • %PTPG—percentage of platelet phosphoglycerides • %PPL—percentage of platelet phospholipids • %RBC—percentage of red blood cell fatty acids

Effects of Low EPA and DHA on Vegetarians

One of the main things that long chain omega-3 fatty acids do, particularly EPA, is reduce blood clotting which protects against heart attacks. There have been some differences noted in blood clotting between vegetarians and meat-eaters.

A 1999 Chile study (18) found that vegetarians had significantly more platelets (242,000 per ul) than non-vegetarians (211,000 per ul) and a shorter bleeding time (4.5 vs. 7.3 min).

In a follow-up 2000 Chile study (4), vegetarians were given 700 mg EPA and 700 mg DHA for 8 weeks. EPA went from .2 to 1.8% and DHA went from 1.1 to 3.0%. Some clotting factors did change, but bleeding time remained lower at 5 1/2 minutes.

In a 1992 UK study (16), only one of eight platelet aggregation parameters in the men (but not the women) was different from the non-vegetarians. Bleeding times were similar.

A 2017 study from the UK compared heart rate variability between a group of 23 adult vegans and 24 omnivores (41). Low heart rate variability reflects a reduced capacity for the heart to respond to the body’s physiological demands and is linked to increased risk for heart disease. As expected, the vegans had lower concentrations of DHA and EPA in both red blood cells and plasma. While vegans had a higher heart rate variability over a 24-hour period, their daytime heart rate variability was lower, and their heart rate was greater. It’s not clear if this is an overall better or worse heart variability for vegans.

Thus, of three studies that looked at clotting factors, the results are mixed.

In terms of cognition, a 2002 study (53) on British mortality found vegetarians to have a barely statistically significant, higher risk of death from mental and neurological diseases (2.21, 1.02–4.78). However, in a 2016 EPIC-Oxford report (54), vegetarian deaths from “mental and behavioral disorders” were not statistically different from non-vegetarians (1.22, 0.78–1.91). The risk of mortality from neurologic disease in a 2013 report from the Adventist Health Study-2 (55) was also not statistically different for vegetarians compared to non-vegetarians (.93, .67-1.29).

Vegetarian Pregnancy and Children

DHA may be important for developing fetuses and infants, and pregnant women more efficiently convert ALA to DHA. Fetuses and infants are able to receive DHA that’s released from the mother’s fat tissues and provided through the umbilical cord or breast milk.

Anthropologist John H. Langdon argues that DHA is not an essential nutrient for the brain development of infants because in cases of very low maternal levels of DHA, infants can utilize other fatty acids for brain tissue which can later be replaced by DHA (32).

A 1994 study measured the DHA levels in umbilical cords of 32 infants born to vegetarian mothers compared to omnivores (30) and found no relationship between the proportions of DHA in plasma or cord artery phospholipids and the birth weight or head circumference of the infants.

In a study comparing breast milk, cow’s milk formula with DHA, soy formula with DHA, and soy formula without DHA, infants who ate soy formula without DHA had indications of slower parasympathetic development, though still within the normal range (87).

Many children have been raised vegan without supplementing with DHA, or even extra ALA, and appear to develop well. Despite this, breastfeeding mothers of vegetarian or vegan children should make sure they’re meeting omega-3 recommendations (see Daily Recommendations) and non-breastfeeding infants should receive an infant formula with 500 mg of omega-3 fats per day.

Traditional Remedy: Increase ALA, Reduce LA

The traditional way that vegetarians were encouraged to raise EPA and DHA levels was by increasing ALA and decreasing linoleic acid (LA), a short chain omega-6 fatty acid. This is because the enzymes that convert ALA into EPA and DHA also convert short-chain omega-6 fatty acids and there is competition between omega-3s and omega-6s. An ideal ratio of omega-6 to omega-3 in the diet is thought be about 3:1 or 4:1.

Most vegetable oils are high in omega-6s and vegetarians tend to get plenty in their diets. A 1981 UK study (17) showed that the dietary ratio of omega-6 to omega-3 fats was 16 for vegans and 6 for meat-eaters. A 1992 UK study (16) showed a ratio of 15.8 for vegan men vs. 10.2 for meat-eating men, and 18.3 for vegan women vs. 8.2 for meat-eating women.

To counteract the high omega-6 to omega-3 ratio, vegetarian health professionals have recommended increasing ALA intakes and decreasing LA intake. Unfortunately, there are no long-term studies looking at vegetarians’ blood EPA and DHA levels after following such recommendations, though we do have some related studies.

A 1999 Australia study (15) of 17 vegetarian men, aged about 26–42 years old, showed that four weeks of 3.7 g of ALA per day (the equivalent of about 1.5 teaspoons of flaxseed oil) didn’t significantly increase the percentages of EPA or DHA in the blood. The same study showed that four weeks of 15.4 g of ALA (the equivalent of about 6.5 teaspoons of flaxseed oil) did increase EPA levels, but still didn’t increase DHA levels. There were no changes in clotting factors. See table below for more details.

Effects of ALA Supplementation: 1999 Australian Study
Baseline After 4 wks of 3.7 g ALA per day After 4 wks of 15.4 g ALA per day
%PTPL %PTPL %PTPL
ALA 0.1 0.1 0.3
EPA 0.3 0.3 0.5
DHA 1.1 1 0.9^
%PPL %PPL %PPL
ALA 0.3 0.4 1.4^
EPA 0.8 0.9 1.4^
DHA 2 2.1 1.9
%PTG %PTG %PTG
ALA 1.2 2.5^ 7.4^
EPA 0.2 0.3 .4^
DHA 0.2 0.2 0.2
%PTPL – percentage of platelet phospholipids
%PPL – percentage of plasma phospholipids
%PTG – percentage of plasma triglycerides
^Statistically significant result compared to baseline

In a 2000 study from The Netherlands (20), the ratio of dietary LA:ALA was lowered from 13.7 to 6.7 by adding 2.0 g of ALA per day to the diets of 9 vegans aged 20 to 60 years old. After 4 weeks, there was no change in blood levels of EPA or DHA.

A 2014 study from the USA (38) put a group of lacto-ovo vegetarians on three different daily regimens for 8 weeks each: 1 oz of walnuts (3.0 g of ALA), 1 regular egg (110 mg DHA), and 1 fortified egg (~500 mg DHA, 40 mg EPA, 1 g ALA). The ratio of n-6:n-3 was 6:1 in the walnut phase and DHA levels did not increase.

In a 1981 UK study (17), 4 vegans aged 26 to 37 years took 6.5 g of ALA per day for 2 weeks. They had some increase in EPA and DHA levels. See the table below.

Effects of ALA Supplementation: 1981 UK Study
Baseline 2 weeks of 6.5 g ALA per day
%PCP %PCP
EPA 0.3 1.4^
DHA 1.3 1.3
%PTPG %PTPG
EPA 0.3 0.3
DHA 0.9 1.2^
%PCP – percentage of plasma choline phosphoglycerides
%PTPG – percentage of platelet phosphoglycerides
^Statistically significant result compared to baseline

A 1992 India study (21), gave 5 vegetarians, aged 25–40 years old, 3.7 g of ALA per day. This regimen increased blood levels of EPA and DHA, and reduced LDL cholesterol and blood aggregation. See the table below.

Effects of ALA Supplementation: 1992 India Study
Baseline After 6 weeks of 3.7 g ALA per day
PPL (umoles/dl) PPL (umoles/dl)
EPA 0.6 2.7^
DHA 2.1 3.0^
%PTPL %PTPL
EPA TR .4^
DHA 1.5 2.2^
LDL Cholesterol (mg/dl) 106 71^
Blood aggregation (%) 72.2 38.8^
PPL – plasma phospholipids
%PTPL – percentage of platelet phospholipids
TR – trace amount
^Statistically significant result compared to baseline

In summary, it appears that 3.7 grams of ALA per day is needed for vegetarians to see an effect in blood DHA percentages in the short-term. But, there isn’t any research in which participants were asked to cut their LA intake at the same time that they increased ALA intake and so we don’t know if that combination would boost DHA levels in the blood or other tissues.

The World Health Organization and Food and Agriculture Organization recommend an LA intake between 2.5-9% of calories, saying that the lower number prevents deficiency and the higher end of the range reduces risk for heart disease (52). Although vegans who don’t ensure sources of ALA tend to have a high ratio of omega-3 to omega-6 fats, their percentage of calories as LA tends has been shown to be 5.1% (41), 7.3% (46), 8.5% (47), and 9.3% (48), well within the range recommended by the WHO. Because of this, we are hesitant to recommend that vegans avoid LA.

Non-vegetarian Conversion Rates

In order to figure out what the ALA to DHA conversion rates are for vegetarians, it might be important to stick with studies that use actual vegetarians rather than meat-eaters (who might have a dietary source of EPA and DHA). But because there’s a lack of long-term studies on vegetarians, we should look at some of the research on meat-eaters.

There have been many studies on meat-eaters’ conversion rates, and for the most part, they’ve shown good conversion rates of ALA to EPA, but very little to DHA. Because small amounts or short terms don’t appear to be promising, we’ll limit this review to two studies that used larger amounts and longer time periods.

A 2010 USA study (49) gave 24 meat-eaters different omega-3 regimens, for eight weeks each, to manipulate their omega-6 to omega-3 ratios. Achieving an omega-6 to omega-3 ratio of 2:1, by adding 8 g ALA per 2,400 kcal via flaxseed oil and walnuts, resulted in higher EPA values than an omega-6 to omega-3 ratio of 2:1 plus supplements of 200 mg EPA and 720 mg DHA per 2,400 kcal. Ratios of 10:1 or 2:1 made no difference for DHA values, but blood DHA levels were already relatively high at 3.9%.

A 2008 Canada study (22) gave meat-eaters 1.2, 2.4, and 3.6 g ALA per day for 12 weeks. This resulted in an increase in the EPA percentage in red blood cell fatty acids for the 2.4 and 3.6 ALA groups, but no increases in DHA. This study used an intention to treat method, which means that subjects who didn’t comply with the regimen were still included in the results. There was little information given on the level of compliance.

A 2007 Canada study (50) found that with an ALA intake of 1% of energy, increasing LA from 3.8% to 10.5% reduced EPA from .93% to .58%. These effects stabilized after 2 weeks.

A 1999 Japan study (23) gave older adults 3 g ALA per day and reduced the omega-6 to omega-3 ratio to 1:1. After 3 months, there was no difference in EPA and DHA levels, but after 10 months, EPA levels had risen from 2.5 to 3.6% of serum lipids, and DHA levels rose from 5.4 to 6.4%—both findings statistically significant.

Welch et al. (24) reported that non-fish-eaters (both vegetarians and meat-eaters) convert ALA to long-chain omega-3s at a slightly greater rate than do meat-eaters, so conversion rates of vegetarians might be greater than these studies on meat-eaters indicate.

In summary, it appears that 3 g (the equivalent of about 1-1/2 teaspoons of flaxseed oil) per day of ALA cannot increase blood percentages of DHA in three months time, but can increase blood percentages in 10 months time, assuming intake of omega-6 is low.

Low Omega-6 to Omega-3 Ratio Foods

The table below lists foods with the lowest omega-6 to omega-3 ratios.

Foods with Lowest Omega-6 to Omega-3 Ratios
Food n-6:n-3 ratio ALA
flaxseeds 1:4 1.6 g / tablespoon
flaxseed oil 1:4 2.5 g / teaspoon
chia seeds 1:3 5 g / oz
camelina oil 1:2
canola oil 2:1 1.3 g / tablespoon
English walnutsa 4:1 – 5:1 2.6 g / oz (14 halves)
walnut oil 5:1 1.4 g / tablespoon
soybean oil 7.5:1 .9 g / tablespoon
black walnuts 10:1 .9 g / oz
aEnglish are the typical walnuts found in most grocery stores.

More information on omega-3 sources can be found in the articles The Fatty Acids and Omega-3s Part 3—Plant Sources.

DHA Supplementation in Vegetarians

A 2014 USA study (39) gave 46 vegans, selected on the basis of having low omega-3 status, a daily dose of 172 mg DHA and 82 mg EPA for 4 months. Percent of total red blood cell fatty acids went from about .6% to .8% for EPA and from about 2.3% to 3.25% for DHA.

In a 2009 review paper (29), Sanders describes a placebo-controlled, randomized trial from the UK in which 39 vegan men supplemented with 200 mg of DHA per day for 3 months and increased the proportion of DHA in plasma by 50% (from about .8 to 1.3% of plasma lipids).

A 2006 German study (35) gave 87 female and 27 male vegetarians 940 mg of DHA per day for 8 weeks. Plasma phospholipids went from 2.8% yo 7.3% DHA.

A 2006 Taiwan study (36) gave 27 postmenopausal vegetarian women 2,140 mg of DHA per day for 6 weeks. LDL fatty acid composition went from 1.4% to 3.7% DHA.

A 2000 Chile study (4) gave vegetarians 700 mg EPA and 700 mg DHA for 8 weeks. Plasma fatty acids went from 1.1 to 3.0% DHA.

A 1996 Canada study (2) gave 1,620 mg of DHA to 12 male and 12 female vegetarians per day for 6 weeks. Serum phospholipids went from 2.4% to 8.3% DHA, and platelet phospholipids increased from 1.2% to 3.9%. A 1997 study from the same researchers (1), and same dose and length, increased serum phospholipids from 2.1% to 7.1% DHA, and increased serum platelet phospholipids from 1.1% to 3.4% in vegetarians.

It’s not clear what percentage blood fatty acids should be DHA, but average meat-eating levels in the studies above indicate that 2.0% should be adequate. Since 200 mg per day raised vegan DHA levels .6%, 300 mg could be expected to raise vegans’ DHA levels .9%, taking them from an average of about 1% to 2%. Therefore, it seems reasonable to recommend 300 mg per day for the average vegan to match the same level as the average omnivores.

Retroconversion of DHA to EPA

A 1996 Canada study (2) showed an 11 – 12% conversion rate of DHA to EPA after 6 weeks of 1,620 mg of DHA in vegetarians. See Table 9 for more details showing that EPA levels increased.

A 1997 Canada study (1) of vegetarians and meat-eaters showed a conversion rate of 9.4% of DHA to EPA from a dose of 1,620 mg DHA per day for 6 weeks, with no differences between groups.

A 1996 French study (3) fed three people 123 mg of DHA one time and found a conversion rate to EPA of 1.4%.

From this research, it’s not clear if someone can rely on modest DHA supplements to create optimal EPA status, but it’s also not clear what an optimal EPA status is or if any effort is required to maintain EPA levels beyond meeting the dietary reference intake for ALA.

Vegan EPA Supplements

If you’re meeting recommended ALA intakes, you shouldn’t need to take an EPA supplement. Fish contain about twice as much DHA as EPA (27), so it’s not unusual for humans to get more DHA than EPA in the diet. But it’s also okay to take a DHA supplement that contains EPA.

You can find a tremendous number of vegan DHA and EPA supplements by doing a Google search. We aren’t able to assess whether any given company is better than another.

Omega-3 Recommendations for Vegans

To sum up the rationale behind our recommendations, it appears that if a vegan is meeting the Dietary Reference Intake for ALA, their EPA status should be adequate. It’s not clear if DHA is a problem for such people but to be safe we recommend either increasing ALA intake or adding a DHA supplement. Please see our article, Daily Needs, for specific recommendations and how to meet them.

Associations of Omega-3s with Increased Risk of Disease

Some studies have associated higher ALA intakes with an increased risk of disease.

Prostate Cancer

A 2009 systematic review and meta-analysis (11) of ALA intake and prostate cancer found:

When examined by study type (i.e., retrospective compared with prospective or dietary ALA compared with tissue concentration) or by decade of publication, only the 6 studies examining blood or tissue ALA concentrations revealed a statistically significant association. With the exception of these studies, there was significant heterogeneity and evidence of publication bias. After adjustment for publication bias, there was no association between ALA and prostate cancer (RR: 0.96; 95% CI: 0.79, 1.17).

A 2010 meta-analysis found that subjects who consumed more than 1.5 g/day of ALA had a significantly decreased risk of prostate cancer (0.95, 0.91-0.99) compared to those who ate less (34).

A 2018 study (44) from Harvard School of Public Health, suggested that the early associations between ALA and prostate cancer might have been due to trans-ALA which has been largely removed from the food supply.

In July of 2013, a study surfaced suggesting the DHA supplementation might cause prostate cancer. This concern is probably unwarranted, though if you are at a high risk for prostate cancer you might want to err on the lower side of the recommendations here. More details can be read in the article, DHA Supplements and Prostate Cancer.

Eyesight

A 2001 analysis (14) from the Nurses Health Study found an almost statistically significant increase in age-related macular degeneration for those with the highest ALA intake. In contrast, a 2013 France study (6) found that higher ALA levels in the blood were associated with a lower risk of late age-related macular degeneration. A 2017 follow-up (45) from the Nurses Health Study found that a high intake of ALA was associated with an increased risk of intermediate age-related macular degeneration before 2002, but not afterward when less trans fats were found in participants’ blood.

A 2005 analysis (12) from the Nurses Health Study found that both the highest intakes of ALA and LA were associated with an increase in lens opacity, which can lead to cataracts. For ALA, the risk ratio was 2.2 (1.2, 4.5) for about 1.26 g compared to .86 g per day. A 2007 analysis (13) of the same group found that the highest category of ALA intake (about 1.26 g per day) was linked to a 16% increase in eye lens nuclear density compared to the lowest category (about .84 g per day) over five years. As of 2018, no follow-up studies appear to have been conducted on ALA and cataracts.

Without more definitive research we don’t believe concerns about eyesight is any reason to avoid plant-based ALA due to the small differences in ALA intake in these studies, the fact that much ALA in meat-based diets comes from animal products, that trans ALA is no longer added to the food supply, and the large number and inconsistencies of associations between different fatty acids and various conditions.

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Also Reviewed

Chong EW, Kreis AJ, Wong TY, Simpson JA, Guymer RH. Dietary omega-3 fatty acid and fish intake in the primary prevention of age-related macular degeneration: a systematic review and meta-analysis. Arch Ophthalmol. 2008 Jun;126(6):826-33.

Craddock JC, Neale EP, Probst YC, Peoples GE. Algal supplementation of vegetarian eating patterns improves plasma and serum docosahexaenoic acid concentrations and omega-3 indices: a systematic literature review. J Hum Nutr Diet. 2017 Dec;30(6):693-699. The six studies included in this review are all summarized in DHA Supplementation in Vegans.

Hooper L, Thompson RL, Harrison RA, Summerbell CD, Ness AR, Moore HJ, Worthington HV, Durrington PN, Higgins JP, Capps NE, Riemersma RA, Ebrahim SB, Davey Smith G. Risks and benefits of omega 3 fats for mortality, cardiovascular disease, and cancer: systematic review. BMJ. 2006 Apr 1;332(7544):752-60. Epub 2006 Mar 24.

Kris-Etherton PM, Hill AM. N-3 fatty acids: food or supplements? J Am Diet Assoc. 2008 Jul;108(7):1125-30. (No abstract available.)

Mangat I. Do vegetarians have to eat fish for optimal cardiovascular protection? Am J Clin Nutr. 2009 May;89(5):1597S-1601S. Epub 2009 Mar 25.

Muskiet FA, Fokkema MR, Schaafsma A, Boersma ER, Crawford MA. Is docosahexaenoic acid (DHA) essential? Lessons from DHA status regulation, our ancient diet, epidemiology and randomized controlled trials. J Nutr. 2004 Jan;134(1):183-6.

Wang C, Harris WS, Chung M, Lichtenstein AH, Balk EM, Kupelnick B, Jordan HS, Lau J. n-3 Fatty acids from fish or fish-oil supplements, but not alpha-linolenic acid, benefit cardiovascular disease outcomes in primary- and secondary-prevention studies: a systematic review. Am J Clin Nutr. 2006 Jul;84(1):5-17.

Williams CM, Burdge G. Long-chain n-3 PUFA: plant v. marine sources. Proc Nutr Soc. 2006 Feb;65(1):42-50. Review.