Battle of the omega-3s: marine vs veggie sources

The health-promoting properties of omega-3 fatty acids are now widely documented. However, issues remain regarding the source, types, safety and adequate intakes. Ernesto Hernandez, PhD, explains

Omega-3 fatty acids are generally derived from either nonmarine sources (vegetable oils) or marine sources.1 Fatty acids from marine oils include eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA), primarily obtained from cold-water fish. Recently, menhaden oil and omega-3 oils from micro algae have been granted GRAS status by the US government. The nonmarine sources of omega-3s are generally in the form of alpha-linolenic acid (ALA), and can be obtained from flaxseed, walnut and other oilseeds such as soybean or canola.

An alternative source of omega-3 currently approved for infant formula and used also as a nutritional supplement is DHA from the micro algae Crypthecodinium cohnii. This algae is grown in fermenters under controlled conditions, and the oil is reported to be free of contaminants. After fermentation, the fermented mass is dried and the oil extracted.

Another alternative for omega-3 fatty acids is stearidonic acid (SDA, 18:4w3). This is an 18-carbon omega-3 oil with four double bonds. It has the advantage that it does not have to compete for a desaturase enzymatic step with the omega-6 pathway. This fatty acid is found in hemp seed and blackcurrant seed oil.

In addition to these marine- and terrestrial-derived omega-3 sources, many new sources of n-3 oils are in development. At the American Oil Chemists? Society annual meeting in Cincinnati in May, Monsanto presented some interesting work on SDA from GMO soybeans, while Japanese researchers presented good work on supplement blends and structured phospholipids. The future of this category looks to remain both buoyant and a source of innovation for some time to come.

Safety issues
Besides the type and source of omega-3 fatty acids, lately there have been concerns over the risks of consuming fish-origin versus plant-origin PUFAs. Some types of fish contain high levels of mercury, PCBs (polychlorinated biphenyls), dioxins and other environmental contaminants. Some species of fish may contain significant levels of methylmercury, considered one of the more dangerous food contaminants today. In general, older, larger predatory fish and marine mammals, such as swordfish and seals, tend to contain the highest levels of these contaminants.2 PCBs and methylmercury are believed to have long half-lives in the body and can accumulate in people who consume contaminated fish on a frequent basis. In the case of PCBs, it is recommended that consumers reduce their exposure to these contaminants by removing the fat from the fish before cooking them; however, methylmercury represents a more serious problem because it is distributed throughout skin, muscle and organs of the fish.

The Food and Drug Administration regulates the safety of all commercial fish, including ocean-caught, farm-raised and imported fish. One example is the recommendation for pregnant women and nursing mothers to limit their consumption of sport-caught fish to one 6oz meal per week. The Environmental Protection Agency recommends that young children consume less than 2oz of sport-caught fish per week.3

Omega-3 fatty acids of plant origin are considered safer from the point of view of contaminants. However, oils like flaxseed in high doses (>30g/day) have been reported to cause loose stools or diarrhoea. Allergic anaphylactic reactions have been reported with flaxseed and flaxseed oil ingestion. Also, serious complications can occur if omega-3s are taken with anticoagulant medications or antihypertensive agents.2

Adequate intakes
The National Academies released in 2002 the Dietary Reference Intakes Report for Energy and Macronutrients that included adequate intakes for linoleic acid (LA) and ALA. Adequate intake is defined as nutrient intake estimates observed in a healthy individual where sufficient scientific data is not available to suggest a recommended daily allowance.

Adequate intakes have been set for LA as 17g/day and 12g/day for men and women aged 19-50 years, respectively. The adequate intake for ALA is 1.6g/day and 1.1g/day for men and women aged 19 to 70 years, respectively.4

The National Institutes of Health published a report in 1999 from a sponsored international workshop on the essentiality and recommended dietary intakes for n-3 and n-6 fatty acids. This working group proposed adequate intakes of 2-3 per cent of total calories for LA, 1 per cent of total calories for ALA, and 0.3 per cent of total calories for EPA and DHA. The group further recommended intakes of EPA and DHA of 650mg/day and a minimum of 300mg DHA/day during pregnancy and lactation.

Other countries such as Health Canada suggest a minimum of 3 per cent of energy from n-6 fatty acids and 0.5 per cent from n-3 fatty acids or 1 per cent for infants who do not receive a preformed source of EPA and DHA. The United Kingdom recommends intakes of 1 per cent of energy from ALA and 0.5 per cent from EPA and DHA combined.

The World Health Organization has also issued recommendations on the basis of the ratio of n-6 to n-3 fatty acids of 5:1-10:1. Sweden recommends a ratio of 5:1, Canada recommends 4:1-10:1, and more recently Japan recently changed its recommendation from 4:1 to 2:1.

Ernesto Hernandez, PhD, is head of the Fats and Oils Processing Program, Food Protein Research and Development Center, at Texas A&M University System.
Respond: editor@ ffnmag.com.
All correspondence will be forwarded to the author.

Adequate intake (AI) for omega-3 fatty acids

Life Stage

Age

Source

Males
(g/day)

Females
(g/day)

Infants

0-6 months

ALA, EPA, DHA

0.5

0.5

Infants

7-12 months

ALA, EPA, DHA

0.5

0.5

Children

1-3 years

ALA

0.7

0.7

Children

4-8 years years

ALA

0.9

0.9

Children

9-13 years

ALA

1.2

1.2

14-18 years

ALA

1.6

1.1

Adults

19 years and older

ALA

1.6

1.1

Pregnancy

All ages

ALA

-

1.4

Breastfeeding

All ages

ALA

-

1.3


Table 2. Sources of ALAs:

Food and Serving Size

% ALA

g/serving

Flaxseed oil, 1tbsp (14g)

57

8.0

Walnuts, 1oz (28g)

14

2.6

Canola oil, 1tbsp (14g)

11

1.6

Soybean oil, 1tbsp (14g)

8

1.0


Table 2. Sources of DHAs and EPAs:

Food and Serving Size

% DHA+EPA

g/serving

Tuna, 3oz (84g)

0.88

0.74

Salmon, Atlantic (84g)

1.5

1.3

Sardines (84g)

0.98

0.84

Fish oil, menhaden (1g)

22

0.22

Fish oil, salmon (1g)

31

0.31


Comparing conversion efficiencies
Essential fatty acids have three distinct metabolic functions: as an energy source, as structural cell membranes and as precursors of eicosanoids. These essential fatty acids share the same enzymatic reactions involved in their metabolic pathways, thus competing for the same elongases and desaturases enzymes.

Linoleic acid (LA) and alpha-linolenic acid (ALA) go through a series of metabolic steps such as a series of elongation and desaturation reactions to be converted ? LA to arachidonic acid and ALA to EPA and DHA. This competition of the omega-3 and omega-6 pathways for enzymatic activity is one of the main arguments in favour of direct consumption of long-chain omega-3 fatty acids such as DHA and EPA in marine oils.5

The resulting fatty acids from the omega-3 and omega-6 metabolic pathways play roles in cell membrane function; in the development and functioning of the brain and nervous system; and in the production of eicosanoids such as thromboxanes, leukotrienes and prostaglandins.

The conversion of LA into arachidonic acid is generally very efficient. However, the conversion of ALA to EPA and DHA is less efficient. It has been reported that in healthy individuals, only 5-10 per cent of ALA is converted to EPA, and 2-5 per cent to DHA. However, in a recent study conducted with women, estimated net fractional conversion of ALA to EPA was 21 per cent and ALA to DHA was 9 per cent.6,7

EH


References
1. Trautwein EA. N-3 Fatty acids ? physiological and technical aspects for their use in food. Eur J Lipide Sci 2001;103:45-55.
2. Center for Food Safety and Applied Nutrition, Food and Drug Administration. Advice for women who are pregnant, or who might become pregnant, and nursing mothers, about avoiding harm to your baby or young child from mercury in fish and shellfish. 2003 Dec 10. Available at www.fda.gov/oc/opacom/mehgadvisory1208.html.
3. Center for Food Safety and Applied Nutrition, Food and Drug Administration. Mercury levels in seafood species 2001 May 11. www.cfsan.fda.gov/~frf/sea-mehg.html.
4. Institute of Medicine. Dietary Reference Intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. Washington, DC: National Academies Press; 2002.
5. Simopoulos AP. Essential fatty acids in health and chronic disease. Food Rev Int 1997; 13(4):623-31.
6. Burdge GC, et al. Eicosapentaenoic and docosapentaenoic acids are the principal products of alpha-linolenic acid metabolism in young men. Br J Nutr 2002; 88(4):355-64.
7. Burdge GC, Wootton SA. Conversion of alpha-linolenic acid to eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in young women. Br J Nutr 2002; 88(4):411-20.

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