A. Recommendations on the Intake of Polyunsaturated Fatty Acids

1. An adequate linoleic acid intake: 2 energy %

Nutrients needed in the diet have three attributes: they cannot be synthesized in sufficient amounts, their inadequacy causes defined symptoms of deficiency, and the deficiency symptoms are prevented or corrected when adequate amounts of the nutrient are present in the diet. Linoleic acid (LA) meets these criteria and has been widely recommended in human diets for about 40 years. Notwithstanding an abundant literature on animal studies purportedly examining dietary requirements for LA (or w6 PUFA), we found no well controlled studies that definitively established the minimum required intake of w6 PUFA in healthy adult humans. At least one of the following three methodological problems confounds clear interpretation of each of the nine most suitable published papers on the subject of LA requirements in adult humans:

Infants were the subject of study in four of the nine reports (Hansen et al 1958, Combes et al 1962, Hansen et al 1963, plus a review by Cuthbertson [1976]).

In eight of the nine reports studies (adult and infant), a dietary source of w3 PUFA was not included. On principle alone the absence of a known essential nutrient compromises interpretation of such reports (Cunnane 2003). However, in addition, because the inclusion of small amounts of ALA reduce the requirement for LA in animals by at least 50% (Cunnane, 2003), it seems likely that LA requirement in infant and adult humans are somewhat overestimated. Only the study by Collins et al (1971) included w3 PUFA but this study was confounded by small size (n=2) and the presence of gastrointestinal disease requiring TPN.

In four of the five reports in adults, LA requirement was estimated for individuals about to undergo surgery for gastrointestinal disease. Only in the study by Wene et al (1975) were healthy adults studied but this study omitted a dietary source of w3 PUFA.

Notwithstanding these issues, the infant studies suggest that 1.0 energy% LA is adequate for healthy human development. The authors of the adult studies generally concluded that LA intakes of 1.0-2.5 energy% would meet requirements, but this conclusion was based mostly on minimizing the plasma level of 20:3w9 (Mead acid; a presumed biochemical marker of w6 PUFA deficiency). The clinical condition of the infants was also considered in one study but otherwise, in these studies, clinical status was not informative. Several authors specifically noted the difficulty in drawing conclusions about LA requirement from measuring plasma fatty acid profiles alone. On the basis of these results, it is concluded that 2 energy % LA is adequate for healthy adult humans

2. A healthy intake of a-linolenic acid : 0.7 energy % There are various international recommendations of PUFA intakes (Meyer et al (2003).  The ALA recommendations are 2.0 g/day (1% en) for five different bodies/countries (NHMRC 1992, Australia; BNF, 1992; Sugano 1996 Japanese; de Deckere et al 1998, EANS; NHF 1999, Australia), 2.2 g/day from one conference (Simopoulos et al 1999), and 1.35 g/day (0.68% en) for the final body (Food and Nutrition Board 2002, USA/Canada).

Actual dietary intakes :The dietary intake of ALA has been reported recently for Australia and France.  In Australia, the ALA intake was determined following completion of a 24-hr recall by 10,851 adults in 1995.  The mean ALA intake was 1.17 g/day with the median intake being 0.95 g/day (Meyer et al 2003).  In France, Astorg et al (2004) have reported the ALA intake in 2099 men and 2785 women collected from ten 24-hr diet records over a 30-month period in the years from 1994 to 1998.  The ALA intakes as a % energy were 0.36 (0.2-1.11) for men [mean (minimum-maximum)] and 0.38 (0.18-1.04) for women. Dolecek (1992) reported the mean ALA intake of 6250 males in USA to be 1.688 +/- 0.736 g/day (1992). In that publication, reference was made to the ALA intake in Japan which had increased steadily from 0.67 g/day in 1946 to 2.08 g/day in 1985.  In Norway, Johansson et al (1998) reported the intake of ALA to be 1.8 g/d for men and 1.2 g/d for women in a national representative study from 1993/4.

Studies examined : This report is based on a survey of ten studies reported in the period from 1968 to 2003 with cardiovascular disease being the primary outcome in nine of the studies.  There were four prospective studies (three conducted in USA and one in Holland), two cross-sectional studies (conducted in USA and Holland), one case-control study (Costa Rica), one primary prevention study (Norway) and two secondary prevention studies (conducted in France and India). Most studies showed benefit from an increasing intake of ALA, often with a significant trend for benefit across quintiles of ALA intake in the case of the prospective, cross-sectional studies and case-control study.  In the Zutphen elderly study there was no beneficial effect of dietary ALA on risk of CAD incidence over 10 years. The authors acknowledged the data was complicated by the positive association of ALA intakes with that of trans fatty acids. The mean ALA intakes (g/day) in the highest quintile in the prospective and cross-sectional studies were 1.5 (males), 1.36 (females), 2.80 (males), 1.14 (males), 0.96 (females), and 1.7 (males and females).  In the two secondary prevention studies, the intervention groups received 1.74 g/day and 2.9 g/day of ALA.  In a primary prevention study from Norway (with no untreated control group), there was no difference in 1 year clinical outcomes in subjects randomized to sunflower oil versus linseed (flaxseed) oil. From these studies, ALA intakes calculated as a percent of energy based on a 2000 kcal/day diet, are shown in Table 1.

These data suggest that a healthy intake of ALA is approximately 0.7% energy. This value is somewhat lower than the 1% energy recommendation of some international groups, but is entirely consistent with recommendations for ALA of the US National Academy’s Institute of Medicine Report.