NutritionMagazine, No.4, 2011, Vol.24

‘Don’t lump natural and industrial trans fatty acids together’

Health effects of natural trans fatty acids

Natural trans fatty acids (TFA) in milk and ruminant meat have not the unfavourable health effects associated with intake of industrial TFA. Moreover, intake of natural TFA in a normal diet is much lower than intake of industrial TFA.

‘It is not reasonable to lump all trans fatty acids (TFA) together in dietary recommendations. Intake of TFA is associated with an elevated risk of cardiovascular disease and cancer. However, that association is only found for industrial TFA. There are indications that, reversely, intake of realistic quantities of natural TFA from milk and ruminant meat is has positive health effects.'
That opinion was expressed by the Canadian researcher Dr Spencer Proctor, the director of the Metabolic and Cardiovascular Diseases Laboratory of the University of Alberta in September 2011 when he presented results of a literature review about the health effects of natural TFA (ruminant TFA, rTFA) in the diet (1). The review, which was published in Advances in Nutrition, was written by a group of experts working in Canada, the USA, France and Denmark. The first author is Dr Sarah Gebauer, a researcher in the US Department of Agriculture.

A usual diet contains little rTFA
Industrially produced TFA (iTFA) may account for up to 60% of all fatty acids in some foods like margarine, pastry and biscuits (2). The association between intake of iTFA and risk of coronary heart disease (CHD) has been demonstrated in many studies including those of Willett et al. (3) and Sun et al. (4). The association with cancer risk is discussed by Thompson et al. (5).
rTFA constitute 2—5% of the fatty acids in dairy fat and 3—9% of the fatty acids in beef and lamb (6). Because of these low levels in a normal diet, the authors of the literature review conclude that it is unlikely that rTFA intake is too high. The most common rTFA are vaccenic acid (trans-11 18:1) and rumenic acid (cis-9, trans-11 18:2). Both TFA are formed during biohydrogenation of unsaturated C-18 fatty acids, notably linoleic acid. In human metabolism vaccenic acid can be converted into rumenic acid.

Intake of rTFA is not associated with cardiovascular risk
In the review eight epidemiological studies of the association between rTFA and risk of CHD including three case-control studies are discussed. In a cross-sectional study no correlation was found between rTFA intake and CHD risk. In one case-control study raised levels of markers of rTFA were found while in another case-control study no correlation was found and in the third one rTFA intake was found to protect against myocardial infarctions.
The four remaining epidemiological studies discussed were prospective cohort studies. The results of these studies were combined in a recent meta-analysis (7). Total mean TFA intake, which ranged in the studies from 2.8 to 10 g/day, was associated with a significant (33%) increase of the risk of CGD incidents (P = 0.002) and a 24% increase in CHD mortality (P = 0.003). Man intake of rTFA, which ranged in the studies from 0.5 to 1.9 g/day, was not associated with CHD risk (P = 0.36).
Gebauer et al. also discuss in their review clinical studies of the effects of intake of iTFA and rTFA on levels of cardiovascular risk factors. To bring iTFA and rTFA to a comparable level, dairy products were used from milk from cows on adapted feed so that their milk had higher-than-usual rTFA levels. A recent review of these intervention studies published in PLoS One (8) presented the conclusion that all TFA raise the LDL/HDL-cholesterol ratio. However, a comment published in PLoS One (9) mentions that this conclusion is not corroborated by all available data. A WHO report (10) concludes that too few clinical studies have been conducted to date to justify solid conclusions. Since a normal diet contains slight quantities of rTFA, Gebauer et al. state, such conclusions are not very relevant after all.

Effect of rTFA on cancer risk is unclear
Seven epidemiological studies have been devoted to the effect of rTFA on cancer risk. In five of these studies the effect of vaccenic acid intake was investigated, in two studies the effect of intake of conjugated linoleic acid (CLA) and in two studies the effects of both vaccenic acid and CLA. In three of the case-control studies a positive correlation was found between serum or erythrocyte vaccenic acid levels and the risk of breast or prostate cancer, and in one study an inverse association was found between serum vaccenic acid levels and breast cancer risk. In the Netherlands Cohort Study on Diet and Cancer, after multivariate correction for possible confounders, intake of vaccenic acid was found to correlate with a raised risk of breast cancer. However, breast cancer risk was not associated with intake of animal fat or consumption of milk, dairy products or ruminant meat, which are the main dietary sources of vaccenic acid. In the paper on that cohort study the authors point to the fact that no information was available about the effect of moments of intake.
In one of the epidemiological CLA studies (12) an inverse correlation was found between CLA intake and risk of colorectal cancer. Women who used four or more servings of full-cream dairy products per day had a 41% lower risk of colorectal cancer than women who used less than one serving per day; the risk was 29% lower for women in the highest than for women in the lowest quartile. In another study serum CLA levels were found to be significantly lower in postmenopausal breast cancer patients than in healthy postmenopausal women. The two remaining epidemiological CLA studies did not find a correlation between either dietary CLA intake or CLA incorporated in adipose tissue with breast cancer risk. No clinical studies have been devoted to date to the effect of rTFA intake and risk of development of cancer.

Still more research needed
Animal studies and in vitro studies have provided some indications for the hypothesis that rTFA may have a beneficial effect on risk of cardiovascular disease and risk of cancer. According to Gebauer et al., there is still much research to be conducted before we know for sure whether this effect holds true for human beings and whether it holds for all TFA or rather for vaccenic acid, rumenic acid or other CLA isomers alone. We also do not know yet at what intake levels this effect is evident.

Jan Blom

[References]

1. S.K. Gebauer, J.-M. Chardigny, M.U. Jakobsen et al. (2011) Effects of ruminant trans fatty acids on cardiovascular disease and cancer: a comprehensive review of epidemiological, clinical, and mechanistic studies. Adv. Nutr. 2: 332—354.
2. S. Stender, A. Astrup, J. Dyerberg (2008) Ruminant and industrially produced trans fatty acids: health aspects. Food Nutr. Res. 52.
3. W.C. Willett, M.J. Stampfer, J.E. Manson et al. (1993) Intake of trans fatty acids an risk of coronary heart disease among women. Lancet 341: 581—585.
4. Q. Sun, J. Ma, H. Campos et al. (2007) A prospective study of trans fatty acids in erythrocytes and risk of coronary heart disease. Circulation 115: 1858—1865.
5. A.K. Thompson, D.I. Shaw, A.M. Minihane et al. (2008) Trans-fatty acids and cancer: the evidence reviewed. Nutr. Res. Rev. 21: 174—188.
6. A. Aro, J.M. Antoine, I. Pizzoferrato et al. (1998) Trans fatty acids in dairy and meat products from 14 European countries: the TRANSFAIR Study. J. Food Compos. Anal. 11: 150—160.
7. N.T. Bendsen, R. Christensen, E.M. Bartels, A. Astrup (2011) Consumption of industrial and ruminant trans fatty acids and risk of coronary heart disease: a systematic review and meta-analysis of cohort studies. Eur. J. Clin. Nutr. 65: 773—783.
8. I.A. Brouwer, A.J. Wanders, M.B. Katan (2010) Effect of animal and industrial trans fatty acids on HDL and LDL cholesterol levels in humans: a quantitative review. PLoS One.
9. J.M. Chardigny, S. Proctor. A.L. Lock, J. Steijns (2010) Conclusions not fully supported by the data at hand; public health implications should be reviewed with greater caution. PloS One.
10. R. Uauy, A. Aro, R. Clarke et al. (2009) WHO Scientific update on trans fatty acids: summary and conclusions. Eur. J. Clin. Nutr. 63: S68–S75.
11. L.E. Voorrips, H.A. Brants, A.F. Kardinaal et al. (2002) Intake of conjugated linoleic acid, fat, and other fatty acids in relation to postmenopausal breast cancer: the Netherlands Cohort Study on Diet and Cancer. Am. J. Clin. Nutr. 76: 873—882.
12. S.C. Larsson, L. Bergkvist, A. Wolk (2005) High-fat dairy food and conjugated linoleic acid intakes in relation to colorectal cancer incidence in the Swedish Mammography Cohort. Am. J. Clin. Nutr. 82: 894—900.

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