Naturally occurring trans fats
Some trans fatty acids occur naturally in the digestive system of ruminant animals such as cows, sheep and goats. Some trans fatty acids are therefore present in meat, milk and other dairy produce. These are mostly C18 monounsaturated trans fatty acids, principally (almost half) vaccenic acid.
According to the The US Department of Agriculture, these trans fats make up 15 to 20 percent of the total trans fat intake in our diet. Others believe that these natural trans fats occur at much lower levels. According to Fran McCullough, author of The Good Fat Cookbook, these natural trans fats occur at "minute" levels, and our total intake of trans fats has increased by 25 times over the introduction of hydrogenated vegetable oils.
Most importantly, however, the naturally occurring trans fats have not, as they occur in animal fats, been shown to share the harmful properties of the synthetic trans fat resulting from hydrogenation. This does not mean that all the trans fatty acids are in themselves harmless, but that any harmful effect is limited and balanced by the beneficial effects of, for example, trans isomers of conjugated linoleic acid (CLA) which are health-promoting with specific roles to play in our bodies. Also vaccenic acid, the main 'natural' trans fatty acid, can be metabolised by humans to CLA.
Thus, COMA reports,
"Willet et al reported findings from the Nurses Health Study, a prospective study involving more than 85,000 women, showing that intake of trans fatty acids was significantly and independently associated with incidence of CHD. The association was only seen for trans fatty isomers from hydrogenated vegetable oils. The mainly different trans isomers from ruminant fats did not show such an association. A case-control study in 239 people suffering an acute myocardial infarction found that after adjustment for age, sex and energy intake, intake of trans fatty acids was directly related to risk of myocardial infarction. Those with the highest intake of trans fatty acids had twice the risk of myocardial infarction as those with the lowest intakes after adjusting for other cardiovascular risk factors. As with the Nurses Health Study, the association was only seen for trans isomers from partially hydrogenated vegetable oils."
From: Department of Health (1994) Nutritional Aspects of Cardiovascular Disease, Report of the Cardiovascular Review Group of the Committee of Medical Aspects of Food Policy (COMA). Report No. 46. London: HMSO. See longer extracts about trans fats here.
The Danish Nutrition Council similarly states in The influence of trans fatty acids on health - 2003 update, 10/03/2003 (Summary):
"Available data suggest that ruminant trans fatty acids, especially concerning the effect on cardiovascular risk, do not possess the same unfavourable effects as industrially produced trans fatty acids. The content of trans fatty acids in industrially hydrogenated fats may reach 60 percent of the fatty acids. The equivalent number for ruminant fat is 2-5 percent."
Likewise, according to the International Dairy Federation (submission to the World Health Organisation, June 2002):
There is a need to differentiate the trans fatty acids. We agree on the negative impact of hydrogenated oils and fats but natural trans fatty acid like CLAs (conjugated linoleic acids) present in meat and dairy products play a specific role on plasma lipids and cardiovascular function. [Khosla P., Fungwe T. V., "Conjugated Linoleic Acid: Effects on Plasma Lipids and Cardiovascular Function"; Curr Opin Lipidol 2001, 12 :1231-4]
CLAs (conjugated linoleic acid) have anticarcinogenic properties, blocking tumour growth and metastasis from the breast (12,13). In the Kuopio Breast Cancer Study, breast cancer patients had lower serum CLA, myristic and trans vaccenic acid (precursor of CLA) than control [Aro A. et al, "Inverse Association Between Dietary and Serum Conjugated Linoleic Acid and Risk of Breast Cancer in Postmenopausal Women"; Nutrition and Cancer 2000, 38: 151-7.]
From: Consultation with Industry Organisations on Diet, Nutrition and the Prevention of Chronic Diseases - Comments from the International Dairy Federation, 15 June 2002. Download (.doc) here.
In conclusion, by eating meat and dairy products, you will be eating naturally-occurring trans fatty acids. But population studies do not indicate that these particular trans isomers are significantly harmful, at least in the quantities and proportions in which they naturally occur in animal products. Also, among these natural trans fatty acids, there may be trans isomers present - such as CLA - which are positively beneficial to health.
This suggests that there may be specific trans fatty acids which arise from industrial processing and hydrogenation of oils which are disproportionately damaging to health. An appreciation of the very different biochemical roles of different fatty acids also suggests that different trans isomers must act differently. One particular concern is the role played by trans polyunsaturates, and in particular the trans isomers of essential fatty acids.
However the broad evidence that natural and synthetic trans fatty acids are not alike has not prevented the food industry and its advocates from conflating the two. By failing to distinguish between the 'good' or at least 'not harmful' natural trans fats, and the unquestionably 'bad' synthetic trans fats, they purport to show that:
- our intake of synthetic trans fats is only adding an additional fraction of trans fat to what has been in the natural human diet for millennia.
- even if we stopped eating all the synthetic trans fats, we would still be eating trans fat anyway, so what's the point?.
One example of this conflation comes from the UK's Food Standards Agency, which wrote in a letter of 21 September 2004 that
"The main issue with hydrogenated fats relates to their content of trans unsaturated fatty acids (trans fats). These trans fats occur in nature in dairy produce and the flesh of ruminants, e.g. beef, lamb."
Studies have failed to show that trans fatty acids are a significant factor contributing to coronary heart disease. The report did not find any evidence that the trans fatty acids found in industrially hardened oils are different to those produced in nature and found in mutton, beef and milk fats, including butter. A Senior Nutrition Scientist at the BNF has stated that Trans fatty acids from different sources are basically the same but the amounts in similar products may differ.
It is really intolerable that these respected orgnisations should be so ready to mislead and obfuscate in this way.