There is not much useful information about trans- fats available, which makes it difficult to avoid them when planning the ketogenic diet. In particular, there is not much information about the trans- fat content of most foods.
The best source of information appears to be Mary Enig , who has been writing about trans- fats since 1978.
Adverse effects of trans- fats
The following are some of the possible adverse effects of trans- fats:
Trans- fats in foods
The trans- fat content of foods is not well documented. There is one study done by Exler et al for the Nutrient Data Laboratory of the US department of Agriculture, which assessed the trans- fat level in 214 food products.
The top ten products for trans- fat as a percentage of total content were:
The important thing to notice is the large variations between the maximum and minimum trans- fat content for each product.
In the case of shortening, spread and margarine, this variation is caused by the manufacturing process, and shows that the trans- fat content can be controlled to some extent during the hydrogenation process. However, unless the trans- fat content is specified on the product, you should avoid using these fats, because of the potentially high level of trans- fats.
The other products are all cooked, and the variation in trans- fats is a consequence of the type of fat that was used in the cooking process. Again, if you want to play safe and reduce the amount of trans- fat in the diet, since the trans- fat content is not usually labeled, it will be necessary to do your own cooking with a suitable fat, and not buy packaged foods.
The next table shows the trans- fat content for products with a high total fat content.
|Total fat||Trans- fat|
For high fat content, with low trans- fats, the vegetable oils, and lard were very good (note, butter should also be good, but was not one of the products tested). The vegetable oils tested were canola oil, olive oil and sun flower oil.
The detailed analysis of fat content for high fat content products was as follows:
|Sunflower oil||Canola oil||Olive oil||Lard||Shortening||Margarine||Spread|
|Mono unsaturated fats|
|Poly unsaturated fats|
|18:3 not n-3||0||0||0||0||0||0||0|
|Total trans- fats||0||0||2||0||22||25||26|
(as % of total fats)
The establishment view
The American Society of Clinical Nutrition/American Institute of Nutrition Task Force published a position paper on trans- fats in the May 1996 issue of the American Journal of Clinical Nutrition.
In this report the task force concluded it was "premature to make new dietary recommendations for the population at large or to change nutrition policy to mandate that trans- fatty acids be listed separately, or be included with saturated fatty acids on nutrition labels, especially in view of the inadequate data base for making or implementing such a change." However, the task force did say that consumers could limit their intake of margarines and shortenings used in frying and baking, and food manufacturers could reduce the trans- fatty acid content of products. Note: the report did not say they should.
From the information given above, it is clear that it will not be possible for users to avoid trans- fats until these are properly labeled on food products. The suggestion that trans- fats should be counted in the total for unsaturated fats is a nonsense, because the biochemical consequences of the two sorts of fats are totally different.
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(update 1.1: 18 July 2002)
(issue 1: 23 May 1998)