in human breast milk fluctuate with maternal consumption of trans fat, and that the amount of trans fats in the bloodstream of breastfed infants fluctuates with the amounts found in their milk. In 1999, reported percentages of trans fats (compared to total fats) in human milk ranged from 1% in Spain, 2% in France, 4% in Germany, and 7% in Canada and the U.S. Trans fats are used in shortenings for deep-frying in restaurants, as they can be used for longer than most conventional oils before becoming rancid. In the early 21st century, non-hydrogenated vegetable oils that have lifespans exceeding that of the frying shortenings became available. As fast-food chains routinely use different fats in different locations, trans fat levels in fast food can have large variations. For example, an analysis of samples of McDonald's French fries collected in 2004 and 2005 found that fries served in New York City contained twice as much trans fat as in Hungary, and 28 times as much as in Denmark, where trans fats are restricted. At KFC, the pattern was reversed, with Hungary's product containing twice the trans fat of the New York product. Even within the U.S. there was variation, with fries in New York containing 30% more trans fat than those from Atlanta. Nutritional guidelines The National Academy of Sciences (NAS) advises the U.S. and Canadian governments on nutritional science for use in public policy and product labeling programs. Their 2002 Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids contains their findings and recommendations regarding consumption of trans fat (summary). Their recommendations are based on two key facts. First, "trans fatty acids are not essential and provide no known benefit to human health", whether of animal or plant origin. Second, while both saturated and trans fats increase levels of low-density lipoprotein (LDL), trans fats also lower levels of high-density lipoprotein (HDL), thus increasing the risk of coronary artery disease. The NAS is concerned "that dietary trans fatty acids are more deleterious with respect to coronary artery disease than saturated fatty acids". This analysis is supported by a 2006 New England Journal of Medicine scientific review that states "from a nutritional standpoint, the consumption of trans fatty acids results in considerable potential harm but no apparent benefit." Because of these facts and concerns, the NAS has concluded there is no safe level of trans fat