More Reasons to Keep Red Meat to Twice a Week

A non-human, cellular molecule is absorbed into human tissues as a result of eating red meat and milk products, according to a study by researchers at the University of California, San Diego (UCSD) School of Medicine, published online the week of September 29, 2003 in Proceedings of the National Academy of Sciences. The researchers also showed that the same foreign molecule
generates an immune response that could potentially lead to inflammation in human tissues.

Several previous studies have linked ingestion of red meat to cancer and heart disease, and possibly to some disorders involving inflammation. However, that research has primarily focused on the role of red-meat saturated fats and on products that arise from cooking. The UCSD study is
the first to investigate human dietary absorption of a cell-surface molecular sugar called N-glycolylneuraminic acid (Neu5Gc), which is found in non-human mammals. Not produced in humans, Neu5Gc occurs naturally in lamb, pork and beef, the so-called "red meats". Levels are very low or undetectable in fruits, vegetables, hen's eggs, poultry and fish. (see * below)

Conducting laboratory studies with human tissue, followed by tests in three adult subjects, the UCSD team provided the first proof that people who ingest Neu5Gc absorb some of it into their tissues. In addition, they demonstrated that many humans generate an immune response against the molecule, which the body sees as a foreign invader.

In the absence of any known molecular mechanism that would produce Neu5Gc in humans, the group reasoned that the small amounts of Neu5Gc found in human tissue could arise from human ingestion of Neu5Gc in dietary sources.

Postdoctoral fellow Muriel Bardor, Ph.D., showed that when human cells in culture were exposed to Neu5Gc, they easily absorbed and incorporated it onto their own surfaces. When the three volunteers drank Neu5Gc purified from pork sources and dissolved in water, there were no immediate ill effects. An analysis of the volunteers' urine, blood, serum (the clear liquid that can be separated from clotted blood), hair and saliva, both before ingestion and regularly for several days after, determined that the human body eliminates most of the Neu5Gc, but retains and metabolically absorbs small amounts of the foreign sugar. At approximately two days following ingestion, the Neu5Gc levels were two to three times the baseline level prior to ingestion. By four to eight days following ingestion, the levels had dropped nearly to baseline. The authors cautioned that a causal relationship between Neu5Gc expression in human tissues with any human disease would be premature and scientifically speculative at best. Instead, they said their findings point to the need for population-level analyses of the presence of Neu5Gc in human tissues in relationship to disease incidence, and the mechanisms of human incorporation and antibody response against this sugar.

Sialic Acid and Neu5Gc content of common food items
Food Neu5Gc intake if eaten at daily recommended servings (Total milligrams)
Cod 27
Salmon 810
Tuna 27
Chicken 27
Turkey 27
Duck 27
Milk (cow, 2%) 711
Milk (cow, raw) 711
Butter 45
Cheese (cow) 600
Lamb 4,860
Pork 5,130
Cheese (goat) 5,544
Beef 11,610
Beef, lean portion 9,720
Beef fat 10,260

** In the late 1990s, the first example of a complete genetic and biochemical difference between humans and apes was reported by a group led by Ajit Varki, M.D., working closely with Elaine Muchmore, M.D. They showed that humans are different from chimpanzees and other great apes in not having the enzyme responsible for producing Neu5Gc, which is also found on the cell surfaces of other non-human mammals. The group showed that this biochemical difference was due to a human-specific mutation in the gene encoding the enzyme CMP-Neu5Ac hydroxylase (CMAH), which is responsible for synthesizing Neu5Gc in apes and other mammals.