031155 Red Meat & Milk Contain “Foreign Molecule”

When people drink milk and eat red meat, they absorb a foreign molecule into their tissues that their immune system views as an invader.

Researchers caution that it is premature to say whether this phenomenon is related to any disease, but they say that over time it may contribute to inflammation in human tissues. Inflammation plays a key role in diseases such as arthritis, lupus and hardening of the arteries.

The molecule, known as Neu5Gc, is a sugar found on the surface of cells in some animals, including cattle and sheep.

Apes also produce it, but humans don't. In fact, the team of scientists from the University of California first became intrigued by the sugar in 1998 when they were searching for genetic differences between humans and great apes.

It turns out that the gene that carries the instructions for making the sugar is the first -- and so far only -- key genetic and biochemical difference between humans and our closest animal relatives, gorillas, chimpanzees and orangutans. We share 98 per cent of our genetic material.

But the researchers believe that some time after Homo sapiens diverged from the common ancestor we shared with the great apes -- about 2.5 million to three million years ago -- we lost our ability to produce the sugar because of a genetic mutation. It became what scientists describe as a "non-human molecule."

Subsequent research found that this non-human molecule is found in the human body, in healthy tissue but predominantly in cancerous tumours. Researchers also found that most people have antibodies circulating in their blood that recognize the sugar. This means that our immune system treats it as an invader.

Since we don't have the gene to make it, it must be that people are eating it. To establish this for certain, the researchers became human guinea pigs.

Team leader Ajit Varki, a professor of medicine at the University of California, San Diego, and two other researchers drank purified Neu5Gc obtained from pork and dissolved in water.

Dr. Varki says he was hesitant to give a potentially harmful substance to human volunteers. None of the three reported any adverse effects.

Tests showed that the sugar was absorbed by their bodies for two to three days, then largely eliminated. However, a small amount was in their body before they drank the pork cocktail and remained there afterward.

Their paper, published in a recent edition of the Proceedings of the National Academy of Sciences, suggests that everyone who doesn't follow a vegan diet is regularly inviting a foreign invader to reside permanently in their bodies, one that sometimes ends up in cancerous growths. This doesn't sound like a great idea, but the researchers say more research is needed to find out if Neu5Gc is actually contributing to disease.

They say there are 10,260 micrograms (one-millionth of a gram) of the sugar in a serving of fatty beef, and 9,720 micrograms in lean beef. The recommended daily serving of 2-per-cent milk has 711 micrograms of it.

The researchers are not recommending that people quit eating red meat or drinking milk.

The sugar is clearly not toxic. However, while it is unlikely it would cause illness, the researchers say it could contribute to the inflammatory process in various diseases that occur when the body's immune system turns on its own healthy cells. This is what happens in arthritis, for example.

It may be that the damage builds up over many years, resulting in problems later in life.

On the other hand, humans have been eating meat for a long time, and may have developed a toler-ance for the sugar. In evolutionary terms, diet is important only if it helps an individual produce offspring. It doesn't matter if it makes people sick after their kids are born and can survive on their own.

"There are clearly advantages to eating red meat. In evolutionary terms, all that matters is that there are advantages in your reproductive life span," says Timothy Johns, a professor of human nutrition at McGill University in Montreal.

Early humans lived much shorter lives than we did, so any risks involved in eating red meat may not have been important. "They may have been there all along, but who cares if you are only going to live for 50 years," says Dr. Johns, an expert in evolution and diet.

The dangers of meat may be more significant now that humans are living much longer, he says.

Dr. Johns says the new findings are intriguing, but not necessarily a reason to change your diet. "This wouldn't be a basis for stopping to eat red meat. Neither does it dispute what we tell people that red meat should be eaten in moderation."

Diets rich in red meat have already been linked to a greater risk of cancer and heart disease. Vegetarian diets, on the other hand, decrease the risk of cancer and heart disease.

Many people believe that this is because meat and dairy products are high in saturated fats. Dr. Varki says Neu5Gc may also play a role. The only way to find out is to do a study that compares the levels of Neu5Gc and its antibodies in healthy people to people who get cancer and heart disease. He also says there is reason to believe that antibodies to the sugar play a role in hepatitis, liver cirrhosis, infectious mononucleosis, rheumatoid arthritis, syphilis and leprosy.

Why would gorillas produce it, but not Homo sapiens? It may have to do with how our immune systems work, and develop resistance to disease. Neu5Gc is found on the surface of animal cells, and it turns out that many disease-causing microbes, including the viruses that cause influenza and whooping cough, gain access to the cells they are invading through similar surface sugars.

The sugar is a form of sialic acid, and humans do make slightly different versions of it. The great influenza pandemic of 1918, which killed more people than both world wars combined, was related to sialic acid. The sugar was the anchor that the virus used to attach itself to the cells it invaded.

Losing the ability to make the sugar may have helped to protect humans from viruses that jumped from species to species. It may have been an evolutionary trade-off: greater protection from infectious diseases but a greater risk of diseases that involve inflammation.

Dr. Varki worries that his findings may have implications for the possibility of using organs from other species in humans. The non-human sugar is common in pigs, which researchers are studying as a potential source of organs for humans. It may make it hard for cross-species transplants to work.

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