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Molecule in red meat & milk viewed as foreign invader

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A difference between humans, apes

 

Humans have lost the ability to make a certain sugar that our closest

animal relatives produce, so when we consume it in milk and red meat, our

immune system views it as an invader

 

By ANNE McILROY

Globe and Mail, Saturday, November 22, 2003 - Page F7

http://www.theglobeandmail.com/servlet/ArticleNews/TPStory/LAC/20031122/IMMUNE22\

/TPScience/

 

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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