Here’s the evidence that red meat is hazardous to your health.
Cancer, heart disease, diabetes… all have been linked to red meat (mostly beef and pork), processed meats (bacon, ham, hot dogs, sausage, and deli meats), or both.
The evidence is stronger for some diseases than others. “But even if only a couple were true, when you put them all together, that would be reason enough to keep red and processed meat consumption pretty low,” says Harvard’s Walter Willett.
Here’s what we know about the risks of eating red and processed meats, and how to minimise them.
In 1995, the NIH-AARP Diet and Health Study sent questionnaires about diet and lifestyle to 3.5 million members of AARP (formerly known as the American Association of Retired Persons). 10 years later, researchers from the National Cancer Institute analysed results from half a million of them.
“At the high end, people who consumed 2,000 calories a day ate about 5 ounces of red meat a day,” notes Barry Popkin, director of the Nutrition Epidemiology Division at the University of North Carolina Gillings School of Public Health in Chapel Hill. “That’s just over 11/2 Quarter Pounders or Big Macs a day.”
Their risk of dying over 10 years — mostly of heart disease or cancer — was roughly 30% higher than the risk of those who ate the least red meat. “People at the low end averaged two-thirds of an ounce of red meat a day,” says Popkin.
Processed meats had less impact than red meats. People who ate the most ate almost 2 ounces a day. “That’s equal to about one hot dog a day,” says Popkin. It’s also a ham or smoked turkey sandwich. Their risk of dying was roughly 20% higher than the risk of those who ate the least processed meat. The “least” group ate the equivalent of one hot dog every 2 weeks, though the risk for the next-to-least group — which ate the equivalent of about one hot dog a week — wasn’t much higher.
And “white meat” — chicken, turkey, and fish — posed no risk. In fact, those who ate the most had slightly lower odds of dying than those who ate the least.
The bottom line: “People really need to cut down on red and processed meat,” says Popkin, who wrote an editorial that was published along with the study. “You can’t stop to get a hot dog every day when you’re walking down the street.”
Of course, it’s also possible that something else about meat eaters in the NIHAARP study raised their risk of dying. “People who eat a lot of red meat eat and drink differently,” says Popkin. “The researchers did all they could to control for the differences.” They looked at everything from weight, education, and smoking to alcohol, vitamin use, fruit consumption, and more. But they could still have missed — or incompletely accounted for — something other than meat that raises a meat eater’s risk of dying.
“This is the biggest study to look at meat and mortality, and it was done as well as you can do it,” says Popkin. ‘”The only way to get around differences in meat eaters is to do a clinical trial” — that is, to randomly assign people to ‘ diets with more or less meat and wait 5 or 10 years. “And you can’t do that on half a million people.”
How might meat raise the risk of dying? Here’s what we know so far.
It’s no shock that meat eaters are more likely to die of heart disease. “A lot of the higher risk is coming from the high amount of saturated fat and cholesterol,” says Harvard’s Walter Willett. It’s clear that saturated fat and cholesterol raise blood cholesterol, which boosts the risk of heart attacks. Although some cuts of meat — like sirloin, filet mignon, and pork tenderloin — are leaner than others, meat is still one of the largest sources of saturated fat in the average American’s diet. And you can’t trim the fat off hamburgers, hot dogs, sausages, or bacon.
“In general, the best evidence comes from a combination of epidemiological studies that look at the disease itself plus risk factors like blood lipids and blood pressure,” explains Willett. “You’ve got that combination of evidence for red meat.” What’s more, what you eat instead of meat may protect your heart. “We’ve published studies showing that replacing meat with chicken and fish would be advantageous,” notes Willett.
That’s partly because chicken and fish have less saturated fat than red meat and partly because the omega-3 fats in fish may lower the risk of heart attacks. “You’re better off replacing meat not just with fish and chicken, but also with some nuts, beans, soy products, and other plant proteins,” says Willett. They’re rich in fibre, vitamins, and minerals, and the unsaturated fat in nuts can help lower blood cholesterol.
“When it comes to red and processed meats, the evidence has always been strongest for colorectal cancer,” says Arthur Schatzkin, chief of the Nutritional Epidemiology Branch of the Division of Cancer Epidemiology & Genetics at the National Cancer Institute in Bethesda, Maryland. Schatzkin was a co-author of the NIH-AARP study. Before NIH-AARP, Swedish researchers who examined 15 studies reported a 28% higher risk of colorectal cancer in red-meat eaters and a 20% higher risk in processed-meat eaters.
“The evidence linking high consumption of red and processed meat to increased risk of colorectal cancer is highly consistent,” says Michael Thun, vice president of epidemiology and surveillance research at the American Cancer Society. When it comes to food, “only the relationship between obesity and cancer is stronger”.
How might red and processed meat cause malignant tumours in the colon and rectum? There are 2 key explanations: N-nitroso compounds (Nitrites are added to processed meats like bacon, hot dogs, sausage, and bologna to preserve and colour the meat). Scientists have known for decades that N-nitroso compounds cause cancer in laboratory animals. So back in the 1970s, when carcinogens were discovered in cooked bacon, researchers came up with a fix. “When the meat industry realised that N-nitroso compounds were being formed in meats that had nitrites added, they started to add ascorbic acid to the meats,” explains Amanda Cross, a co-author of the NIH-AARP study who works with Schatzkin at the National Cancer Institute.
Nearly all processed meats now contain either sodium ascorbate (Vitamin C) or its cousin sodium erythorbate. They prevent the N-nitroso compounds from forming in the meat, but that doesn’t entirely solve the problem. Recent studies indicate that N-nitroso compounds can form in the gut when people eat processed meats, and giving them Vitamin C doesn’t stop it. In fact, any meat may trigger the reaction. “When we did some feeding studies with non-processed red meat, we found that a considerable amount of N-nitroso compounds still form in the gut,” says Cross.
For N-nitroso compounds to form, you need amines and amides, which come from protein, plus a nitrosating agent, which can come eitherfrom nitrites in processed meats or from nitrates that can be converted to nitrites in the mouth and elsewhere in the body. “Nitrates can come from water and some vegetables,” Cross explains. At first Cross thought that more amines and amides in the gut would mean that more N-nitroso compounds would form. “But when we tried feeding a high-protein white meat diet or a high protein vegetarian diet, we didn’t see an increase in N-nitroso compound formation,” she says. N-nitroso compounds only increased after red meat, suggesting that the meat’s heme iron might be at fault. (Heme iron gives red meat its colour.) “We think heme iron is catalysing the reaction between the amines and the nitrosating agent,” explains Cross. “When we fed non-heme iron, we didn’t see an increase in N-nitroso formation in the gut at all.” Non-heme iron is found in grains, beans, leafy greens, and iron-containing vitamin supplements, cereals, and other fortified foods. “So we think it has more to do with how much heme iron and bacteria are there,” says Cross. The normal bacteria that live in your gut seem to play a key role. “If you feed these nitrates to rats that have no bacterial flora, you do not see N-nitroso compound formation,” says Cross. “If you add bacteria, you see an increase.” And some people have bacteria that produce far more N-nitroso compounds than others.
Cooked meat mutagens. Grilling meat or cooking it at high temperatures creates two classes of mutagens — heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs). (A mutagen causes changes in DNA, which can lead to cancer.) To the body, they’re foreign substances that need to be expelled. “Because the body really wants to get rid of HCAs, the liver adds oxygen to them and other parts of the body add acetyl groups to make them more soluble,” says Rashmi Sinha, deputy chief of nutritional epidemiology at the National Cancer Institute. Once soluble, they can be washed out in the urine.
Ironically, the efforts to get rid of HCAs can make them more dangerous. “In some cases, that can make them more active and damage DNA,” says Sinha. Her research is trying to identify the genes that make HCAs more active. “It’s not one gene,” she notes. “It’s a multistep pathway, so it would have to be a combination of genes.” In the meantime, consumers can avoid cooking meat at high temperatures and for a long time. “The more brown the meat is, the more likely it is that heterocyclic amines have formed on the surface,” says Sinha. They’re there, she adds, “when it’s that nice crusty brown.” Marinating meat before cooking it may remove compounds that turn into HCAs. So does microwaving. “It gets rid of some of the precursors,” explains Sinha. The goal is to minimise high-temperature cooking. “A simple way to think about it is if water is available — like when you’re stewing — the temperature of the meat never rises above the boiling point of water,” says Sinha.
Also critical: “You have to minimise how long you keep the meat on the grill.” Both less time and lower temperatures matter. And it’s not just red meat. “Chicken produces a lot of heterocyclic amines,” says Sinha. “But in the NIH-AARP study, we didn’t see an increased risk from poultry.” In fact, people who ate more white meat — chicken, turkey, and fish — had a slightly lower risk of dying. One explanation: “The majority of the population — close to 60% — doesn’t eat grilled chicken,” says Sinha. “They eat roast chicken, which doesn’t have these compounds, even if it’s well done.”
Another problem occurs when the fat from grilled meat or poultry drops onto the fire or electric element. The heat creates polycyclic aromatic hydrocarbons (PAHs), which travel via smoke to the meat’s surface. It’s a double whammy. “Smoked and grilled meat and poultry have both heterocyclic compounds and polycyclic hydrocarbons,” says Sinha.
Overall, the evidence that points to N-nitroso compounds and mutagens is compelling, but it’s not enough. “What carries the most weight is not the potential mechanisms but the consistency,” says the American Cancer Society’s Michael Thun.
“The relationship between meat consumption and colorectal cancer has been replicated in enough studies and enough different situations that it holds up.”
Meat mutagens and N-nitroso compounds may damage more than the gut. “In animal models, both HCAs and N-nitroso compounds are multi-site carcinogens,” says the National Cancer Institute’s Amanda Cross. “Once they’re in the blood, there’s no reason they can’t act at any site.” But so far, the evidence for other cancers isn’t as strong as it is for colorectal.
Here’s a summary of some key cancers:
Prostate. In the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial, men who averaged at least a third of an ounce of well-done meat a day had a 40% higher risk of prostate cancer than men who ate no well-done meat. And in the Agricultural Health Study, men who ate well-done meat had twice the risk of advanced prostate cancer than men who ate no well-done meat. But overall, most experts are waiting for more evidence. “We’re following up on it,” says Cross.
Pancreas. In an earlier report from the NIH-AARP study, men — but not women — who ate more red meat or more grilled or barbecued meat had a 40-50% higher risk of pancreatic cancer. And in April, researchers reported (but haven’t yet published) data from the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial showing a 60% higher risk in people who preferred well-done steak. “If you could find a risk factor that would reduce your risk by even 20%, that would be a big deal” because pancreatic cancer is so deadly, says the National Cancer Institute’s Arthur Schatzkin. But, he adds, “I don’t think we’re there yet.”
Breast. Studies looking at meat and postmenopausal breast cancer have come up empty. “In two studies that looked at breast cancer, we didn’t see any association,” says the National Cancer Institute’s Rashmi Sinha. And a Canadian study of nearly 50,000 women found no link between breast cancer and heme iron. Harvard’s Walter Willett concurs. “Meat consumption was not related to breast cancer in postmenopausal women in our studies.”
However, a link did show up with less common premenopausal breast cancer. In the Nurses’ Health Study, women who averaged more than 11/2 servings of red meat a day had twice the risk of estrogen-positive premenopausal breast cancer than women who ate red meat no more than 3 times a week. “We also saw that meat consumption during high school years is related to risk,” says Willett. But that hasn’t been confirmed, in part because “there’s not another good study in premenopausal women.”
When researchers dug into data from the Women’s Health Study — which monitored more than 37,000 women for roughly 9 years — they found that those who ate processed meat at least 5 times a week had a 38% higher risk of Type 2 diabetes than those who ate processed meat less than once a week. The most common processed meats: bacon and hot dogs.
“The adverse effect of processed meats shows up repeatedly,” says Willett. “The nitrites may be damaging the islet cells in the pancreas.” (In Type 2 diabetes, the islet cells continue to secrete insulin, but they can’t keep up with the demand because the body becomes insulin-resistant.)
The Women’s Health Study also found a 28% higher risk of Type 2 diabetes in those who ate the most red meat of any kind. As with colorectal cancer, heme iron may be partly to blame. In the Nurses’ Health Study, women who consumed the most heme iron had a 28% higher risk of diabetes than women who consumed the least. “There’s pretty good evidence that some of the adverse effect of red meat consumption is related to its high iron content, which probably damages the islet cells,” says Willett.
The non-heme iron in grains, beans, and vegetables is not under suspicion. “That’s an important difference because heme iron often gets an A+ because it’s more bioavailable,” says Willett. But that bioavailability is a double-edged sword. “The problem with heme iron is that we continue to absorb it even if we don’t need it,” he notes. “It overrides the regulatory mechanisms.” That doesn’t happen with non-heme iron. “If you have adequate iron stores, you won’t absorb non-heme iron very well,” says Willett. “That’s actually good.”
— Source: Nutrition Action, June 2009