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Most reputable sources of nutrition information recommend a diet high in fruits and vegetables and low in red meat. Vegans go much further. Strict vegans reject all animal products including fish, eggs, and milk. Some vegans come across like religious zealots. Here are some comments recently posted by vegans on Facebook:

Right now the biggest social issue facing the world is the violence and suffering of animals.

The dairy industry is the number one feminist issue facing our modern society.

I expect within a generation that milk will be viewed as the most unhealthy habit after cigarettes. I bet it is responsible for more disease than anything else in the US. Dairy products promote all stages of cancer. [In fact, low fat dairy can be protective against some types of cancer]

against some types of cancer] Milk contains blood and puss[ sic ]

] Humans are not omnivores; they are herbivores. [Most biologists would disagree.]

I was even told that that anyone who really cares about the welfare of others must promote veganism. It seems I am an evil, uncaring person if I waste my time writing about any other subject.

Vegans offer some good arguments based on ethics, environmental protection, cruelty to animals, and sustainability. I won’t get into those issues here. I’ll only address the scientific evidence behind the health claims. How does this description of a video strike you?:

Death in America is largely a foodborne illness. Focusing on studies published just over the last year in peer-reviewed scientific medical journals, [Hardly any of the studies he cites were published over the last year.] Michael Greger, M.D., offers practical advice on how best to feed ourselves and our families to prevent, treat, and even reverse many of the top 15 killers in the United States.[emphasis added]

That video was recommended to me by a vegan activist. I dipped into it randomly and noticed statements that I knew were not true, at least not as stated without any qualifications. I prefer to get my information from the medical literature rather than from videos. But I was eventually browbeaten into watching the whole thing when the activist impugned my objectivity and my ethics.

“If only you would watch this video”

I hear that all the time from people who have been overwhelmed by the information presented in a video that supports their beliefs. They assume that the evidence presented is incontrovertible, and that anyone who agreed to watch it would necessarily be converted to their beliefs. These videos tend to fall into an easily recognizable pattern. They feature a charismatic scientist with an agenda who makes sweeping statements that go beyond the evidence, makes unwarranted assumptions about the meaning of studies, and omits any reference to contradictory evidence. I recognized this pattern by briefly sampling the video, and my initial opinion was only confirmed by watching it in toto.

The leading causes of death

He starts with a table showing the leading causes of death and goes through them one by one, presenting his evidence that a diet devoid of animal-based foods can prevent and cure each of them.

To evaluate the accuracy of his information, let’s ask:

What exactly is he claiming?

What references does he supply in support of that claim?

Do those reference really say what he says they say?

Have those studies been replicated?

How good was the methodology of those studies?

Are there other studies that came to different conclusions?

Is there any good evidence comparing total avoidance of animal-based foods and a diet that is mainly plant-based but includes small amounts of animal products?

Cardiovascular disease (CVD)

He cites a reference showing that “a plant-based diet of primarily whole grains, fruits, vegetables, and legumes can completely prevent heart attacks.” This is a quotation from an article in the Food and Drug Law Journal, and the footnotes there only send us to Caldwell Esselstyn’s flawed research. Esselstyn studied only a small number of patients who already had heart disease, and he treated them with statin drugs in addition to diet, and their diet included skim milk and low-fat yogurt. You can read my criticism of his research here. It is ludicrous to interpret that research as showing that a plant-based diet can completely prevent heart attacks. A more accurate interpretation is that patients (only a few patients in one study) who had already had a heart attack did not have a second heart attack while being treated with cholesterol-lowering medications and a diet that was largely plant-based but also included foods derived from animals.

Cancer

That same article claims that up to 75% of cancers can be prevented, but the supporting reference indicate smoking accounts for 30% of cancers and diet alone might prevent somewhere between 20-42% of all cancers, and as little as 10% of certain individual types of cancer. It points out that “making quantitative estimates at this time is treacherous, as the available evidence can only be interpreted roughly,” because of confounders like exercise, methodological difficulties, and the need to rely on unreliable memory for recall of intake. They conclude that “one can sensibly recommend an abundant consumption of fruits and vegetables and low intake of red meat.” This supports mainstream nutrition advice, not veganism.

Inflammation

He cites a study showing that a single meal high in animal fat can paralyze our arteries and “cripple” them. This was a small study of 10 volunteers with no control group. It measured flow-dependent vasoactivity. It’s not clear what that means, but surely it’s an exaggeration to say that the arteries were paralyzed or crippled. It would be interesting to compare the results to those of vegans who ate a meal with an equal number of calories. And what we really want to know is whether the observed changes have any practical clinical significance.

Chronic obstructive pulmonary disease (COPD)

COPD “can be prevented and even treated with a plant-based diet.” He relies on a study that measured exhaled NO as a marker for inflammation, showing that it increases after a high fat meal. He describes it as causing internal damage. The study’s conclusion was “This suggests that a high fat diet may contribute to chronic inflammatory disease of the airway and lungs.” But this study showed no association between airway inflammation as measured by exhaled nitric oxide and systemic inflammation as measured either by CRP or fibrinogen. And it said nothing specifically about COPD or about the effect of removing animal foods from the diet.

Alzheimers

“We’ve known for 20 years that those who eat meat are 2-3 times as likely to become demented as vegetarians.” This claim is based on an old Adventist health study that has not been replicated. It studied two groups: matched and unmatched subjects. The data he cites are from the matched group. There was no difference in incidence of dementia between meat eaters and vegetarians in the unmatched study. Adventists are lacto-ovo-vegetarians who eat milk and eggs. And they are also a rather unique group with other healthy lifestyle practices. So it is disingenuous to claim this study as definitive evidence for veganism.

He neglects to tell us about studies that got different results, like the one showing that fish consumption reduces the risk of Alzheimer’s.

What is his evidence that Alzheimer’s can be treated with a plant-based diet? He offers a phase II study from Iran that compared saffron extract to a low dose of a drug that has only a small clinical benefit. The authors only claim it provides “preliminary evidence of a possible therapeutic effect of saffron.” Not very convincing, and certainly not evidence that a plant-based diet can treat Alzheimer’s. Saffron extract was being studied here as an herbal medicine, not as a food.

Kidney failure

Can kidney failure be prevented and treated with a plant-based diet? He points to a study showing that diets lower in red meat and animal fat may decrease the risk of microalbuminuria. It also showed a reduced risk with low fat dairy!

Other claims

He cites a study concluding “Our results suggest that a decrease in meat consumption may improve weight management.” Suggest, may, decrease. Not veganism.

He compares raw meat to hand grenades, because of bacterial contamination. If you don’t handle them safely, it’s like pulling the pin. Are we selling hand grenades in grocery stores? This is a ridiculous comparison, and it ignores the fact that plant-based foods can be a source of contamination too.

Flu: kale stimulates the immune system.

Eating just a few fruits and veg can improve the body’s ability to fight off pneumonia.

Suicide prevention? Restriction of meat fish and poultry improves mood.

I’m bored, and I’m sure you are too. There is more, much more. But I have made my point.

What Do Other Studies Show about the Benefits and Risks of Veganism?

This study showed mortality from ischemic heart disease was 26% lower in vegans and 34% lower in lacto-ovo-vegetarians (in other words, it’s better not to eliminate milk and eggs). “There were no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, lung cancer, breast cancer, prostate cancer, or all other causes combined.”

Another study showed that the healthiest people in Europe, the inhabitants of Iceland, Switzerland, and Scandinavia, consume large amounts of animal foods.

This study found no significant differences in mortality between vegetarians and nonvegetarians.

There are risks. A vegan diet can lead to deficiencies in various nutrients: vitamin B12, vitamin D, calcium, iodine, iron and omega-3 fatty acids. Careful planning can help avoid that; but anecdotally, the vegan who recommended the video to me recently found out he was deficient in B12 despite supplementation.

Confounders

The elephant in the room is weight loss. Vegans weigh less than meat eaters, and many of the benefts claimed could be consequences of weight loss, particularly in diabetes. And they could be a consequence of eating more fruits and vegetables, rather than avoiding meat and milk.

The data he finds most convincing are from the 7th Day Adventist study. One of his tables shows reduction in blood pressure and diabetes that are right in line with decrease in body mass index (BMI). This contradicts other data that supposedly removed BMI as a confounder. Adventists are a select group with other healthy lifestyle behaviors. Until we have confirmatory data from other studies in a general population, I don’t think it is wise to hang our hat on these decades-old Adventist studies.

Can diabetes be cured?

He says it can be cured by a plant-based diet, but the general consensus of medical experts is that diabetes can’t be “cured.” Diabetes can be treated and controlled with diet, weight loss, medication, obesity surgery, and even islet cell transplants. The symptoms subside, the blood sugar normalizes, and some patients no longer need their medication after they lose weight and make other lifestyle changes. But we don’t consider it “cured.” If you’re going to say diet “cures” diabetes, it would only be fair to say medication also “cures” it. It’s better to think of it not as cured but as controlled and requiring continuing attention.

Other diet beliefs

Surprise, surprise! Not everyone agrees that the vegan diet is best. Gary Taubes has written a book (with far more references than this video) advocating a low-carb diet. William Davis, the “Wheat Belly” doctor, tells us we must avoid a whole category of plant-based foods. He says meat is OK but wheat is addictive. The pH balance contingent tells us it is the acid in the food that is important, not whether the food comes from a plant or animal. The lacto-ovo-vegetarians don’t eat meat, but have no objection to eggs and milk. The paleo diet accepts meat, as does the Bible diet. Fruitarians reject animal products and also all vegetables and grains; they can be considered an even more restrictive form of vegans. Some of these diet beliefs are based mainly on ideology, but most of them claim to be based on solid science. There is only one science. If the evidence were really so clear-cut in favor of veganism, we wouldn’t have all these differing approaches.

What about the Eskimos?

Some people eat meat almost exclusively and seem to thrive on it. All the nutrients the human body requires are found in meat, even vitamin C when the meat is eaten raw.

Vegans tell us the Inuit, who lived almost exclusively on food of animal origin, had a short life span. That’s not true. Statistics on the Inuit between 1822 and 1836 showed that their average life expectancy was about the same as that of European peasants of the time who ate a diet overwhelmingly based on bread. 25% of Inuit lived past 60, and some lived into their 80s and 90s.

The Inuit ate meat out of necessity. It was all they could get for most of the year. Their diet was very high in fat. If it had not been, they could not have survived in one of the coldest, most barren, most hostile environments anywhere on Earth. Even today, it would not be wise for people living in the Arctic to try to follow a vegan diet.

Blubber is a staple of the Inuit diet, and it contains large amounts of antioxidants. Atherosclerosis is practically unknown in Greenland. In Uummannaq, Greenland, a population of 3000 residents had no deaths due to CVD in the 1970s. And the average 70 year old Inuit with a traditional diet of whale and seal has arteries as elastic as that of a 20-year old Danish resident. Why didn’t Dr. Greger mention that research? I think I can guess why.

Veganism prevents harm to animals

That sounds like a slam-dunk, but it’s more complicated. Some people think veganism is not really the lifestyle of least harm to animals.

Conclusion

The video confirmed what I already knew from evaluating the published evidence: it is healthier to eat more plant-based foods and less red meat. It didn’t convince me that we should categorically eliminate all animal products. The vegan diet can be a healthy one, and I wouldn’t discourage anyone from following it; but the evidence for health benefits is nowhere near as impressive or definitive as the true believers think. Death is not “a foodborne illness” and eliminating all animal products is not a cure-all.

As Ben Goldacre said in Bad Science :

The most important take-home message with diet and health is that anyone who ever expresses anything with certainty is basically wrong, because the evidence for cause and effect in this area is almost always weak and circumstantial…