Research linking vegetarianism to heart benefits doesn’t tell us what we really need to know

Yesterday, someone sent me an email alerting me to this BBC news report of a study which found that vegetarians were found to be at lower risk of heart disease compared to non-vegetarians [1]. The study looked at about 44,500 adults living in England and Scotland over an average period of about 11½ years. Hospital admissions for heart disease (e.g. angina and heart attacks) were monitored, as were deaths due to heart disease. Vegetarians, it turned out, were 32 per cent less likely to suffer from heart disease-related issues than non-vegetarians. The authors put this apparent benefit, at least in part, down to factors such as lower blood pressure, cholesterol and body mass index in vegetarians compared to non-vegetarians.

If you’ve read this blog before, you’ll likely be familiar with the idea that studies of this nature (so-called ‘epidemiological’ studies) may tell us that two things are associated with each other, but not that one is causing the other. In this instance, we simply don’t know if the vegetarians had lower risk of heart disease because, say, they are generally more health-conscious than non-vegetarians. In this study, the researchers attempted to ‘control for’ (take into account) factors that might queer the pitch (known as ‘confounding factors’) such as smoking and physical activity.

However, the fact remains this can only ever be a quite imprecise science, and at the end of the day we’re still left with evidence which can only prove association (but not ‘causality’). The authors of this study do not mention this major limitation at all, and only give the briefest of mentions to the fact that their efforts to control for confounding factors might have come up short.

For me, there’s another glaring problem with this study: the fact that it focuses on heart disease only. The problem here is that heart disease only affects a minority of people, and it’s generally much more informative to take a wider view. If we really want to do this as objectively as possible then I suggest we should, wherever possible, assess the relationship of any lifestyle factor or intervention has with overall risk of death (sometimes referred to a ‘total mortality’ or ‘overall mortality’). This encompasses all illnesses and the diagnosis is never in doubt (unlike heart disease, where the diagnosis may not be clear cut at all).

We actually have evidence from the same group of researchers (based at Oxford University in the UK) that vegetarians do not have reduced mortality compared to non-vegetarians [2,3]. One of these studies [3] looked at death from heart disease specifically (as well as overall risk of death), and found it was not lower in vegetarians either.

But let’s not get too hung up on the Oxford researchers, and perhaps cast our net further afield. Last year a group of Chinese researchers performed a ‘meta-analysis’ [4] of 7 relevant studies [5-11] (including more than one from the Oxford researchers) which included data relating to diet and overall risk of death in almost 125,000 people. Remember, this is still epidemiological evidence and incapable of discerning cause and effect. This caveat noted, the meta-analysis did show a lower risk of death from heart disease and cancer incidence in vegetarians.

However, when it came to the all-important measure of overall risk of death there was no apparent protection afforded by a vegetarian diet.

It seems odd to me that the Oxford researchers would publish their latest study, when better evidence regarding the possible effects of a vegetarian diet is already available, including their very own evidence. Their focus has narrowed and, I think, risks people losing sight of the overall picture. I’d say the scope and relevance of the Oxford researchers’ work is on a distinctly downward slide.

References:

1. Crowe FL, et al. Risk of hospitalization or death from ischemic heart disease among British vegetarians and nonvegetarians: results from the EPIC-Oxford cohort study. Am J Clin Nutr epub 30 January 2013

2. Key TJA, et al. Dietary habits and mortality in 11000 vegetarians and health conscious people: results of a 17 year follow up. Br Med J 1996;313:775-9

3. Key TJ, et al. Mortality in British vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). Am J Clin Nutr. 2009;89(5):1613S-1619S

4. Huang T, et al. Cardiovascular disease mortality and cancer incidence in vegetarians: a meta-analysis and systematic review. Ann Nutr 2012;60(4):233-40

5. Chang-Claude J, et al. Lifestyle determinants and mortality in German vegetarians and health-conscious persons: results of a 21-year follow-up. Cancer Epidemiol Biomarkers Prev 2005;14: 963–968

6. Key TJ, et al. Mortality in British vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). Am J Clin Nutr 2009;89:1613S–1619S.

7. Beeson WL, et al.Chronic disease among Seventh-day Adventists, a low-risk group – rationale, methodology, and description of the population. Cancer 1989;64:570–581

8. Thorogood M, et al.Risk of death from cancer and ischaemic heart disease in meat and non-meat eaters. BMJ 1994; 308: 1667–1670.

9. Berkel J, et al. Mortality pattern and life expectancy of Seventh-Day Adventists in the Netherlands. Int J Epidemiol 1983;12:455–459.

10. Ogata M, et al. Mortality among Japanese Zen priests. J Epidemiol Community Health 1984;38:161–166.

11. Key TJ, et al. Dietary habits and mortality in 11,000 vegetarians and health conscious people: results of a 17 year follow up. BMJ 1996;313:775–779.

18 Responses to Research linking vegetarianism to heart benefits doesn’t tell us what we really need to know

  1. Pauline 31 January 2013 at 6:31 pm #

    Do you think this could have anything to do with iron status? Vegetarians have a much lower iron intake, and too much iron is a risk factor for heart disease, I believe. When it comes to causes, it’s guesswork, even if it is educated guesswork.

  2. Georgia Ede MD 1 February 2013 at 4:13 pm #

    Dr. Briffa, thank you for calling attention to this topic. This “study”, like all epidemiological studies of vegetarians and vegans that I am aware of, suffers from a tragic flaw: it does not take refined carbohydrate intake into consideration. To exclude the single most common nutritional risk factor for heart disease (and chronic disease in general) renders this already weak study completely useless. Isn’t it possible (and rather likely) that the vegetarians in this study had less heart disease simply because they tended to eat less refined carbohydrates–sugars, refined flours, etc? Many vegetarians (but certainly not all) tend to be more health conscious in general and may simply eat more whole foods and less junk food.

    I have not been able to find a single shred of scientific evidence that eating more vegetables improves health–as crazy as that sounds! [I gave a presentation about this very subject at the Ancestral Health Symposium at Harvard last August, which is available on vimeo if anyone is curious.] I doubt that vegetables were what protected this group of people, and doubt that simply adding more vegetable matter to an otherwise unhealthy diet will reduce anyone’s risk for any particular disease.

  3. martin 1 February 2013 at 4:49 pm #

    Too much inorganic (supplements) iron can kill you pretty quickly. I’m fairly certain organic iron from plants, which you get body gets very little, will not. Veggie diets are great but only 1 in a 100 knows what they are doing…..yes I’ve researched and experimented since the early 90′s. Most people have an agenda and zero belief that vegan-ism works…But Vegan-ism does work that’s way more powerful if you really know via evidence. (Find yours) With a few good supplements it works better than any piece of solid blood. :) IMO

  4. Megan 1 February 2013 at 5:00 pm #

    As the price of meat goes up I’m sure we can expect many more research findings about the health benefits of going without it. I have recently read that grain-based diets (aka cheap carbs) “prevent” Alzheimers and help diabetics to control blood sugar all of which, I’d like to bet, is nonsense. Agribusiness, food manufacturers and researchers collude with governments to keep profits healthy, project money flowing and the rest of us under tight control.

  5. Albedo 1 February 2013 at 5:21 pm #

    Most studies in the medical field overlook the sociological factors. You don’t find a lot of vegetarians among the poorer layers of the population, those people who also tend to die a bit younger. Vegetarians tend to be, in average of course, a bit richer and more educated.

  6. Craig 1 February 2013 at 6:10 pm #

    Iron by its very nature is prone to oxidation – which is bad, as we all know.

    But if it was completely down to iron, then likely only male meat-eaters would be in the high-risk category because vegetarians get very little iron by way of their diet (both men and women) and women who eat meat lose blood regularly through menstruation, and thus iron along with it.

    I am actually surprised that vegetarians don’t have greater incidences of heart disease and cancer, seeing as they typically have a very low intake of B-vitamins to tackle homocysteine (a reliable marker for heart disease), carbohydrate-reliant diets (which have to take up the slack for the lack of protein and carbohydrates) which fuel sugar-loving tumours as well as diabetes and its complications, and the reliance on non-animal fats, most of which will be polyunsaturated (which are pro-inflammatory, decrease immune system function, and cause mutations due to their chemical instability – which are the three ideal factors for an environment that is cancer friendly).

    Just my 2c.

  7. mike 1 February 2013 at 6:34 pm #

    This is one of those dopey headlines the Mail and the Express love to trumpet every every other week (tomorrow I’m guessing). One week it’s cholesterol bad, the next… In this case, for me John has summed it up, but to simplify, it’s folk on the one hand who do give a damn about their health, and on the other the smokers, boozers and the bewildered.

  8. John Walker 1 February 2013 at 6:37 pm #

    Quote from embarrassing fat bodies:

    “Cholesterol is vital for the body. However, too much of it can lead to blocked arteries, strokes and heart attacks.” With advice like this from medical practitioners, what chance does anyone stand of ever being able to lose weight, following their eating plans?
    This program was shown on Channel 4 on Monday evening. If you want a good laugh at the nonsensical ”facts” imparted watch it on 4 od. It’s so sad for the obese people they feature, who are trying to lose weight, but you just know they will never get anywhere with conventional dietary wisdom. Just think of the money that the NHS could save, if they didn’t have to perform surgery to make stomachs unnaturally small, or to fit “gastric bands”. It’s a criminal shame that the ‘experts’ just don’t see that obese people, who “can’t’ lose weight”, are victims of misguided advice.

  9. Lucy 1 February 2013 at 6:44 pm #

    I’m a vegetarian and don’t have low iron levels – in fact, the last time I gave blood they commented on the good levels. I do eat a lot of spinach though!

  10. Judy Barnes Baker 1 February 2013 at 6:51 pm #

    Don’t forget they they are comparing the vegetarains to those eating the standard diet which is high in carbs, grains, sugar, and junk food. These studies often show that eating more red meat is dangerous, but most of the red meat consumed is hamburger. What goes along with a typical hamburger? A bun, french fries or chips, and a soft drink. Vegetarians are obviously concerned about health, so I think they would also be less likely to drink alcohol or soft drinks or eat cookies, fried foods, etc. Tom Noughton’s analyses of this study also pointed out that the vegetarians in the study were younger than the other group.

  11. Chloe 1 February 2013 at 7:02 pm #

    I know plenty of vegetarians who live on carbs and cheese. Not the best!

    @Lucy – interestingly, spinach doesn’t contain any more iron than other leafy greens. This misconception was based on a wrongly placed decimal point in a book detailing the nutritional contents of foods.

  12. Anonymous 1 February 2013 at 7:24 pm #

    I’ve found that vegetarians are far more likely to suffer from various mental illnesses (of vice-versa – mentally ill people are more prone to be vegetarians). Studies have shown the correlation, but my personal experience makes me believe it’s the former.

  13. Martin 2 February 2013 at 1:05 am #

    Eating meat can be healthy but unlikely seeing how animals are treated and the source is always suspect……Animals if treated better you would see a drop in diseases. If you see the pain and suffering how the hell do you think that is not transferred in some way via blood filth and hormones…..crazy to eat meat for any reason these days. you don’t live in the Serengeti, you live in hackney….or anywhere else for that matter. Cholesterol myth; It is the arteries not being able to attain enough Vitamin C, as the body needs all it can get to clear the fats away from the cell wall and rebuild it.

  14. david manovitch 2 February 2013 at 2:16 am #

    What is meant by risk of death? This term is meaningless without defining it. We all have a 100% risk of death do we not? What is important is years of life free from significant illness and overall life span. It is always difficult to exclude the confounding variables in this kind of research, not to mention inheritance. Access to ‘medical salvage’, that can artificially extend life needs to be corrected for too. Overall these studies prove little though it is a worthwhile question to ask.

  15. Reijo Laatikainen 2 February 2013 at 10:26 am #

    Nutrition science is always imprecise, ie. randomized trials produce conflicting evidence too. It’s frustrating to see how carb diet proponents embrace prospective cohort studies when the outcomes suit their purpose. But when not, the cohort studies are ridiculated. This is very consistent trend, not much conflicting evidence around ;-)

  16. tony kerstein 2 February 2013 at 8:58 pm #

    Spinach (and soy products) contains oxalic acid,the crystals of which cause kidney stones, as well as phytic acid which binds and neutralises much of the useful minerals, including iron.

  17. Stabby 3 February 2013 at 8:01 am #

    The new meta analysis for red meat and cardiovascular disease/diabetes is in and it doesn’t look like there is an association between fresh red meat and heart attack risk http://www.ncbi.nlm.nih.gov/pubmed/23001745 And not a high enough association with diabetes to suggest causation (though I think that overcooking meat can be a problem due to some controlled trials).

    However -processed- meat was associated quite strongly. This too is epidemiological and prone to confounding and the neglect of important contextual factors, but with regards to the totality of epidemiological evidence we can’t say that being a vegetarian reduces heart risk, only that being a no-processed-meat-a-tarian reduces CHD risk.

    That’s assuming that epidemiology proves causation, which it doesn’t. But just for the sake of argument I think that this demonstrates that vegetarianism reducing CHD risk is untenable based on epidemiology. The data suggests that it could be vegetarianism, but it just as strongly suggests that the benefit of vegetarianism is simply not eating processed meat.

    And Dr. Briffa is absolutely right, EPIC showed no improvement in all-cause mortality in the vegetarian group. One has to wonder if eating unprocessed meat that isn’t overcooked is a benefit to all-cause mortality! I’m not going to say it, but this data justifies the hypothesis.

  18. david manovitch 3 February 2013 at 6:32 pm #

    I can’t provide the references but is there not abundant evidence that vegetables or plant foods are good for us? Is there not evidence form the USA to show that a predominantly plant food diet reverses atheroma? Georgia Ede’s comments are puzzling. Surely she could not suggest that vegetables are an irrelevance as far as health is concerned. Many people now agree that the Hunter gatherer diet is that to which most humans are still adapted to, and which promotes optimal health. Vegetables are easier to gather in a technologically bereft society, than meat is to hunt, so it is logical that plant foods would play an important role in the pre-tool age. Meatv per se surely cannot be harmful provided it comes from healthy wild animals rather than farmed and certainly we should not eat the processed crap that is consumed today in western style societies. I would agree that vegetarian diets are only likely to be healthy if they avoid processed foods and particularly sugars and starch rich foods.

    The old adage of ” all things in moderation” still seems sensible to me, and as far as food is concerned I would add the words unprocessed/unrefined.

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