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Vegetarians live longer and have better health. While you don't necessarily have to become a vegetarian to benefit, even moderate reductions in animal protein intake are likely to improve your health.

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Vegetarians are healthier and live longer than omnivores. There have been many suggestions in the past that people who consumed less meat lived longer. Many isolated communities that were predominantly vegetarian have been observed to have long life expectancies of far exceeding 70years (Hunzakuts, Vilcabambas, mountain dwellers of Turkey, Russian Caucuses). In Scandinavian countries, lack of meat during both world wars caused a drop in mortality which returned to usual levels when meat became available again. The diet of the Mediterranean countries which has far less meat than other European countries is associated with a greater life expectancy of up to 2 years (Singh 03).

On examination, vegetarians are more likely to be thin, have lower cholesterol levels: 4.88 vs 5.31mmol/l. While this difference may seem small it represents a 24% reduction in the rate of ischaemic heart disease (narrowing of the coronary arteries). In the overview study of Key in 1998 where the results of five studies were combined with information about 76,000 people, those with higher meat intakes had greater risk of death. When regular meat eaters were assigned a relative risk of death of 1, semi-vegetarian (meat less than once a week) had a lesser relative risk of dying of 0.8. Vegetarians had a relative risk of 0.6 compared to meat eaters, that is a 40% lower death rate in the observation period (Key 99). (Relative risk is calculated by dividing the group into subgroups representing the presence or absence of a risk factor. The number of deaths in each group is corrected for the age of the various participants and then the proportions of death/total number in each group are compared.) 

A further report in 2003 of the analysis of six studies by Singh et al (with considerable overlap with the previously quoted study) came to similar conclusions (Singh 03).

Study (Main authors) Follow up yrs Vegetarian (number) Comparison (number) Adj Mortality
-Relative Risk (95% CI)

Oxford Vegetarian Study (UK) (Appleby)

22

Zero meat 
(n = 6000)

Meat eater
(n = 5000)

1.01
(0.89, 1.14)

Health Food Shoppers Study (UK) (Key)

17

Vegetarian
(n = 4627)

Non-vegetarian
(n = 6144)

1.04
(0.93, 1.16)

Germans
(Chang-Claude)

11

Vegetarian (1904)

General population

Men 0.44
(0.36, 0.53) Women 0.53 (0.44, 0.64)

Adventist Mortality Study (US) (Kahn/Singh)

26

Zero meat intake
(n = 7918)

Meat eaten once or more 0.88
per wk
(n = 6958)

0.88
(0.82, 0.93)

Adventist Health Study (US) (Frazer/Sing/Fraser and Shavlik)

12

Zero meat intake
(n = 7191)

Meat eaten once or more per wk
(n = 7463)

0.85
(0.76, 0.94)

Italians (Forbes)

5

Meat  <once per wk (NR)

Meat >once per wk (NR)

0.55
(0.28, 1.10)

These analyses were controlled for most of the major confounding factors: smoking, alcohol consumption, body mass index, exercise and education level when the data were available. Four of the six showed statistically significant decrease in risk of death for the very low meat intake, with the other two showing either non-statistically significant improvement or no difference. None showed a worse outcome for low meat intake groups. The longer that the low meat diet was adhered to, the greater the benefit. The major reduction in mortality was related principally to a lower incidence of coronary artery disease. The rate of cancers was unaffected. It has been noted in the past that red meat is associated with colonic cancer but this was not demonstrated (Singh03).

Does eating meat, particularly red meat predispose to cancer? There now very convincing evidence of the association between consumption of red meat as well as processed meat and cancer. Following on from the WHO 2002 report on diet and disease (WHO Report #916), the authoritative World Cancer Research Fund & American Institute for Cancer Research report on the relationship between lifestyle and cancer risk, further strengthens the link between some foods along with such factors as overweight/obesity/physical activity and cancer risk as set out in the following table (World Cancer Research Fund 07):

Evidence Decreased Risk Increased Risk
Convincing Physical activity (colon) Body fatness (oesophagus, pancreas, colorectum, breast, in post menopausal women, endometrium, kidney)
Abdominal fatness (colorectum)
Alcohol (Oral cavity, pharynx, larynx, oesphagus, liver, breast
Aflatoxin (liver)
Red meat (colorectum)
Preserved meat (colorectum)
Salt-preserved foods and salt (stomach)
Probable Fruits and vegetables (oral cavity, oesophagus, stomach, colorectum)
Physical activity (breast and endometrium)
Body fatness (breast - premenopause)
Milk (colorectum)
Body fatness (gall bladder)
Abdominal fatness (pancreas, breast -post menopausal, endometrium)
Alcohol (liver colorectum)
High calcium (prostate)
Salted or salty foods (stomach)

A study published in 2009 by the National Institutes of Health and the American Association of Retired People of over a half a million people clearly showed the increased risk of both cancer and heart death in those who ate the most red meat (68g/1000KCal) versus those who ate the least (9.3g/1000KCal) as shown in the following table (RR= relative risk) (Sinha 2009):

Disease type Lowest consumption Highest consumption
Cancer death risk 1(Reference RR) 1.22 RR
Heart disease death risk 1 (Reference RR) 1.27 RR

In fairness, the above study showed that eating white meat was protective for both heart disease and cancer.

Another convincing study has been published linking premenopausal hormone sensitive (ER+/PR+) breast cancer to red meat consumption. This study done in more than 90,000 nurses from 1991 to 2003, showed that the risk of this particular type of cancer (the most common form in this age group) increased progressively with the amount of red meat consumed from the lowest level of 3 or less servings/week to the highest of more than 1.5 servings per day. The relatively risk at the highest intake was just under twice that for the lowest intake as shown in the following table (Cho 06):

Servings red meat 3 or less per week More than 1.5 per day
Cases / Total number 121 / 26,787 52 / 8649
Crude rate per 10,000 45 62
Multivariate* Relative Risk 1.0 1.97

*Multivariate risk is adjusted for age and other risk factors such as smoking, parity, body mass index, family history, age of menarche, alcohol intake etc.

A general association of the level of meat consumption with the rate of breast cancer was shown in the 2007 UK Women's Cohort Study of over 35,000 women, with a higher risk for the post menopausal compared to pre-menopausal women (Taylor 07):
 
Meat consumption Total population HR* Pre menopausal HR Post menopausal HR
None
0 g/day
1.00 1.00 >1.00
Low
<62g/day
1.10 0.72 1.68
Medium
62-103g/day
1.3 1.00 1.81
High
>103g/day
1.4 1.16 1.87


*Hazard Ratio measures the change of risk from the baseline group (in this case zero meat consumption). The higher the number, the greater the risk. A HR of 2 for example doubles the risk of getting the disorder being measured.

Red meat consumption in mothers may cause fertility problems in male children. A 2007 study from the US (Swan 07) showed an inverse  lowering of sperm counts in relation to increasing maternal red meat consumption during pregnancy. It was speculated that the mechanism may be due to the presence of anabolic steroids and other xenobiotics in the meat.

Maternal Beef consumption in servings 7 or less per week More than 7
Sperm count in sons 85 X 106/mL 68.3  X 106/mL

There are a lot of vegetarians in the community. By default, if vegetarianism led to significant medical problems, it would become rapidly apparent. A survey done by the Harris group on behalf of the Vegetarian Resource Group in the US revealed the following statistics: (VRG)

Adults 18 and older

%
Never eat red meat 6.7
Never eat poultry 6.3
Never eat seafood 16.6
Who are vegetarian 2.3>
Who are vegans 1.4

This means that there are around 4.7 million adult vegetarians in the US, with vegans accounting for around one third to one half of these. Similar proportions are found the UK.

Intellectual development and vegetarianism. Leonardo da Vinci, Pythagoras, Einstein and Edison were all vegetarians. This would seem to cut across the assertion that we need a supply of the LC-PUFAs, DHA and EPA, from animal sources to supplement the claimed meagre endogenous production for proper brain development. See the section on brain development and LC-PUFAs. In a recent study in the UK, higher IQ was associated with an increased likelihood of being vegetarian. More than 8000 people from 1970 were studied. Four and a half percent reported being vegetarian at the age of 30. Higher IQ at the age of 10 increased the odds of being vegetarian at the age of 30 by 1.38 to 1. This remained true after social class, education level and sex were controlled for (Gale 06).

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References:

(Cho 06) Eunyoung Cho, Wendy Y. Chen, David J. Hunter, Meir J. Stampfer, Graham A. Colditz,  Susan E. Hankinson,  Walter C. Willett. Red Meat Intake and Risk of Breast Cancer Among Premenopausal Women. Arch Intern Med. 2006;166:2253-2259

(Gale 06) Catharine R Gale, Ian J Deary, Ingrid Schoon, G David Batty. IQ in childhood and vegetarianism in adulthood: 1970 British cohort study. 2006; DOI:10.1136/bmj.39030.675069.55

(Key 99) Timothy J. Key, Gwyneth K. Davey and Paul N. Appleby. Health benefits of a vegetarian diet. Proceedings of the Nutrition Society 1999; 58:271–275

(Pramil 03) Pramil N Singh, Joan Sabaté, and Gary E Fraser. Does low meat consumption increase life expectancy in humans? Am J Clin Nutr 2003;78(suppl):526S–32S.

(Singh 03) Pramil N Singh, Joan Sabaté, and Gary E Fraser. Does low meat consumption increase life expectancy in humans? Am J Clin Nutr 2003;78(suppl):526S–32S.

(Sinha 09) Rashmi Sinha, Amanda J. Cross, Barry I. Graubard, Michael F. Leitzmann, Arthur Schatzkin. Meat intake and Mortality. Arch Intern Med 2009; 169:562-575

(Swan 07) S.H.Swan, F.Liu, J.W.Overstreet, C.Brazil N.E.Skakkebaek. Semen quality of fertile US males in relation to their mothers’ beef consumption during pregnancy. Human Reproduction 2007 doi:10.1093 humrep/dem068.

(Taylor 07) EF Taylor, VJ Burley, DC Greenwood, JE Cade. Meat consumption and risk of breast cancer in the UK Women’s Cohort Study. Br J Cancer 2007; 96:1139-1146 (VRG) Vegetarian Resource Group. www.vrg.org

(WHO Report #916) WHO Technical Report Series #916. Diet, Nutrition and the Prevention of Chronic Diseases. 2002. (World Cancer Research Fund 07) Cancer Research. Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. 2007 Downloadable from http://www.dietandcancerreport.org/downloads/Second_Expert_Report.pdf