Analysis Interpretation

In general, these studies of vegetarian populations have revealed lower cancer mortality (or incidence) in vegetarians than in non-vegetarians. In comparing vegetarians with the general population, in comparing vegetarians specifically with non-vegetarians, in comparing different cancer sites for different genders, ages groups, and time periods, a total of 204 SMRs or relative risks have been evaluated. More than 70% (71%) of the

Cancor site

Figure 4.13 Standardized incidence ratios for selected cancer sites in SDA males age > 35 years in Copenhagen, 1943-1977 (Jensen, 1983).

Cancor site

Figure 4.13 Standardized incidence ratios for selected cancer sites in SDA males age > 35 years in Copenhagen, 1943-1977 (Jensen, 1983).

SMRs or relative risks were decreased (i.e., <100 or 1.0) when comparing vegetarians with non-vegetarians.

Of the 145 SMRs or relative risks that were less than 100, 39% were deemed to be statistically significant (i.e., not due to chance). These figures varied when evaluating specific cancer sites, however. For colon cancer, out of a total of 20 studies, 19 (95%) reported an SMR/RR less than 100 (or 1.0). Six of these were statistically significant. For the other cancer sites evaluated, this figure was lower. For prostate cancer, only 55% of the SMRs reviewed were <100 (none significantly so) while for breast cancer 67% were <100, of which one was significantly less than 100 and two were significantly greater than 100. Table 4.3 summarizes the percent of SMRs and/or RRs less than the null value for all cancer sites combined as well as for colon, prostate, breast, lung, stomach, pancreas and ovarian cancer as well as for the lymphatic/hematopoietic cancers.

The difference experienced by vegetarians compared with non-vegetarians appeared to be moderated by gender. Males appeared to enjoy a stronger degree of protection from cancer mortality/incidence than females. For example, four out of six studies (67%) that evaluated col-orectal cancer risk by gender revealed lower SMRs for males than females. For lung cancer, six out of six (100%) SMRs were lower in males and for all cancer sites combined, six out of six studies (100%) showed lower SMRs in males than females.

Table 4.3 Standardized Mortality (Morbidity) Ratios and Relative Risks for Vegetarian vs. Non-Vegetarian Populations for Several Types of Cancer

Study

Years

Subgroups

Sites

Colon

Breast

Prostate

Lung

Stomach

Lymph/ Hem.

Pancreas

Ovar)

Kinlen's Nuns10

1911-1978

No Meat

96

99

87

-

45*

95

-

-

98

Some meat

104

93

117

-

20*

76

-

-

116

Kinlen's

Males

101

81

116

78

105

189

282*

-

Vegetarians11

1936-1970

Females

139*

78

215*

83

168

125

126

-

Health Food

Males

50*

64*

-

106

27

37*

-

79

-

Shoppers13

1973-95

Females

76*

87

88

-

37*

66

-

100

90

Health Food

Veg vs.

-

90

174*

131

113

123

-

-

-

Shoppers14

1973-1995

Non-Veg.

Oxford

Vegetarians15

1981-1995

Both Sexes

50*

-

-

-

-

-

-

-

-

Oxford

Veg. vs.

Vegetarians14

1981-1995

Non-Veg.

-

94

110

42

66

46

-

-

-

German Veg.16

1979-89

Males

48*

44

-

-

8*

78

-

137

-

Females

74

78

-

-

89

63

-

-

-

SDA (U.S.)19

1955-59

Males

69*

75

-

73

33*

76

72

92

-

Females

81*

83

77

-

55

67

95

108

-

SDA (U.S.)21

1960-76

Males

51*

56*

-

86

11

98

107/59*

-

-

Females

68*

51*

89

-

28

67

104/54

-

-

SDA (U.S.)28

1976-82

Males

73*

64*

-

125

25*

50*

100

68

-

Females

92

76*

91

-

36*

16*

87

76

129

SDA (Dutch)35

1968-77

50*

43*

50*

-

45*

59*

-

330

  • Copenhagen)37 Japanese Vegetarians17
  • SMR< 100

1943-1977 Males

1966-81 Males

107 91 74 91

  • 83 .95 .67
  • SMR/RR statistically significant (p<0.05)

98 89

62* 112

82 126

106 150

101 154

VI VI

A possible explanation for this phenomenon is the lifestyle habits of the general population. Traditionally, the prevalence of smoking and drinking is lower among females than males in the general population. When comparisons are made between female vegetarians and female non-vegetarians, therefore, the contrast is not as dramatic as when vegetarian males are compared with non-vegetarian males, who generally consume more tobacco and alcohol than females. This may explain the apparent stronger effect of vegetarianism on cancer risk among males.

Overall, the protective association with vegetarianism seems most pronounced for colon, stomach, and lung cancer and less pronounced for ovarian, pancreatic, and the lymphatic/hematopoietic cancers.

When within-group comparisons are evaluated where dietary variation exists (e.g., within the SDA populations), several patterns emerge regarding consumption of various foods and food groups. In the SDA mortality studies in the U.S., in which high fat foods of animal origin are examined (e.g., meat, milk, eggs, and cheese), no strong consistent pattern of increasing risk of cancer attendant to increasing consumption of these foods is observed. For several cancer sites including prostate, ovary, colon, and breast cancers, relative risks are either close to the null value (i.e., 1.0) or only slightly higher (e.g., 1.3-1.8). Of the 21 relative risks evaluated in regard to cancer mortality associated with animal-product consumption in the Adventist Health Studies (Table 4.1), only 19% were >1.5, while 52% were between 1.0 and 1.5. Another 29% were actually <1.0 in value. Values of this magnitude (i.e., close to 1.0) must be interpreted cautiously, as they might be explained by misclassification, confounding, or sampling error.

Data from the SDA incidence study provided slightly more evidence for increased cancer risk with increasing meat consumption (particularly for colon and prostate cancers), but no strong evidence for a positive association with breast cancer. On the other hand, several strong reductions in cancer risk were observed in association with frequent consumption of several types of vegetables and fruits, as well as for consumption of soy-based foods, legumes, and nuts. These patterns were observed for most of the cancer sites evaluated in the incidence study including lung, prostate, colon, bladder, and pancreas.

Although initial interest in the diet-cancer relationship focused on dietary fat as a potential culprit, studies conducted among vegetarian populations during the last two decades have not, in general, supported the dietary fat hypothesis for many forms of cancer. Rather, a pronounced protective effect of fruit and vegetable consumption has emerged.

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