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Clinical Review ABC of breast diseases

Breast cancer—epidemiology, risk factors, and genetics

BMJ 2000; 321 doi: https://doi.org/10.1136/bmj.321.7261.624 (Published 09 September 2000) Cite this as: BMJ 2000;321:624

Rapid Response:

Body mass index (BMI) and breast cancer risk

Dear Sir

I feel I must comment on the statement made about Body Mass Index
(BMI) in the table on established or probable risk factors for breast
cancer in the paper Breast cancer – epidemiology, risk factors, and
genetics by McPherson K, Steel CM and Dixon JM (BMJ 2000; 321: 624-628)
(1). Apart from the obvious mis-statement that the high-risk group for
premenopausal women are those with a BMI > 35 I have grave doubts about
the usefulness of an arbitrary dividing value of 35 for BMI in the
discussion of risk factors for breast cancer.

It seems clear that the implication of the statements made here is
that for post menopausal women there are two groups: a low risk group with
BMI of 35 or under, and a high risk group with BMI of over 35, and the
relative risk comparing the two groups is 2.

It is also implied that for premenopausal women the situation is
reversed: there is a low risk group with BMI over 35 and a high-risk group
with BMI of 35 or under, and the relative risk is 0.7.

A few moments thought suggests that a BMI of 35 or more indicates a
high level of obesity. The results of NHANES, (Kuczmarski RJ, 1997) (2)
in the USA suggests that BMI above 25 indicates overweight and over 30
indicates obesity. In a pooled analysis of cohort studies on breast
cancer risk by van den Brandt et al (2000) (3), the proportion of
premenopausal cases with BMI values above 35 was not given but the
proportion of cases with BMI above 33 was 20/703 = 2.84%. This would
imply that at least 100 – 2.84 = 97.16% of cases are part of the high-risk
group.

The paper (3) also indicates that for post-menopausal women the
proportion of cases with BMI over 33 is 185/3206 = 5.97%, so the
proportion in the high-risk group here with BMI > 35 must apparently be
smaller than this.

This seems to indicate that dividing women into two high/low risk
groups according to BMI above or below 35 is not helpful.

What the paper (3) does point out is that in premenopausal women the
relative risk for those with a BMI above 31 is 0.54 (95% CI : 0.34, 0.85),
compared to premenopausal women with a BMI of less than 21. For post-
menopausal women, compared with those with a BMI of less than 21 that the
relative risk for those whose BMI was greater than 28 was 1.26 (95% CI :
1.09, 1.46) and that it did not increase further for BMI values greater
than 28.

When relative risks are given the groups being compared should always
be clearly defined.

Yours faithfully

Alan C C Gibbs

Lecturer in Medical Statistics

REFERENCES

1. McPherson K, Steel CM, Dixon JM. Breast cancer – epidemiology
risk factors and genetics. BMJ 2000; 321: 624-8.

2. Kuczmarski RJ, Carroll MD, Flegal KM, Troiano RP. Varying body
mass index cut-off points to describe overweight prevalence among US
adults NHANES III (1998 to 1994). Obes Res 1997; 5: 542-8.

3. van den Brandt PA et al. Pooled analysis of prospective cohort
studies on height, weight and breast cancer risk. Am J Epidemiol 2000;
152: 514-27.

Competing interests:  
None declared

Competing interests: No competing interests

16 May 2003
Alan C Gibbs
lecturer in medical statistics
Centre for cancer epidemiology,Christie hospital trust.Manchester,M20 4QLBreast cancer incidence