Obesity and overweight in relation to organ-specific cancer mortality in London (UK): findings from the original Whitehall study

Abstract

OBJECTIVE:

To examine the relation of obesity and overweight with organ-specific cancer mortality.

METHODS:

In the Whitehall prospective cohort study of London-based government employees, 18 403 middle-age men participated in a medical examination between 1967 and 1970. Subjects were followed up for cause-specific mortality for up to 35 y (median: interquartile range (25th–75th centile); 28.1 y: 18.6–33.8).

RESULTS:

There were over 3000 cancer deaths in this cohort. There was a raised risk of mortality from carcinoma of the rectum, bladder, colon, and liver, and for lymphoma in obese or overweight men following adjustment for range of covariates, which included socioeconomic position and physical activity. These relationships held after exclusion of deaths occurring in the first 20 y of follow-up.

CONCLUSION:

Avoidance of obesity and overweight in adult life may reduce the risk of developing some cancers.

Introduction

Although elevated rates of cancer mortality in individuals with higher body weight were first documented almost a century ago,1 most attention has focused on the role of obesity in the aetiology of coronary heart disease (CHD). In large-scale prospective studies, findings are generally consistent: a positive association is apparent such that obese and overweight persons, as indexed by their body mass index (BMI), experience a higher risk of CHD than their leaner counterparts.2 This relation may be largely ascribed to mediation via the established CHD risk indicators of blood pressure, blood lipids and glucose tolerance.2

In the last two decades, the cohort studies on which these observations are based have matured, so accumulating sufficient events to allow investigators to examine the link between obesity and some organ-specific cancers. There is a consensus that obesity and overweight are associated with an increased risk of cancer of the breast (in postmenopausal women), endometrium, kidney, colon (strongest in men), oesophagus and pancreas.3, 4 However, studies examining the influence of obesity and overweight on other malignancies—prostate, liver, stomach, bladder, lymphoma and leukaemia—reveal inconclusive findings.3, 4, 5 These discrepant results may be explained by variability in definition of obesity and overweight across reports, so complicating comparison; and a failure to adjust for important covariates, such as socioeconomic position6 and physical activity.7 Additionally, in cohort studies, the presence of subclinical malignancy at baseline may lead to low body weight. It is likely, therefore, that the positive obesity/overweight–cancer gradient seen for some sites would, in fact, be steeper if deaths occurring in the early years of follow-up were excluded from analyses. However, the few extant prospective cohort studies have a sufficiently high number of cancer cases with which to examine this issue of reverse causality.

In the Whitehall study, over 18 000 middle-aged London-based government employees participated in a medical examination in the late 1960s, which included an assessment of their BMI and a range of covariate data.8 In an extended (maximum 35 y) mortality surveillance of this cohort, there have been over 3000 cancer deaths, enabling us to address these issues of data scarcity and methodological shortcomings. In earlier (≤15 y) follow-ups of this cohort, raised risks of total mortality,9, 10 cardiovascular disease9, 11 and total cancers9 were reported in overweight and obese groups. In this most recent follow-up we examine, for the first time, the link between obesity and a range of organ-specific malignancies.

Materials and methods

In the Whitehall study, data were collected on 18 403 non-industrial London-based male government employees aged from 40 to 64 y when examined between September 1967 and January 1970, representing a 74% response. This involved the completion of a study questionnaire and participation in a medical examination, both of which have been described in detail elsewhere.8 In brief, the questionnaire included enquiries regarding civil service employment grade (an indicator of socio-economic position),12 smoking habits,13 intermittent claudication,14 angina,15, 16 chronic bronchitis,17 marital status,18 physical activity,19 unexplained weight loss in the preceding year and the use of drug therapy for heart problem or high blood pressure.8 Forced expiratory volume in one second (FEV1) adjusted for height,20 ischaemia,21 fasting plasma cholesterol,22 postchallenge 2-h blood glucose,23 blood pressure,24 and triceps skinfold thickness8 were determined using standardised protocols. In addition, in a representative sample of the cohort, 1669 men participated in a dietary survey. This involved the completion of a 3-day semiquantitative record of all food and drink consumed.25

Assessment of obesity and overweight

Height was measured with the subject wearing shoes and standing with his back to a measuring rod; readings were taken to the nearest ½ in. (approximately 12.7 mm) below.8 Weight was recorded with the subject wearing shoes but with jacket removed; readings were taken to the nearest ½ lb (227 g).8 Following conversion from imperial to metric units, BMI (weight (kg) divided by height squared (m2)) was computed. To facilitate comparability of our findings with those from other studies,26, 27, 28 we defined underweight (<18.5 to 25.0 kg/m2), normal weight (BMI 18.5 to <25.0 kg/m2), overweight (25.0–29.99 kg/m2) and obesity (≥30.0 kg/m2), according to criteria advanced by the World Health Organisation.29

Ascertainment of cancer mortality

The records of 18 245 men (99.1% of subjects) were traced and flagged using the procedures of the National Health Service Central Registry (NHSCR) until 31 December 2002. Among the 11 710 men who died, 91.6% of death certificates were coded according to the eighth revision of the International Classification of Diseases (ICD),30 7.0% according to the ninth revision31 and 1.4% according to the tenth revision.32 The category of all malignant neoplasms (ICD8: 140–208; ICD9: 140–209; ICD10: C00–C97)—referred to as ‘all-cancers’—was divided into individual organs. These were: oesophagus (ICD8/9: 150; ICD10: C15); stomach (ICD8/9: 151; ICD10: C16); colon (ICD8/9: 153; ICD10: C18); rectum (ICD8/9: 154; ICD10: C19); liver (ICD8/9: 155–156; ICD10: C22-C24); pancreas (ICD8/9: 157; ICD10: C25); trachea, bronchus and lung (ICD8/9: 162; ICD10: C33–C34; referred to as ‘lung cancer’); prostate (ICD8/9: 185; ICD10: C61); bladder (ICD8/9: 188; ICD10: C67); kidney (ICD8/9: 189; ICD10: C64–C66, C68); brain (ICD8/9: 191; ICD10: C71); lymphoma (ICD8/9: 200–203; ICD10: C81–C90); and leukaemia (ICD8: 204–207; ICD9: 204–208; ICD 10: C91–C95).

Data analyses

A total of 17 347 men identified in the NHSCR had data for BMI and all potential covariates. The cause of death for 41 of these was unknown and they were excluded from all analyses. In addition, we excluded 204 men classified as underweight (see later explanation) leaving an analytical sample of 17 102 men (92.9% of those recruited). In analyses of baseline characteristics according to the level of obesity and overweight, the prevalence of the former was adjusted for age (5 y age groups) by the direct standardisation method. Trends in proportions were tested for statistical significance using the Mantel–Haenszel test; for continuous variables, least-squares means were used to present the age-adjusted means, and tests for trend across obesity, overweight and normal weight groups were computed by fitting a linear trend term. In the examination of the relation of dietary characteristics with weight in a subsample of the present cohort, the distribution of alcohol intake was highly skewed; therefore, analysis was conducted on the logarithmically transformed data after adding 0.5 to all data points to overcome values of zero.

Models fitted with a BMI by follow-up time interaction term confirmed that the proportional hazards assumption was not violated. Thus, hazard ratios and accompanying confidence intervals were computed for the relation of obesity/overweight with each mortality outcome using Cox's proportional hazards regression model33 with follow-up period as the time scale. These models were initially adjusted for age and then for other potential covariates. For the purposes of statistical adjustment, age, triceps skinfold thickness, plasma cholesterol, height-adjusted FEV1 and systolic blood pressure were fitted as continuous variables; while unexplained weight loss in the last year (two levels), employment grade (5), marital status (4), blood pressure-lowering medication (2), blood glucose levels (3), disease at study entry (2) and physical activity (6) were fitted as categorical variables. During the baseline study, the physical activity enquiries on the questionnaire were modified. Levels of this behaviour were therefore determined from either an item about travel activity19 (administered to approximately two-thirds of men) or from leisure activities34 (administered to the remainder). Analyses of the obesity/overweight–cancer relation indicated that there was no confounding effect due to questionnaire type. Smoking status was grouped into four categories (never, ex-smoker, current pipe or cigar smoker, current cigarette smoker) together with additional adjustment for the number of cigarettes smoked per day in current smokers. Existing disease at study entry was defined as a positive response to enquiries regarding a range of health conditions: myocardial ischaemia, intermittent claudication, physician-diagnosed heart problems or high blood pressure (one question), dyspnoea and bronchitis. The existence of ischaemia was determined from ischaemic signs on an ECG trace, or positive responses to either the Rose angina questionnaire or a report of severe pain across the front of the chest lasting 30 min or more.15 Men with diabetes comprised those who gave a positive response to the questionnaire enquiry ‘are you, or have you been, diabetic?’, or those who had blood glucose level 2 h after the glucose load of ≥11.1 mmol/l (≥200 mg/100 ml). A blood glucose of 5.4–11.0 mmol/l (96–199 mg/100 ml) was used to designate participants with glucose intolerance, with all remaining men termed normoglycaemic.23

To address the problem of reverse causality—for some cancers, tumour presence may lead to weight loss, so attenuating the obesity–cancer relationship—we took three approaches. Firstly, we dropped the underweight group from our analyses on the understanding that this group would contain some men with undetected cancer that may have resulted in weight loss. Secondly, we adjusted for unexplained weight loss in the preceding year and existing illness at study entry. Finally, in subsequent analyses, we excluded deaths in the first 10 and 20 y of mortality surveillance. In so doing, we reasoned that a significant proportion of deaths attributable to cancer, if present at study induction, would have occurred within this time frame.35 All statistical analyses were conducted using SAS computer software.36

Results

In Table 1 the relation of obesity and overweight with baseline characteristics are presented. Men with obesity comprised 4.2% (717) of the analytical sample. The most unfavourable level of each baseline characteristic was apparent in the obese and overweight study participants, the only exception being cigarette smoking where normal weight men had the highest prevalence.

Table 1 Obesity and overweight in relation to baseline characteristics (1967–70)

In Table 2 the relations of overweight and obesity to a range of dietary characteristics, assessed in a subgroup of the study population, are depicted. Higher quantities of alcohol consumption were apparent in the men classified as obese, while lower levels of fibre and fat (total and saturated) intake were seen.

Table 2 Overweight and obesity in relation to self-reported dietary intake over a 3-day period (N=1652)

In 17 102 men there were 10 901 (63.7%) deaths over a maximum of 35 y follow-up. Of these, 3051 deaths were ascribed to all-cancers, the most common site being that of the lung (26% of all cancer fatalities). Of all cancers, 498 occurred within the first 10 y of follow-up and 1401 within the first 20 y of follow-up. In Tables 3 and 4 the relationships between obesity, overweight and mortality from various cancer sites are presented.

Table 3 Mortality rates and hazard ratios for selected site–specific cancer deaths in relation to obesity and overweight
Table 4 Mortality rates and hazard ratios for selected site-specific cancer deaths in relation to obesity and overweight

The suggestion of a raised rate of total cancers in the obese and overweight groups in the age- and multiply-adjusted analyses was essentially eliminated when deaths occurring in the first 20 y of follow-up were excluded from the analysis. Owing to the low prevalence of obesity in the present study population, there were few cancer deaths in this group for some anatomical sites; some of our findings should therefore viewed with caution. In an age-adjusted analysis, the lowest risk of lung cancer deaths was apparent in obese men; however, following adjustment for confounding factors which included smoking—the highest prevalence of this behaviour was evident in the normal weight group—this relation was lost. There was a suggestion of elevated rates of carcinoma of colon and lymphoma in the obese and overweight groups following full adjustment and exclusion of deaths in the first 20 y of follow-up, although statistical significance at conventional levels was not always evident.

Of the individual cancer sites depicted in Table 4, there were raised rates of carcinoma of liver, rectum and bladder in the obese or overweight groups, although most confidence intervals included unity. In contrast, there was a suggestion of a non-significant inverse weight–kidney cancer relation although confidence intervals were wide owing to a low number of cases. There was little evidence of an obesity/overweight–malignancy gradient for any other cancer site featured in Tables 3 and 4. When we included the underweight group in age-adjusted analyses with total cancers as the end point of interest (data not shown), we found an elevated rate in this group (HRunderweight vs normal weight (95% CI): 1.61 (1.21, 2.13)) that was heavily attenuated when remaining covariates were added to the multivariable model (1.22 (0.91, 1.62)). There were too few site-specific cancer cases in the underweight group (N=204 men) to facilitate further analyses.

Discussion

The main finding of this study was an elevated risk of mortality from carcinoma of the rectum, bladder, colon, and liver, and for lymphoma in men who were obese or overweight in comparison to those in the normal weight group, although statistical significance was not always apparent. There was little evidence of an weight–cancer gradient for other malignancies.

Comparison with other studies

Some investigators combine colon and rectal cancer into a single disease category when examining their aetiology; however, owing to their dissimilar epidemiology,37 separation is warranted. Previous studies in men suggest a direct obesity/overweight–colon cancer association,3, 4 as apparent herein, although this is not a universal result.38 We also found a positive non-significant gradient for rectal cancer, an association that has previously been suggested to be null.39, 40 Three other studies26, 41, 42 have reported an excess occurrence of malignant neoplasm of the liver in overweight persons. While we made the same observation in the present study, these findings should be viewed with caution owing to the very low number of cases; a caveat that should also be applied to some of our other results. In a recent meta-analysis of observational studies, adiposity was associated with increased pancreatic cancer risk.28 That this effect was modest—HR (95% CI); 1.19 (1.10, 1.29)—implicates unmeasured or residual confounding as a likely alternative explanation.28 Indeed, in the present analyses in which we utilised a wide range of confounding factors, we found no evidence of an association between BMI and this cancer.

Plausible mechanisms

Several mechanisms have been invoked for the higher occurrence of some cancers in the obese or overweight. Differences in incidence of some cancers across body weight categories may reflect differences in dietary characteristics, such as fat and fibre intake.43 In the Whitehall study, a small representative subgroup of 1669 participants completed a 3-day dietary record during the period of baseline examination.25 While there were insufficient site-specific cancer cases to examine the potential confounding effect of dietary characteristics on the obesity/overweight–cancer association in this group, when we related intake to overweight and obesity, the differences across these groups were small and not always most favourable in the leanest subjects. It is unlikely, therefore, that these small and inconsistent differences in diet were large enough to explain differences in later cancer risk across the body weight groups.

A more specific explanation for the obesity/overweight–cancer relation that has received much recent attention concerns the role of insulin and insulin-like growth factors. These hypotheses suggests that obesity precipitates insulin resistance and the resulting prolonged hyperinsulinaemia in itself acts as a tumour growth promoter.44 Additionally, higher levels of insulin-like growth factor (IGF-I)—a multifunctional, circulating peptide that encourages tumour growth through its mitosis and antiapoptosis properties45—are positively associated with prostate and colorectal cancer risk,46 both energy-related malignancies. While IGF-I is also positively related to BMI, the gradient is in fact non-linear (at obese levels it decreases).47 Although we found an elevated risk of colon and rectal cancer mortality in the obese and overweight groups herein, there was no evidence of a relationship with prostate cancer.

Study strengths and limitations

The strengths of the present study include its size—superior to most, if not all,48 reports in the literature—which facilitated an examination of reverse causality; its prospective design; the measurement of a range of covariate data including physical activity and socio-economic position; and the definition of obesity and overweight which matches WHO criteria. These strengths notwithstanding, this study is not without its weaknesses. The assessment of obesity and overweight was based on BMI, an imperfect measure of adiposity. Although skinfold thickness was measured in the Whitehall study participants, readings were only taken at the triceps, rendering the data of little practical use. In contrast to many previous reports that focus on a single cancer site rather than a range (perhaps in the interest of generating a greater number publications), we related obesity and overweight to 16 mortality outcomes. Clearly, not all of the relationships we examined were hypothesis driven. In so doing we wished to gain insight into specificity of association,49 an important determinant of causality in observational epidemiology.50 However, one disadvantage of this approach is that some of our findings may have arisen by chance. Additionally, the cancer outcomes reported herein were based on mortality surveillance. Thus, our results reflect the combined effect of weight on survival and incidence. The suggestion has been made that the relation of weight to survival may be higher than that for incidence of some cancers (eg breast).48

In conclusion, an elevated risk of mortality from carcinoma of the rectum, bladder, colon, and liver, and for lymphoma in men who were obese or overweight in comparison to those in the normal weight group was apparent in this cohort of London-based government employees. Avoidance of obesity and overweight in adult life may therefore reduce the risk of developing these cancers. However, prevention and treatment of obesity is unlikely to be achieved through attempts to alter physical activity and dietary habits at the level of the individual.51 Rather, fundamental changes in environmental structure that includes ready access to amenities conducive to physical activity (eg parkland) and retail outlets that provide a range of micronutrient-dense food sources is crucial.52

Contributions

David Batty generated the idea for this paper; Martin Shipley conducted all data analyses. David Batty wrote the first draft of the manuscript to which co-authors contributed.

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Acknowledgements

We are grateful to the civil servants who gave of their time to take part in the baseline study. The original screening of participants in the Whitehall study was funded by the Department of Health and Social Security and the Tobacco Research Council. Martin Shipley and Elizabeth Breeze are supported by the British Heart Foundation; Michael Marmot by the UK Medical Research Council (MRC). When work on this report began, David Batty was financed by the UK MRC at the London School of Hygiene and Tropical Medicine and subsequently by a University Senior Research Fellowship at the University of Copenhagen. He is now the recipient of a Wellcome Advanced Training Fellowship.

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Batty, G., Shipley, M., Jarrett, R. et al. Obesity and overweight in relation to organ-specific cancer mortality in London (UK): findings from the original Whitehall study. Int J Obes 29, 1267–1274 (2005). https://doi.org/10.1038/sj.ijo.0803020

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Keywords

  • overweight
  • cancer
  • Whitehall
  • cohort study

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