The initial results of the Linxian General Population NIT, published in 1993, showed no association between factors A, B, or C and overall mortality, total cancer mortality, or mortality from esophageal or gastric cancers (5
). However, factor D, which included selenium, vitamin E, and beta-carotene, statistically significantly reduced total mortality, total cancer mortality, and mortality from gastric cancer (5
). An important question remained, however, whether the preventive effects of factor D would last beyond the trial period. The results of the continued follow-up show that hazard ratios, as indicated by moving hazard ratio curves, remained less than 1.0 for each of these endpoints for the majority of the follow-up period; 10 years after completion of the trial, the group that received factor D still showed a 5% reduction in total mortality and an 11% reduction in gastric cancer mortality. Overall, one in 70 people who took factor D was spared death from all causes, and one in 227 was spared death from gastric cancer.
Stratification of results by sex and age was planned a priori. There were no statistically significant interactions with sex. However, when stratified by age, factor D had a strong protective effect in individuals younger than 55 years but almost no effect on subjects aged 55 years or older. This pattern was seen consistently for total mortality, total cancer mortality, gastric cancer mortality, and esophageal cancer mortality. Indeed, the effect of factor D on esophageal cancer was reversed by age, showing a protective effect for younger but a harmful effect for older individuals.
Because this trial provided selenium, vitamin E, and beta-carotene as one factor, it was not possible to disentangle the effects of these three supplements. However, observational case–cohort studies using subjects in this cohort and patients with upper gastrointestinal tract cancers that developed during the intervention period showed inverse associations between risk of esophageal cancer and baseline serum levels of selenium and alpha-tocopherol, but not beta-carotene (13
). Higher baseline serum selenium also was associated with reduced risk of gastric cardia cancer (13
). These results suggest that the protective effects seen in the randomized trial were due to the selenium and vitamin E components. In a subcohort of 1103 subjects from this trial followed through May 31, 2001, higher baseline serum selenium levels also were associated with statistically significant reductions in esophageal and gastric cardia cancer mortality (16
). A separate randomized controlled trial in Linxian (17
) gave further support for a preventive effect of selenium in subjects with preexisting esophageal squamous dysplasia, the precursor lesion of esophageal squamous cell carcinoma. Compared with control subjects, those with mild dysplasia who received 10 months of daily supplementation with 200 μg of selenomethionine were more likely to have regression and less likely to have progression of their esophageal squamous dysplasia.
In addition to evaluating the durability of the beneficial effects observed during the trial period for factor D, we also evaluated other postintervention events in this trial to look for late effects, and several were noted. When the full 15.25 years of follow-up were considered, nutritional supplementation with factor A (vitamin A and zinc) was associated with increased total mortality, mainly due to an increase in stroke deaths among subjects given factor A compared with those who were not given factor A, whereas supplementation with factor C (vitamin C and molybdenum) was associated with a decrease in stroke deaths and with a slight increase in esophageal/cardia cancer deaths.
Increased mortality among factor A recipients was not expected, given the low retinol levels of the population at the start of the trial, the modest doses of both vitamin A and zinc, and the generally null or protective results from previous observational studies in this cohort relating high serum retinol and tissue zinc concentrations to various cancer endpoints (15
). In fact, a previous analysis of stroke (the main contributor to the observed increase in total mortality) in this trial showed a protective effect for persons who took the combination of factor A and factor D (HR
0.71, 95% CI
0.50 to 1.00, for group AD vs placebo) (19
We were surprised that factor C, a combination of vitamin C and molybdenum, appeared to be associated with increased risk for the combined esophageal and cardia cancer endpoint, given the well-known role of vitamin C as an antioxidant and inhibitor of carcinogenic N
-nitroso compound production in the stomach (20
). Furthermore, many epidemiological studies have shown reduced risk of these cancers in persons with high consumption of fruits and vegetables and rich sources of vitamin C, and the only prospective study of plasma vitamin C and gastric cancer showed a protective effect for high concentrations (21
). No similar prospective studies of plasma vitamin C values in esophageal cancer are known.
The decreased risk of stroke among trial participants who received factor C was not wholly unexpected. Higher plasma vitamin C levels have been associated with reduced risk of stroke in prospective epidemiological studies (22
), although randomized trials that have included vitamin C as part of an antioxidant vitamin treatment arm have not shown any effect (25
). There are no data on molybdenum and stroke.
The effects of vitamin and mineral supplements on reduction of total mortality and cancer mortality have been heavily debated over the past 25 years. The results of this study need to be interpreted in the context of other trials of vitamin and mineral supplementation. By the 1980s, it was established that antioxidants could quench free oxygen radicals and potentially reduce the risk of cancer by preventing DNA damage by these radicals. Observational epidemiological studies showed inverse associations between cancer incidence and dietary intake of several vitamins and minerals, but more definitive evidence awaited the completion of randomized trials (27
). It was generally assumed that prescribing pills would be a more convenient and acceptable way to prevent cancer than proscribing carcinogens. These facts and assumptions motivated the design and conduct of the first generation of randomized controlled cancer prevention trials, including the Linxian General Population NIT, to reduce cancer risk using vitamins and minerals. The results of the largest and most informative of these trials (ie, those with more than 10 000 participants) were often contrary to the initial expectations. Beta-carotene supplementation increased total mortality in both the Alpha-Tocopherol, Beta-Carotene (ATBC) Cancer Prevention Study (28
) and the beta-CArotene and Retinol Efficacy Trial (CARET) Study (29
), whereas no mortality benefit for beta-carotene was seen in either The Physicians’ Health Study (PHS) (30
) or the Women’s Health Study (31
). The most likely explanation for the unexpected findings from these four large trials conducted in the West is a mismatch of the design of the trials with the population attributes: each of these trials tested pharmacological doses of micronutrients in already well-nourished populations (32
A recent meta-analysis of randomized trials of antioxidant supplements for prevention of cancer and other diseases (33
) combined the results of these large-scale studies with many smaller studies and concluded that treatment with beta-carotene, vitamin A, or vitamin E likely increased total mortality, and the effect of vitamin C or selenium on total mortality needed further study. The results of this study agree in part with the meta-analysis conclusions. In this trial, factor A (which included vitamin A) increased mortality, factor C (which included vitamin C) was not associated with overall mortality, and factor D (which included selenium) decreased mortality. However, beta-carotene and vitamin E, two supplements that were associated with increased mortality risk in the meta-analysis, were in factor D, which reduced mortality in this trial. Several potential explanations exist for these apparently discrepant results. First, the protective effect of selenium may have been stronger than the possible deleterious effects of beta-carotene and vitamin E in this trial, so the overall effect of factor D was beneficial. This hypothesis is supported by medium-sized trials that have shown beneficial effects for selenium in reducing mortality and cancer risk (34
A second possible explanation for these discrepancies is that baseline nutritional status of the populations studied influenced the supplementation effects. The people of Linxian are nutritionally deficient (3
), so vitamin and mineral supplements may be more beneficial to them than to other populations that have been studied. The results of the Dysplasia NIT (38
), a medium-sized randomized nutrition intervention trial that was conducted among subjects with cytologically diagnosed esophageal squamous dysplasia in Linxian at the same time as the General Population NIT, showed that supplementation with 26 minerals and vitamins was associated with a non–statistically significant 7% reduction in mortality risk. Results from the Nutritional Prevention of Cancer Trial showed that the benefits for selenium supplementation on total cancer mortality (39
) and the development of prostate cancer (36
) were essentially limited to participants with lower selenium levels at the start of the trial. Further support for this hypothesis comes from the data of the meta-analysis of antioxidant supplement trials and total mortality itself (33
). We classified the 68 study populations included in this analysis as Western (n
58), East Asian (n
8), or other (n
2) and found statistically significant heterogeneity between the results of the studies performed in Western and East Asian populations. The Western studies had a combined odds ratio (OR) of 1.04 (95% CI
1.01 to 1.06) and the East Asian studies had a combined OR of 0.92 (95% CI
0.84 to 1.02) (χ2
for heterogeneity P
.02). Because nearly all of the events in the East Asian group came from Linxian, which we know has borderline or deficient nutrition, this difference in meta-analysis results may well reflect differences in the baseline nutritional status of the populations evaluated.
A third possible explanation for the heterogeneity of results observed among studies that have evaluated the association of vitamin supplements and total mortality or gastrointestinal cancer risk is effect modification by the stage of disease at study entry. Our results show that only individuals younger than 55 years benefited from factor D. This result may indicate greater benefit earlier in the course of carcinogenesis and is consistent with a “point of no return,” beyond which supplementation with vitamins is not useful and may be harmful, preferentially benefiting the developing tumor more than the host. This hypothesis may help explain why observational studies, which reflect long-term intake of vitamins and vitamin-containing fruits and vegetables, have usually shown beneficial associations, whereas trials, which have largely been conducted in older patients, have sometimes shown harmful effects from vitamin interventions.
Participants in the ATBC and CARET studies, in addition to being older than those in the Linxian general population trial (ages 50–69 years for ATBC and 45–74 years for CARET), were heavy smokers and some were exposed to asbestos, both powerful carcinogenic exposures that may have put them beyond the point in the disease process that they could benefit from supplements. Detailed analyses of both of these studies have shown that the increased risk associated with vitamin use was almost exclusively seen in current (as opposed to former) smokers (40
) and in those who smoked most (41
). In contrast, the PHS study, which included fewer than 10% smokers, showed no adverse effect of beta-carotene (30
In addition to this report, three other cancer prevention nutrition intervention trials have reported results from continued follow-up after the termination of intervention. Follow-up of the participants of the ATBC Study for up to 8 years after the end of the intervention showed that both the harmful effects of beta-carotene (ie, increased total mortality and lung cancer incidence) and the beneficial effect of vitamin E (ie, decreased prostate cancer incidence) disappeared, albeit slowly (42
). However, analyses of cerebral infarcts (80% of all strokes) among vitamin E recipients in the ATBC Study showed reversed effects during the trial (relative risk [RR] = 0.86, 95% CI = 0.75 to 0.99) (43
) and the 6 years posttrial (RR = 1.13, 95% CI = 1.00 to 1.27) (44
). After 6 years of postintervention follow-up in the CARET study, the relative risk of total mortality remained greater than 1.0, but this elevated risk diminished and was no longer statistically significant (45
). Lung cancer mortality, however, was still statistically significantly increased. The Calcium Polyp Prevention Trial reported that the protective effect of calcium supplementation on colorectal adenoma recurrence found during the trial period (RR = 0.81, 95% CI = 0.74 to 0.98) (46
) continued up to 5 years after supplementation ended and was, if anything, stronger after than during the intervention itself (RR = 0.63, 95% CI = 0.46 to 0.87) (47
Durability of cancer prevention effects after cessation of intervention has also been observed with nonnutritional agents. In fact, the most consistent example of a sustained cancer prevention effect reported to date from any cancer prevention agent tested in trials is for tamoxifen in the primary prevention of breast cancer. Posttrial follow-up from three tamoxifen trials (48
) consistently found benefit after the conclusion of active treatment, and in one trial (50
), statistically significant reduction in risk (among patients with estrogen receptor-positive tumors) was seen only after treatment had ceased.
This study has several strengths. It was a randomized double-blind design and had excellent compliance and long-term follow-up with virtually complete ascertainment of cases in a well-defined population.
This study also has limitations. Interventions with factors containing multiple agents do not allow evaluation of the effects of individual agents alone, nor were we able to evaluate more than one dose for each of the agents supplemented. The people of Linxian are deficient in many micronutrients, which may limit the generalizability of these results to well-nourished populations. If the protective effects of this study are due to replacement of essential nutrients in a nutritionally deprived population, then similar interventions might be useful in similarly deprived populations in the West, including the United States, although populations with low rates of esophageal and gastric cancer mortality are unlikely to avert as many deaths as high-rate populations such as that in Linxian. Finally, the smoothed hazard ratios that we presented were intended to provide an alternative visual representation of the effects at specific points in time and to complement the cumulative view offered by the Kaplan–Meier curves. These smoothed hazard ratios should be interpreted with caution, however, because the confidence intervals around these curves nearly always include 1.0. Thus, such curves are affected by the play of chance and may be biased by choice of smoothing parameters, edge effects, and other factors.
It should be noted that the follow-up period occurred during a time of dramatic economic progress in China as a whole. Although documented improvements in dietary intakes in Linxian during follow-up were modest (37
), more substantial undocumented changes almost certainly occurred. Effects of dietary improvements should have been evenly distributed across all participants in the various randomized treatment groups in the trial. Thus, if the effects of the supplements and the dietary micronutrient intake are additive, any dietary changes that might have occurred should not bias the treatment group effects in the postintervention period. If instead the benefit from supplementation was to correct a deficiency state to exceed some minimum required threshold, then, if all people started to become less deficient because of dietary improvements over time, the observed treatment effects would be expected to weaken. It is all the more remarkable then that benefits persisted despite this likely improved nutrition and its attendant attenuation of treatment effects. It is also possible that improved diet may have modified effects in the postintervention period, including the enhanced benefit widely observed in younger participants and the emergence of late effects, most notably the benefit for factor C on cerebrovascular deaths.
In summary, 10 years of postintervention follow-up of participants in this cancer prevention trial demonstrated the durability of previously observed beneficial effects on mortality from supplementation with selenium, vitamin E, and beta-carotene. The persistence of risk reduction for up to 10 years after treatment in this trial reinforces the validity of the original trial findings and is consistent with an emerging new paradigm in cancer prevention, namely, that prevention may be achievable with short-term as opposed to life-long treatment. Striking age interactions were seen, suggesting that supplements may be more beneficial in younger age groups. Late beneficial and harmful effects on mortality not observed during the trial period of supplementation were also seen for other supplementation groups. Durability and late effects should be examined in other prevention trials.