Plasma levels of retinol in cancer patients supplemented with retinol

Antioxidant supplements for preventing gastrointestinal cancers

BACKGROUND

Oxidative stress may cause gastrointestinal cancers. The evidence on whether antioxidant supplements are effective in preventing gastrointestinal cancers is contradictory.

OBJECTIVES

To assess the beneficial and harmful effects of antioxidant supplements in preventing gastrointestinal cancers.

SEARCH STRATEGY

We identified trials through the trials registers of the four Cochrane Review Groups on gastrointestinal diseases, The Cochrane Central Register of Controlled Trials in The Cochrane Library (Issue 2, 2007), MEDLINE, EMBASE, LILACS, SCI-EXPANDED, and The Chinese Biomedical Database from inception to October 2007. We scanned reference lists and contacted pharmaceutical companies.

SELECTION CRITERIA

Randomised trials comparing antioxidant supplements to placebo/no intervention examining occurrence of gastrointestinal cancers.

DATA COLLECTION AND ANALYSIS

Two authors (GB and DN) independently selected trials for inclusion and extracted data. Outcome measures were gastrointestinal cancers, overall mortality, and adverse effects. Outcomes were reported as relative risks (RR) with 95% confidence interval (CI) based on random-effects and fixed-effect model meta-analysis. Meta-regression assessed the effect of covariates across the trials.

MAIN RESULTS

We identified 20 randomised trials (211,818 participants), assessing beta-carotene (12 trials), vitamin A (4 trials), vitamin C (8 trials), vitamin E (10 trials), and selenium (9 trials). Trials quality was generally high. Heterogeneity was low to moderate. Antioxidant supplements were without significant effects on gastrointestinal cancers (RR 0.94, 95% CI 0.83 to 1.06). However, there was significant heterogeneity (I(2) = 54.0%, P = 0.003). The heterogeneity may have been explained by bias risk (low-bias risk trials RR 1.04, 95% CI 0.96 to 1.13 compared to high-bias risk trials RR 0.59, 95% CI 0.43 to 0.80; test of interaction P < 0.0005), and type of antioxidant supplement (beta-carotene potentially increasing and selenium potentially decreasing cancer risk). The antioxidant supplements had no significant effects on mortality in a random-effects model meta-analysis (RR 1.02, 95% CI 0.97 to 1.07, I(2) = 53.5%), but significantly increased mortality in a fixed-effect model meta-analysis (RR 1.04, 95% CI 1.02 to 1.07). Beta-carotene in combination with vitamin A (RR 1.16, 95% CI 1.09 to 1.23) and vitamin E (RR 1.06, 95% CI 1.02 to 1.11) significantly increased mortality. Increased yellowing of the skin and belching were non-serious adverse effects of beta-carotene. In five trials (four with high risk of bias), selenium seemed to show significant beneficial effect on gastrointestinal cancer occurrence (RR 0.59, 95% CI 0.46 to 0.75, I(2) = 0%).

AUTHORS' CONCLUSIONS

We could not find convincing evidence that antioxidant supplements prevent gastrointestinal cancers. On the contrary, antioxidant supplements seem to increase overall mortality. The potential cancer preventive effect of selenium should be tested in adequately conducted randomised trials.

Antioxidant supplements for preventing gastrointestinal cancers

BACKGROUND

Oxidative stress may cause gastrointestinal cancers. The evidence on whether antioxidant supplements are effective in preventing gastrointestinal cancers is contradictory.

OBJECTIVES

To assess the beneficial and harmful effects of antioxidant supplements in preventing gastrointestinal cancers.

SEARCH STRATEGY

We identified trials through the trials registers of the four Cochrane Review Groups on gastrointestinal diseases, The Cochrane Central Register of Controlled Trials on The Cochrane Library (Issue 1, 2003), MEDLINE, EMBASE, LILACS, and SCI-EXPANDED from inception to February 2003, and The Chinese Biomedical Database (March 2003). We scanned reference lists and contacted pharmaceutical companies.

SELECTION CRITERIA

Randomised trials comparing antioxidant supplements to placebo/no intervention examining the incidence of gastrointestinal cancers.

DATA COLLECTION AND ANALYSIS

Two reviewers independently selected trials for inclusion and extracted data. The outcome measures were incidence of gastrointestinal cancers, overall mortality, and adverse events. Outcomes were reported as relative risks (RR) with 95% confidence interval (CI) based on fixed and random effects meta-analyses.

MAIN RESULTS

We identified 14 randomised trials (170,525 participants), assessing beta-carotene (9 trials), vitamin A (4 trials), vitamin C (4 trials), vitamin E (5 trials), and selenium (6 trials). Trial quality was generally high. Heterogeneity was low to moderate. Neither the fixed effect (RR 0.96, 95% CI 0.88 to 1.04) nor random effects meta-analyses (RR 0.90, 95% CI 0.77 to 1.05) showed significant effects of supplementation with antioxidants on the incidences of gastrointestinal cancers. Among the seven high-quality trials reporting on mortality (131,727 participants), the fixed effect (RR 1.06, 95% CI 1.02 to 1.10) unlike the random effects meta-analysis (RR 1.06, 95% CI 0.98 to 1.15) showed that antioxidant supplements significantly increased mortality. Two low-quality trials (32,302 participants) found no significant effect of antioxidant supplementation on mortality. The difference between the mortality estimates in high- and low-quality trials was significant by test of interaction (z = 2.10, P = 0.04). Beta-carotene and vitamin A (RR 1.29, 95% CI 1.14 to 1.45) and beta-carotene and vitamin E (RR 1.10, 95% CI 1.01 to 1.20) significantly increased mortality, while beta-carotene alone only tended to do so (RR 1.05, 95% CI 0.99 to 1.11). Increased yellowing of the skin and belching were non-serious adverse effects of beta-carotene. In four trials (three with unclear/inadequate methodology), selenium showed significant beneficial effect on gastrointestinal cancer incidences.

REVIEWERS' CONCLUSIONS

We could not find evidence that antioxidant supplements prevent gastrointestinal cancers. On the contrary, they seem to increase overall mortality. The potential cancer preventive effect of selenium should be studied in adequately conducted randomised trials.

Laboratory evaluation during high-dose vitamin A administration: a randomized study on lung cancer patients after surgical resection

The laboratory findings in patients receiving high-dose vitamin A as adjuvant treatment for stage I lung cancer are here reported. A group of 283 patients were randomized to either treatment with retinyl palmitate (300,000 IU daily for 12 months) or standard follow-up, and are now evaluable after a median observation period of 28 months. At regular intervals, all the patients underwent a physical examination, chest roentgenogram, blood chemistries, haematological assays, hepatic and renal function tests and determinations of serum triglycerides and cholesterol. Serum transaminase abnormalities were of similar magnitude in cases and controls, while gamma-glutamyltransferase levels were abnormally elevated in 69% of the treated patients compared to 39% of controls (mean values 149 vs 57 IU/l at 24 months, P less than 0.05). Serum triglyceride concentrations over 150 mg/dl were seen in 74% of treated patients compared to 43% of controls at 12 months, the average concentration was 283 mg/dl compared to 179 mg/dl (P less than 0.05). Cholesterol levels showed a modest, non-significant rise with time in both groups, and there was no other laboratory evidence of toxicity attributable to vitamin A. Serum retinol and retinol-binding protein, assessed on a limited sample of patients, were higher in the treatment arm (P less than 0.05) at 12 months. In our experience 300,000 IU/day of retinyl palmitate can be administered as a possible chemopreventive agent with reasonable safety for up to 2 years.