Vitamin A and beta-carotene — when do they help, and when can they be harmful?

Aim of the study: Age-related macular degeneration (AMD) is a degenerative condition of the back of the eye that occurs in people over the age of 50 years. Antioxidants may prevent cellular damage in the retina by reacting with free radicals that are produced in the process of light absorption. Higher dietary levels of antioxidant vitamins and minerals may reduce the risk of progression of AMD. This is the third update of the review.

Results: We included 26 studies conducted in the USA, Europe, China, and Australia. These studies enrolled 11,952 people aged 65 to 75 years and included slightly more women (on average, 56% women). We judged the studies that contributed data to the review to be at low or unclear risk of bias. Thirteen studies compared multivitamins with control in people with early and intermediate AMD. Most evidence came from the Age-Related Eye Disease Study (AREDS) in the USA. People taking antioxidant vitamins were less likely to progress to late AMD (odds ratio (OR) 0.72, 95% confidence interval (CI) 0.58 to 0.90; 3 studies, 2445 participants; moderate-certainty evidence). In people with early AMD, who are at low risk of progression, this means there would be approximately four fewer cases of progression to late AMD for every 1000 people taking vitamins (one fewer to six fewer cases).

Aim of the study: Vitamin A deficiency (VAD) is a major public health problem in low- and middle-income countries, affecting 190 million children under five years of age and leading to many adverse health consequences, including death. Based on prior evidence and a previous version of this review, the World Health Organization has continued to recommend vitamin A supplementation (VAS) for children aged 6 to 59 months.

Results: The updated search identified no new RCTs. We identified 47 studies, involving approximately 1,223,856 children. Studies were set in 19 countries: 30 (63%) in Asia, 16 of these in India; 8 (17%) in Africa; 7 (15%) in Latin America, and 2 (4%) in Australia. About one-third of the studies were in urban/periurban settings, and half were in rural settings; the remaining studies did not clearly report settings. Most studies included equal numbers of girls and boys and lasted about one year. The mean age of the children was about 33 months. The included studies were at variable overall risk of bias; however, evidence for the primary outcome was at low risk of bias. A meta-analysis for all-cause mortality included 19 trials (1,202,382 children). At longest follow-up, there was a 12% observed reduction in the risk of all-cause mortality for VAS compared with control using a fixed-effect model (risk ratio (RR) 0.88, 95% confidence interval (CI) 0.83 to 0.93; high-certainty evidence).

Aim of the study: Whether vitamin A and β-carotene as nutrients are protective factors against cataracts remains unclear. Thus, the aim of this study was to summarize the evidence from epidemiologic studies of vitamin A and β-carotene with the risk for cataract.

Results: Twenty-two articles were included in this meta-analysis. The relative risk (RR; 95% confidence interval [CI]) for cataract for the highest versus lowest category of serum β-carotene levels was 0.827 (95% CI, 0.736-0.930), and the association was significant between β-carotene intake and cataract risk (RR, 0.937; 95% CI, 0.880-0.997). Significant association of cataract risk with vitamin A intake was found overall (RR, 0.831; 95% CI, 0.757-0.913). However, no significant association was found between serum vitamin A and cataract risk (RR, 0.925; 95% CI, 0.675-1.266; I(2) = 63.1%), but an inverse association was observed with risk for cataract with serum vitamin A after sensitivity analysis (RR, 0.765; 95% CI, 0.654-0.894; I(2) = 29.1%). Greater vitamin A and β-carotene intakes might be inversely associated with risk for cataract.

Aim of the study: dietary β-carotene and vitamin A intake have shown some potential effect in the development of esophageal cancer. This meta-analysis was performed to investigate the association of β-carotene and vitamin A intake on the risk of esophageal cancer.

Results: this meta-analysis included 14 articles. The highest category of β-carotene intake may significantly reduce the risk of esophageal cancer compared with the lowest category (OR = 0.62, 95 % CI = 0.50-0.77). Similar significant results were found in American and European populations but not in other populations with β-carotene intake. An inverse association was found between vitamin A intake and esophageal cancer risk (OR = 0.79, 95 % CI = 0.63-0.99). No potential publication bias was detected. our study suggested that dietary β-carotene and vitamin A intake may reduce the risk of esophageal cancer. More relevant studies are needed to further explore this association, as there were some limitations in our analysis.

Aim of the study: Whether dietary β-carotene and vitamin A intake protect against lung cancer risk is not clear. Therefore, we performed this meta-analysis to investigate the association between them.

Results: Evidence from 18 studies suggested that higher category of dietary β-carotene intake could reduce lung cancer risk (0.768 (0.675-0.874)).The associations were also significant in American and Asian populations. In conclusions, higher category of dietary β-carotene and vitamin A intakes could reduce the risk of lung cancer. However, the dose-response analysis was not performed due to the limited data in each individual study. Due to this limitation, further studies with detailed dose, cases and person-years for β-carotene and vitamin A of each category are wanted to assess this dose-response association.