Ninety trials were identified. The number of participants was not always reported in the studies. Twenty-seven of the studies were published and 63 were on-going.
Colorectal cancer (5 published, 19 ongoing studies):
Published studies evaluated calcium, anti-oxidant micronutrients and fibre supplementation as treatments. All had small sample sizes and most considered indirect and early indicators (such as labelling index) of colon cancers as outcomes. One study looked at individuals with high fat diets, two at individuals with familial adenomatous polyposis, one at patients with previous adenomas and one at colectomy patients. Ongoing studies were mainly of chemoprevention (different doses of calcium or fibres, different types of fibres, prostaglandin inhibitors and trace elements). In general the ongoing studies included populations with pre-cancerous lesions, with the outcome of polyp recurrence (secondary prevention). Only one ongoing study looked at healthy participants (primary prevention). There was little published or ongoing evidence to suggest that any of the treatments had significant effects long term.
Oral cancer (6 published, 4 ongoing studies):
Both published and unpublished studies aimed to achieve chemoprevention of early markers of oral carcinogenesis (micronuclei) or treatment of oral leukoplakias (secondary prevention) mainly using retinoid-related chemicals (vitamin A, 13-cis-retinoic acid, beta-carotene) in very high risk groups or people already suffering from leukoplakia. The evidence in favour of vitamin A and cis-retinoic acid was quite strong in terms of the frequency of micronuclei and recurrence or development of leukoplakias. A less conspicuous effect was obtained with beta-carotene, which was the least toxic of the three treatments.
Oesophageal cancer (3 published, 5 ongoing studies):
The studies mainly focused on Asian populations, which generally have a higher risk of developing oesophageal cancer. None of the studies looked at high-risk Western populations. Published trials suggested a possible protective effect of combinations of vitamins and trace elements (zinc) on chronic oesophagitis with or without hyperplasia and dysplasia (thought to be pre-cancerous lesions). The ongoing studies should provide more information concerning cancer incidence and mortality.
Cervical cancer (2 published, 4 ongoing studies):
All of the studies deal with chemoprevention of precancerous lesions with vitamins or related compounds (oral or topical). No study took into account cervical cancer as an end point. The only study for which results have been published was negative; therefore the studies showed no evidence of efficacy.
Lung cancer (4 published, 11 ongoing studies):
The ongoing studies varied greatly from the published studies in terms of size, study design and end points. The published studies suggested some evidence of a possible positive effect of beta-carotene, selenium, folate+vitamin B12 on some early indicators of DNA damage in bronchial cells or on squamous metaplasia. The ongoing studies aim to evaluate the effect of treatment on lung cancer incidence in high-risk groups using mainly vitamin A (including its precursors and derivatives).
Non-melanoma skin cancer (3 published, 7 ongoing studies):
Results from two small randomised trials and several non-randomised clinical studies support the hypothesis that some carotenoids can reduce the development of non-melanoma skin cancer in very high-risk groups affected by xeroderma pigmentosum, actinic keratosis or previous primary skin cancer itself. The most promising of these agents were certain synthetic derivatives of vitamin A. The preliminary results from one beta-carotene trial were non significant, and the effect of selenium was unknown, although two ongoing trials were noted.
Head and neck cancer (1 published, 2 ongoing studies):
These were multi end point studies aimed at preventing new primary cancers in individuals previously affected by head and neck cancers. Treatment with 13-cis-retinoic acid seems to be protective, but toxicity is very high. New strategies in ongoing trials included lower doses of retinoic acid or identification of alternative treatments and less toxic treatments using other retinoid-related chemicals.
Breast cancer (1 published, 6 ongoing studies):
Six out of the seven trials (1 prevention, 6 chemoprevention) used treatments based on the proposed link between estrogenic activity/metabolic activity and breast cancer. Final results were not available from many of the studies as they were large and still on going.
Stomach cancer (0 published, 3 ongoing studies):
All of the studies were based on the chemoprevention of pre-cancerous lesions with anti-oxidants plus, in one, anti-Helicobacter treatment. There was no evidence of effectiveness as all of the trials were still on going.
Liver cancer (2 published, 2 ongoing studies):
Both published studies were chemoprevention RCTs looking at the protective effect on primary liver cancer of selenium in high-risk individuals. No comment was made as to the significance of the findings, which generally showed fewer incidences of cancer in the treatment as compared to control groups. There was evidence available from the ongoing studies. One looked at the affect of hepatitis B vaccine in childhood for preventing the incidence of cancer, and the other (a RCT) at the affect of green tea and radix salvia miltiorrhizae herb in preventing primary liver cancer in a Chinese population.