Taking multivitamins may help stop HIV infection developing into full-blown Aids, researchers from the Harvard School of Public Health in the US say.
Aids drugs are in short supply in Africa
In a six-year study, 538 African women with HIV were given a daily supplement of a multivitamin or a dummy pill.
Of the 267 taking dummy pills, 12% developed Aids compared with 7% of the 271 on a multivitamin pill.
The 271 also suffered fewer late-stage complications, the researchers told the New England Journal of Medicine.
They said their findings suggested vitamin supplements could be given to people with HIV in the developing world to delay the need to start treating them with Aids drugs.
Although Aids drugs have been shown to be effective, they are powerful, and do have side effects.
Researcher Dr Wafaie Fawzi said: "It's a low-cost intervention that could result in major savings and be helpful to many individuals in terms of better quality of life."
The research was conducted in Tanzania, where until recently Aids drugs were not available to most people
A total of 1,078 women received either a multivitamin, a multivitamin with vitamin A, vitamin A alone or a dummy pill.
Eighteen of the 271 women (7%) who took multivitamins developed Aids.
In contrast, 31 of the 267 women (12%) who took a dummy pill went on to develop full-blown symptoms.
The women on multivitamins also had fewer late-stage complications such as fatigue, diarrhoea and mouth ulcers and lower levels of the virus in their blood.
The researchers suspect that multivitamins not only boost the immune system, but also hinder the virus' ability to reproduce itself.
They found no significant benefit to vitamin A alone, and adding vitamin A to multivitamins somewhat reduced their benefits.
Dr Fawzi said that although men were not studied, multivitamins are likely to benefit them as well.
The high-dose multivitamins with vitamins B, C, and E used for the study cost about $15 for a year's supply. Aids drugs in Tanzania cost about $300 a year.
Dr Fawzi said further research was needed to find out whether lower-dose vitamins would have the same effect and whether vitamins would help people who were already taking Aids drugs.
Dr Cate Hankins, chief scientific advisor to UNAIDS, welcomed the study.
She told BBC News Online: "These findings are very encouraging and very positive.
"This is a relatively low-cost intervention that could be started immediately someone was found to be HIV positive."
Dr Hankins said the fact that multivitamins reduced HIV-linked symptoms such as oral thrush, mouth ulcers and painful swallowing was also important.
These problems made it difficult for people with the virus to eat properly, at a time when taking on board sufficient calories was more important than ever.
Dr Hankins said ensuring people with HIV were properly nourished was key, as it helped to maximise the effect of anti-retroviral drugs.