The Netherlands is about to begin a major experiment in which people in jail will be given food supplements to determine if certain vitamins make them less aggressive.

Aggressive prisoners in five Dutch jails and two youth offender institutions will be offered extra fish oil, vitamins and minerals to see if it reduces their violent behaviour from early next year. Previous experiments at a Dutch jail involving 200 prisoners resulted in a 34% drop in incidents, the Volkskrant said on Saturday.

Britain is also experimenting with giving prisoners food supplements and the results of a follow-up project are due shortly, the paper said.

If the method proves successful, the justice ministry plans to offer it to more prisoners who have difficulty controlling their aggression, researcher Ap Zaalberg told Radio 1 news.

However, although food supplements may help, the ministry should really be focusing on prison food, Wageningen University professor Frans Kok told broadcaster Nos. Nevertheless, food supplements do affect behaviour, he said.

Kok also warned that the earlier experiments were open to manipulation and that measuring aggressive behaviour is difficult.