The Unilever-owned spread has been asked to alter ads for its Pro-Activ margarine brand after the Advertising Standards Authority ruled there was no proof behind its claims to "lower cholesterol" and "keep blood vessels healthy". Bartle Bogle Hegarty handles the account.
The ruling comes days after the ASA upheld a complaint against a St Ivel milk brand. A series of press ads featured Professor Lord Robert Winston claiming Omega-3 in its "clever milk" helped children to learn and concentrate.
A spokesman at the ASA said: "With the increase of functional foods, these claims are becoming more prevalent. The brands must be able to substantiate their claims with documentary evidence."
The ASA's decision comes as European Union countries prepare to implement laws that will make it more difficult for products to make health claims in their ads.
The ASA spokesman added: "There's no call for an outright ban, but it does become problematic if ads are wholly misleading."
In April, the Broadcast Committee of Advertising Practice ruled that advertisers should not imply that any alcoholic drink could make health claims. The ruling stated that ads could not suggest alcohol could be drunk as part of an exercise, fitness or weight-control regime.