Halo effect of a weight-loss trial on spouses: The DIRECT-Spouse study

Objective We examined the halo effect of a 2-year weight-loss diet trial, the Dietary Intervention Randomized Controlled Trial (DIRECT), on the weight and nutritional patterns of participants' spouses.Design DIRECT participants in a research centre workplace were randomly assigned to one of three diets: Low-fat, Mediterranean or Low-carbohydrate. A sample of wives of the DIRECT participants, who attended support update meetings specific to their husband's diet during the first 6 months, were followed for 2 years.Setting South Israel.Subjects Seventy-four women (mean age = 51 years, mean BMI =26·6 kg/m²).Results Among the wives of husbands randomised to the Low-fat, Mediterranean and Low-carbohydrate diet, self-reported weight change was respectively -14·8 kg, -2·30 kg and -4·62 kg after 6 months, and +0·39 kg, -3·00 kg and -2·30 kg after 2 years. Weight loss among wives whose husbands were in the alternative diet groups combined (Mediterranean+Low-carbohydrate) was significantly greater than among wives whose husbands were in the Low-fat group after 6 months (P = 0·031) and 2 years (P = 0·034). Overweight wives experienced more weight loss. The weight change of couples was significantly correlated (r = 0·42, P < 0·001). Across all dietary groups, wives had significant improvement in their dietary patterns in all food groups according to their husbands' diets, mainly by a larger significant decrease in carbohydrate consumption in the Low-carbohydrate group (P = 0·013 compared to Low-fat). Six-month weight change among the seventy-four DIRECT participants whose wives took part in the group support sessions was -5·2 kg, compared to -3·5 kg among the 248 DIRECT participants whose wives did not take part in these sessions (P = 0·020).Conclusions Focusing on the couple as a unit could provide a cost-effective approach to weight-loss programmes.