The invention provides methods and kits for designing a diet with a desired fat content for an individual in need thereof to allow the individual to, for example, maintain or reduce healthy weight, manage diabetes, for example by managing weight or accommodate a food allergy. The method comprises determining whether the individual carries APOA5 -1131T and/or C allele or both or any allele in chromosome 11 that is in a tight linkage disequilibrium with said-alleles, wherein if the individual is a homozygote for APOA5 -1131T allele or an allele in tight linkage disequilibrium with APOA5 -1131T allele, the designing of a diet comprises reducing a total fat content of the diet below 30% of total calorie intake and/or reducing the amount of monounsaturated fatty acids in the diet under about 11% of total calorie intake. We surprisingly found that these methods and kits apply to both females and males and to a variety of ethnic backgrounds.