Current nutritional and genetic epidemiological methods yield 'risk factors' on the basis of population studies. Risk factors, however, are statistical estimates of the percentage reduction in disease in the population if the risk were to be avoided or the gene variant were not present. These measures are often assumed to apply to individuals who are likely to differ in genetic make-up, lifestyle, and dietary patterns than to the individuals in the study population. Developing individual risk factors in light of the genetic diversity of human populations, the complexity of foods, culture and lifestyle, and the variety of metabolic processes that lead to health or disease is a significant challenge for personalizing dietary advice for healthy or individuals with chronic disease.