While ‘retrospective studies’, for example, can be inaccurate, as they rely on data that have already been collected in the past or need to be recalled by study participants, ‘prospective studies’ can be more accurate, because they describe current occurrences (e.g., nutritional behavior and disease risk) over the course of time. While ‘observational studies’ can only observe associations and suggest (hypothesis-generating), ‘randomized controlled trials (RCT)’ can establish cause-and-effect relationships (hypothesis-testing). But even study results of RCTs can be misleading when, for example, results are pooled in a ‘meta-analysis’, mixing data from disease prevention in healthy people with data on slowing down disease progression in patients. Frequently, effects of high-dose micronutrient supplement use in patients (therapy) or vulnerable groups (e.g., smokers) are transferred to the average consumer who uses fortified foods and/or dietary supplements to prevent micronutrient deficiency (prophylaxis); this is not accurate.
It is important to note that many studies using dietary supplements do not show positive effects on health or disease prevention because they test individuals with an already existing disease or an increased risk of developing a certain disorder. In addition, these studies may not include enough individuals (too small) or may not run long enough (too short) to show a significant effect. Moreover, health status is influenced by so many individual factors (e.g., lifestyle) that it is extremely difficult to show a significant effect of micronutrients.
However, it should be kept in mind that dietary supplements are meant to “supplement”, but not “substitute” a healthy diet.