Table 1 Advantages and Limitations of Various Types of Nutrition Studies

Cell culture – in vitro

Food item is placed in cells or other tissues in culture conditions.

Help determine mechanisms of action.

Provide clues for further investigation to define mechanisms responsible for how food components interact with host cells.

What occurs in cells may be different from what occurs in human body.

Not conclusive.

Animal – in vivo

Food item is fed to laboratory animals (e.g. rat, mouse, guinea pig, rabbit). Tests the effects of food on certain diseases, physiological conditions, and behaviors

Can be tightly controlled for testing the metabolism, specificity, and reproducibility of the effect of a certain food component.

Tests the toxicity and safety of food components added to the diet.

Humans differ from animals in many aspects of their physiology, such as food digestion, nutrient absorption, genetics, lifestyle, etc.

Not conclusive.

Case studies

One individual’s experience with food or a food component is documented.

Help determine how certain food/components may affect clinical conditions or disorders in humans.

Focus on social, psychological, or medical conditions.

Provide clues for further investigation.

Not scientifically rigorous – only one person’s experience.

Not conclusive but is more evidence-building.

Epidemiological/observational

Groups of subjects, typically living in one geographical area, who have developed a disease or condition are compared with a similar sample of subjects who have not developed the disease.

Address whether a certain food/component could cause a disease but not whether it did cause a disease.

Memory recall is often used to assess how a food/component might have affected an individual – long-term memory recall may not be accurate.

Regionally biased – differences observed between regions may be related to different dietary of cultural preferences or genetic differences.

Not conclusive but is more evidence-building.

Prospective cohort studies

A large group of individuals with a similar background who are healthy when the study begins are followed over time. Diets are assessed at the beginning, during, and end of the study. Eating habits of those who get a disease are compared to those who do not acquire a disease.

Provide clues about the risk/benefits of a given diet, food component or lack of an important vitamin/mineral (e.g., development of iron deficiency and neurodevelopmental impairment) over time.

The controls may not be similar to those who acquire a disease with respect to demographic and health factors.

Not conclusive but is more evidence-building.

Randomized, controlled clinical trial (RCT)

Evaluates the efficacy of a specific nutritional intervention (e.g., food/component) within a population. Subjects with similar backgrounds are randomly allocated to receive either the test item or placebo.

Represents the “gold standard” for clinical trial methodology.

Randomization minimizes allocation bias by balancing both known and unknown social and health factors.

Sample sizes are often large to detect subtle differences in treatments, diet or food component when they exist.

Provides most conclusive evidence.

Unethical to randomize breastfed infants into an infant formula feeding group.

Meta-analysis

Review of the existing scientific literature. Pooled data from several studies are subjected to a statistical meta-analysis.

Meta-analysis of trials provides a more precise estimate of the treatment effect because of the increased sample size and statistical power.

Results can be generalized to a larger population.

Inconsistency of results across studies can be quantified and analyzed.

The validity of the meta-analysis depends on the quality of the systematic review and studies included in the analysis.

If confounding variables are not controlled for in the primary studies, there will be potential bias in the meta-analysis (e.g., differing definitions of breastfeeding).