If you're trying to make sense of health and medical research, it helps to understand the differences between three basic types of studies.
Basic science/laboratory studies: These are conducted in cells, tissue or animals, typically mice or rats. This research is essential because it can be rigorously controlled. However, what happens to cells in a test tube or in a rodent may not happen in humans.
Epidemiological studies, sometimes called population or observational studies: These observe a group of people over a period of time to identify factors, such as nutrient intake or blood levels, that may increase or decrease a person’s risk of developing certain diseases or conditions, such as cancer or fractures. These types of studies have helped scientists identify many potential benefits of various nutrients, for instance. But these types of studies merely identify associations, rather than prove causation.
Clinical trials: These “gold standard” studies evaluate the effectiveness of treatments or preventive measures. Clinical trials should be rigorously controlled, so that extraneous (“confounding”) factors don’t distort the results. The best trials randomize volunteers (like a flip of a coin) to a treatment or a control group (who usually receive a placebo) and are double-blinded so that neither the study participants nor the researchers know who is receiving which treatment. A trial must last long enough and have enough participants to produce what is referred to as “statistical power” to show that the results didn’t just happen by chance.