Studies that track very large numbers of people who already eat a certain way can be very valuable. For example, that is how scientists learned about the dangers of smoking and the benefits of exercise.

But observational studies are rife with uncertainty. These studies aren't controlled like experiments, and they're less precise and noisy. Health outcomes could also be the result of a higher income or better educated, or being more health-conscious.

Many observational studies rely on surveys and have subjects report on their diets.

An obvious challenge is that people don't remember what they ate for lunch yesterday or if they sprinkled something over the salad or snacked afterwards. Yet, much nutrition research rests on people's self-reporting from memory of what they ate. Many people also lie about what they eat, offering more socially acceptable answers.

Researchers discovered that different bodies have very different responses to the same food. This makes nutrition research more difficult.

Israeli scientists tracked 800 people over a week, monitoring their blood sugar levels to see how they responded to the same foods. Every person responded differently to identical meals, suggesting that health and nutrition cannot be simply understood by assessing what people eat.

Nutrition science is horribly underfunded by the government, so food companies and industry groups sponsor research.

Industry-funded studies tend to have results that are more favourable to the industry.

Researchers have used all these imperfect tools to learn important things over the years.

Nutritional research contributed to the knowledge that folate deficiency among pregnant women causes congenital disabilities or that drinking too much soda increases the risk of diabetes and fatty liver disease.

• Always consider all the available research on a question, not just single studies. Meta-analyses or systematic reviews are helpful.
• Different types of studies on a question, such as clinical trials, observational data, and lab studies, should all point toward a common conclusion.
• Paying attention to the source of funding behind the research is key. Research funded by independent government agencies or foundations tends to be more credible.