Here’s a helpful page on different types of studies (Literature Reviews: Types of Clinical Study Designs).
Some types studies are more helpful than others.
A good article on the subject (Correlational Studies – A Closer Look at Correlational Research).
The correlational method involves looking at relationships between two or more variables. While researchers can use correlations to see if a relationship exists, the variables themselves are not under the control of the researchers.
Another point is that while correlational research can reveal if a relationship exists between variables, this kind of research cannot prove that changes to one variable lead to changes to another variable. In other words, correlational studies cannot prove cause-and-effect relationships.
This paper (What Is Correlation?) is helpful in understanding the numbers.
Metabolic Ward Studies
Metabolic ward studies confine people to a controlled medical setting. Their intake is controlled and activity levels can be monitored.
Here’s an example of the typical set of controls for this kind of study (Carbohydrate Study Leaves Diet Researchers Divided – As expected, first study from Nutrition Science Initiative makes waves):
The food was prepared in a lab, frozen, then shipped to the study sites, where it was then prepared along with fresh produce. The baseline diet didn’t have a large amount of sugar, so it may have “differed somewhat from the customary diets” of the participants, wrote the authors.
The participants were not allowed to leave or to eat other foods, and were told they had to eat all of the food in front of them. They were allowed to meet with visitors, but only under the supervision of a nurse or research staff. The metabolic chamber measurement times were all at least 23 hours. Blood and urine measurements were also taken.
Food Macro Percentage Studies
Studies which vary the percentage of each macro (Fat, Protein, Carbs) and hold calories constant are also challenging due to the differing thermic effect of food (TEF). The thermic effect of food is the energy required for digestion, absorption, and disposal of ingested nutrients. It’s not possible to hold calories in constant and vary the macro mixture without affected calories out. And TEF even varies within a particular macro. The magnitude of TEF depends on the composition of the food consumed:
- Carbohydrates: 5 to 15% of the energy consumed
- Protein: 20 to 35%
- Fats: at most 5 to 15 %
Limits of Observational Epidemiology
Here’s a good article (Commentary: The hormone replacement–coronary heart disease conundrum: is this the death of observational epidemiology?
Debbie A Lawlor, George Davey Smith, Shah Ebrahim. International Journal of Epidemiology, Volume 33, Issue 3, 1 June 2004, Pages 464–467) which looks at the limits of these sorts of studies.