Dear Sandip, 

I don't know what exactly do you mean with diet studies. Could you provide more information about the context where the question is set?

For instance, there is a wide variety of methods to assess diets. Usually we use different tools to gather dietary data such as 24-recalls, Food Frequency Questionnaires and other questionnaires to measure food security or hunger. 

That data is analyzed in different ways depending on the indicator that you need/want. For example, if you would like to know how diverse is a diet (i.e. if the person is eating all the groups of foods/micronutrient) you shoul use the household dietary diversity score defined by the FAO or the Food Consumption Score used by the WFP. 

So all depends on what are you really seeking with the diet studies.

if you  are planning to do diet studies  for a specific set of products then you can go through the nutrient availability of the product, see what are the major functional nutrients present in the product , see the nutrients which are having antioxidant properties . Also see the limiting nutrient/s of the product . 

We are completing an experimental study where macronutrients have the same percentages, but pellets differ for the quality of nitrogen provided. We are exploring how EAA/Non EAA ratios (Essential Amino Acids) may influence lifespan. The more % NEAA  contained into pellets, the shortest is lifespan. EAA only containing pellets near doubles lifespan observed by control laboratory diets. Covariance is quite linear. Epigenetic modifications are  very different according to organs examined, and what found in liver, as an example, cannot be translated to what happens in muscles.  Adipose tissues modifications are prominent, and independent by dimensions of spontaneous food intake.