In: Math
See , I can suggest you some ideas how can a diet plans change affect someones health, you are collecting less number of variables as you need to come up with some more variables to measure if a person is healthy or not with respect to his diet plans, you can make a diet plan for both vegetarian and non-vegetarian and before the experiment you can measure some standard measurements, Weight, Body mass index, Blood pressure, Heart rate , Waist measurements etc . After collecting the relevant data for people willing to participate in your program, tell them the which diet chart to follow vegetarian or non-vegetarian, after some regular intervals go and collect the mentioned variables again, do that for atleast a month with regular measurements of weight,blood pressure,Heart rate etc. You can choose as many variables to want, but i would only suggest you to stick to the above mentioned in bold. After that analyze and summarize your data and see which diet affected ones measurements and which not, this way you can capture a trend and conclude by validating your findings, and term that this diet plan can make a person health more better.
I am attaching a sample data where you might understand my context