I've been answering the Nurses Study questionnaires for over 3 decades now. The questionnaire always asks your current weight & height. As they state. this is an annual BMI asked over the years before major illnesses. Also, they inquired about my weight in my teens, 20s, etc...to compare the trend. Most of the studies have not been funded to include diagnostic blood tests or saliva swabs for genetic tests.
Interestingly, the past two years have included an online cognitive test (flash card memory).