About the habits of participants in long term lifestyle studies

Part GQ, part IMHO.

I am thinking mostly of nutrition, but my question applies to fitness/exercise as well.

The format is this: “In a long-term study that followed x# of individuals for y# of years, results suggested that people who ______ had a greatly increased rate of ______”

Some of these studies extend over 5, 10, 15 years. My question is, how does natural human variability affect those studies, given that I suspect very few people maintain the same habits across decades?

I mean, my nutritional habits vary greatly over time according to my finances, my age, my emotional well-being, and my understanding of current nutritional theory.

Are the long-term values averaged? Would my years of 1 veg serving/day in my thirties be averaged against the 7 veg servings/day I eat now, and be treated as an overall intake of 4 veg serving per day? I used to think it was fine to drink a quart of fruit juice every day; now I hardly touch the stuff. I’m not the only one now eating more fish, fewer potatoes.

For exercise, does anyone really maintain a constant rate/intensity over the course of years, or wouldn’t a lot of people get off track for months at a time after the birth of a child, a change in their hours/place of employment, having to care for aging family members, bad breakups, injuries?

The IMHO component to my question is: how discouraged should I be when I read these studies, thinking, wow, I don’t have a decade of (whatever the research is looking at), I wonder if it is even worth making changes? But I think I could better assess that for myself if I understood how the data in those studies gets analyzed, and there should be some protocol for that.

I have some experience in being in studies, and they closely monitor all the variables, and draw conclusions from every change . . . and compare you with other people who have (or don’t have) similar changes. For example, the changes you make at 50 are compared with other people who’ve made the same change at 50, and also with people who’ve made the same change at 40 or 60. As long as people are being honest, it’s relevant.