Unfortunately that's not always possible as we have selectively bred over time for appearance, size and/or sugar content in our fruits, vegetables and even meat at the expense of macro- and micronutrients.
We have also begun overly sanitizing our foods, pasteurizing everything and adding antibiotics to the detriment out our gut microbiome.
In many cases the food our great grandmas ate doesn't exist anymore.
I like that!
I had some friends over for a small dinner party a few weeks back. One of them was helping me in the kitchen butter some sour dough bread. Mid-way through she stopped agog just staring. I looked to where her eyes were staring and realized she was shocked at how much butter I was smearing on each slice of bread.
I'm active and believe in eating a lot of natural fats, but I realized that even young adults subscribe to the demonization of saturated fat. (I might also be a little too inclined to eat butter and coconut oil - so moderation - as always is important if we're to not alienate everybody).
Snarky "lead paint?" aside, this only really works if you're also as active as great-grandma was. Chances are she got a lot more exercise than we modern folk do.
"There has been no commensurate decline in physical activity, in either country – in the UK, exercise levels have increased over the last 20 years."
Sure, exercise is important, but a potential lack of it is not the problem. The most significant way you can interact with you body is through the food/drink you put in your mouth.
Food has changed so much over the past few decades that most people no longer eat food but instead consume food like products.
I think a better heuristic, is "Just eat what your great-grandma ate". Or avoid the middle of the supermarket as much as possible.
Simply eating more fat and less sugar/carbs, I have lost 10+ pounds and feel much better. These nutrition fads have done more harm than good. My mother switched from butter to Crisco when I was a kid, then she banned salt from the house, and bought (high sugare, low fat) snack wells for us. All based on poor/pseudo science.
The diabetes and obesity are modern problems caused by a diet that our bodies haven't evolved to handle.