If you wander through the woods and fields picking wild berries and gathering wild herbs, that's natural food. That's it. Eat it raw and that's as natural as it gets. As we can't safely eat raw meat, natural food has to be vegetable, digestible raw (that lets a lot of things out) and arrived at its place of growth quite by chance, seeds blown on the wind of dropped by birds. It rather reduces your options, but if you live in a warm climate you could survive like this.
The next step is to plant the stuff yourself. At the most basic level, you clear away other plants, make a hole, drop a seed in, and water it if there's insufficient rain. Then you know exactly where to go to gather your food from, and as long as you don't neglect it, it's a pretty safe bet it'll be there when the time comes. Storing it until the next harvest is a separate issue.
Cooking and preparation
This gives you access to hard grains and vegetables which may need to be soaked in salt water, boiled, or roasted to be digestible or to remove toxins. The first potatoes were horribly poisonous until boiled like crazy, and the local farmers selected different strains to get the modern tuber that's just about safe (if not very pleasant) to eat raw. Cooking also introduces meat into the available list, which is a jolly good thing at the end of Winter. This isn't natural now, it's been altered. Is this as far as you'd take it?
Having decided to cultivate your plants are you going to feed them? Are you going to offer some manure or compost, or other organic matter to give them a boost? If you start giving them modern fertilizer it may not be considered natural food by exponents of organic growing. If we are now including meat where does that come from? Hunting? If you keep livestock you have to feed them. In the wild they'd eat grass and other plant materials. Fowl would add insects to that diet. Pigs would eat anything, including you if you didn't run fast enough. If you feed them you then have to consider the source of their feed. It may not be "natural". Even giving large quantities of grain may not be considered a natural diet.
You can bury root vegetables over Winter, and grain can be milled and stored dry. Other things, even meat, can be dried to store. But that often requires the addition of salt, which you didn't just dig up, did you? And canning, freezing, pickling, adding preservatives etc are modern World methods well away from nature.
Now we really get away from our "roots" (sorry). You can buy processed food in a store that according to the ingredients has no actual food in it at all. What is food? Nutrients that keep you healthy by enabling your body to replace cells, build and repair tissue, and allow your immune system to work. When you eat your body breaks it down to compounds it can absorb, so theoretically, it doesn't matter if you eat food that is a list of chemical compounds, does it?
Well here's where a chemist and your doctor just might disagree. The chemist would say that processed laboratory created food, if "complete" is the same as raw veggies.....chemically. Your doctor would prefer you to eat the raw veggies, but he may not really be able to explain why. They would probably have a jolly professional disagreement on what constutues natural, and neither of them is really right or wrong
Live hopefully, and hopefully you'll live.