Every hunter gatherer culture was/is different, but I don't think any consume large quantities of grains or seeds- that was mostly only made practical as a major source of calories by the development of agriculture.

It's not clear that humans actually need much insoluble fiber (what's mostly found in grains and seeds) for good health. Starchy tubers (which make up as much as 70% of calories in many equatorial hunter gatherer societies such as the Kitvavans) are a good source of soluble fiber, which acts as a substrate for gut bacteria.

Compared to agricultural diets, I suspect that most hunter gatherers had much higher consumption of soluble fiber (primarily from leafy vegetables and tubers), and much lower consumption of insoluble fiber.