What effect did the large number of berries have on Brian in Hatchet?

Brian gets a stomach ache from eating too many berries.

One of Brian’s first concerns when he finds himself the only survivor of a plane crash in the Canadian wilderness is where to find food.  Brian has only a hatchet and some clothing.  He does not have any food.  When daydreaming about his next meal, he decides to look for berries.

The sun was still high and that meant that he had some time before...

Brian gets a stomach ache from eating too many berries.

One of Brian’s first concerns when he finds himself the only survivor of a plane crash in the Canadian wilderness is where to find food.  Brian has only a hatchet and some clothing.  He does not have any food.  When daydreaming about his next meal, he decides to look for berries.

The sun was still high and that meant that he had some time before darkness to find berries. He didn't want to be away from his—he almost thought of it as home—shelter when it came to be dark. (Ch. 6)

Brian does find berries, but they are not very tasty.  They are tart and make his mouth dry.  He eats them anyway because he is hungry and there is nothing else.  He hasn’t eaten in two days.  The berries turn his stomach, but he eats too many of them just the same.

Brian wakes up with serious stomach pains.  The berries, which he later names gut cherries, are almost not worth the price.

It was as if all the berries, all the pits had exploded in the center of him, ripped arid tore at him. He crawled out the doorway and was sick in the sand, then crawled still farther and was sick again, vomiting and with terrible diarrhea for over an hour, for over a year he thought, until he was at last empty and drained of all strength. (Ch. 7)

Although the plan to look for berries and use the birds to locate them is a good one, the berries that Brian finds are hardly helpful.  He realizes that he has to eat them, but he tries to focus on the ripe ones and only eat them a few at a time.  Brian finds some raspberries, which are much better, but for them he has competition from a bear.