If you look at our distant ancestors (and wild animals in general), their version of a "balanced" diet wasn't meat and three veg every day.
Kill an animal, and eat it...if it took you three days, then you ate meat for three days. Blackberries in season, and they would eat them until the fruit was exhausted. Go hungry for a few days, whatever.
Eating a truckload of local seasonal produce when it's in season will be balanced by not having it in those quantities for the balance of the year.