Fiber + All About Fiber

In a whole vs processed foods fiber comparison, whole foods win at almost every swap: an apple has about 10 times the fiber of a cup of apple juice, and US diets highest in ultra-processed foods carry about 30% less fiber per calorie. Even when fiber is matched, an NIH trial found people ate about 500 more calories a day on an ultra-processed diet.

Sometimes, but it depends on the step: food processing fiber loss is large when milling, juicing or peeling removes part of the plant, and small when food is only cooked, canned or frozen. Refining wheat strips out virtually all of its fiber, while canned pumpkin actually has more fiber per gram than fresh.

Reheating food fiber losses are small: a second round of heat doesn’t strip fiber from leftovers, and the main effect of heat is a gradual breakdown of fiber chains that depends on how harsh the cooking is. Chilled and reheated pasta and potatoes can even end up with more resistant starch than when they were fresh.

Freezing food fiber barely changes: studies of frozen corn, peas and beans found no significant change in soluble, insoluble or total fiber. What the freezer does change is texture, and over many months some vitamin C, so the fiber in your frozen food is about what it was on the day it went in.

Good news for meal prep fiber retention: fiber holds up well in the fridge and freezer, and research on stored produce finds minimal change for intact fruits and vegetables. The bigger losses come from peeling and trimming during prep, while chilling and reheating starchy foods like rice can actually add resistant starch.