The broader definition of fiber includes non-digestible carbohydrates present in human milk. In fact, oligosaccharides make up the 3rd major macronutrient by weight and their fermentation products (short chain fatty acids) are an important energy source as well as providing an acidic environment in the infant's colon that suppresses harmful bacteria. Human Milk Oligosaccharides (HMOs) are complexes of multiple sugar molecules not digested by the infant but serve as fermentation substrates for preferred gut microbes, bifidobacteria, our most important probiotics.
The presence of 15-20 g/L of HMO’s in human milk begs the question why have humans evolved this way and are there lessons for the best choice of macronutrients for children and adults.
The most immediate question for breastfeeding mothers is what foods to wean the infant onto and bring into question the current practice of switching from breast milk to high glycemic carbohydrates devoid of fiber. My personal bias from thousands of studies on outcomes and biological mechanisms is that our best macronutrient profile should more closely mirror that of breast milk i.e. a high fiber diet, with most energy coming from fat with modest amounts of protein and a fiber to carb ratio corresponding to our most healthy fruits and vegetables. There is also evidence that autoimmune diseases of childhood, celiac and type 1 diabetes can be prevented by increased short chain fatty acids from higher fiber consumption.