This invention is a device and method for adjustably and reversibly reducing nutrient absorption from food passing through a person's gastrointestinal tract using an Adjustable Gastrointestinal Bifurcation (AGB) and a flow-control member. The Adjustable Gastrointestinal Bifurcation (AGB) includes: a first food-flow route (comprising a first branch of the AGB) through the person's gastrointestinal tract wherein there is normal absorption of nutrients from food; and a second food-flow route (comprising a second branch of the AGB) through the person's gastrointestinal tract wherein there is reduced absorption of nutrients from food. The flow-control member adjusts the types and/or amounts of food which are diverted through the second food-flow route versus the first food-flow route.