Effects of Weight Loss Surgery on Digestion and Nutrient Absorption

All kinds of weight loss surgery change your digestive tract, or gastrointestinal tract (GI tract), in one or more ways. Understanding normal digestion and nutrient absorption is helpful for understanding the changes that occur with each of the various weight loss surgeries

Digestion occurs when your body breaks food down into smaller components. Digestion begins in your mouth, when you chew, and continues as digestive enzymes mix with food and break it down further in your stomach and intestines. Digestion makes nutrients (proteins, fats, carbohydrates, vitamins and minerals) available to for your body to absorb.

Absorption is when your body takes nutrients from the GI tract and brings them into your body. Weight loss surgery can interfere with digestion and/or absorption to promote weight loss.

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Digestion, Absorption, the GI Tract and Weight Loss Surgery

This table describes the passage of food in the GI tract from the beginning to end. It also describes how weight loss surgery can change the normal process.

Organ/Location Process(es) Possible Effect(s) of Weight Loss Surgery
Mouth
  • Chewing and grinding food
  • Mixing with saliva to prevent choking and begin the breakdown process
No direct effects, but all weight loss surgery diets require slower eating and thorough chewing. This can lead to you to eat less.
Esophagus Food passes from mouth to stomach No direct effects, but all weight loss surgeries shrink your stomach pouch. Overeating overfills the pouch and can cause heartburn and esophagitis.
Stomach
  • Expandable pouch that stores food
  • Continued digestion with hydrochloric acid
  • An empty stomach releases more ghrelin, a hormone that tells your brain that you're hungry
  • A full stomach releases less ghrelin
  • Reduction in usable stomach size means fullness signals are sent to your brain faster—before you've eaten as much food
  • 15% of your stomach remains above the band with the lap-band
  • 15% of your stomach remains in use in sleeve plication, RYGB and gastric sleeve surgeries
  • 50% of your stomach remains in use in the BPD-DS
  • Reduction in ghrelin (a hunger hormone) after gastric sleeve surgery
Small intestine
  • The majority of nutrient absorption
  • Food travels (from top to bottom) through the duodenum, jejunum and ileum
  • Food mixes with digestive enzymes from your pancreas, liver and gallbladder
  • The duodenum and part of the jejunum can be bypassed in RYGB, leading to less nutrient absorption
  • The BPD-DS bypasses more of the small intestine, leading to even less nutrient absorption
  • BPD-DS allows digestion but prevents some absorption
  • Your gallbladder may be removed during weight loss surgery to prevent gallstones. You can still digest and absorb food without a gallbladder.
Large intestine
  • Storage of remaining food as waste
  • Some fermentation of dietary fiber
Unchanged by bariatric surgery
Rectum and anus Excretion of waste as feces Unchanged by bariatric surgery