Digesting and absorbing nutrients

WLS procedures change the digestive system (GI tract) in different ways. It helps to know how the GI tract works for a better understanding about how surgery can help you to lose weight.

Weight loss surgery changes the normal process of your gastrointestinal tract (GI tract). The weight loss is achieved by a surgical intervention that interferes with the way digestion works and nutrients are absorped. Different procedures change the digestion process in different ways. It is helpful to know how the normal GI tract works to absorb nutrients to understand how the different procedures can change your digestive system. You can read more here


Digestion is the process of breaking down food, by mechanical and enzymatic action in the alimentary canal, into substances that can be used by the body. The digestive process begins when you start to chew food in your mouth and enzymes help to break down that food so you can swallow. Food breaks down further in your stomach and intestines. Digestion is the process which turns food into nutrients (protein, fat, carbohydrates, vitamins and minerals) for your body to absorb. 

The effects of surgery on the function of digestive organs


There are no direct effects of weight loss surgery on the function of your mouth. However, being mindful to slow down the chewing and swallowing process is part of any weight loss surgery dietary plan. Chewing food thoroughly and taking time over eating allows us to feel fuller more quickly, leading us to eat less.  


There are no direct effects of weight loss surgery on the function of your esophagus. However, all types of weight loss surgeries shrink your stomach pouch. If you eat too much food, hence put too much food through your esophagus in a single sitting, your stomach pouch will overfill, causing heartburn and esophagitis.


The stomach is an expandable pouch that stores food. Digestion is continued here with hydrochloric acid. When your stomach is empty it releases ghrelin, a hormone which tells your brain you are hungry. A full stomach releases less ghrelin. Depending on which weight loss surgery you have, the effects on your stomach and the way it functions after surgery will vary. While all weight loss surgery types have the effect of reducing the stomach’s size and capacity, the surgeries differ in ways they achieve the reduction.  When the stomach size is reduced through surgery, fullness signals are sent to the brain much faster before overeating occurs.

Different surgery types change the stomach in specific ways: 

  • Lap Band: 15 per cent of your stomach remains in use above the gastric band.
  • RNYGB & Gastric Sleeve surgeries: 15 per cent of your stomach remains in use and the rest is removed.
  • BPD-DS: 50 per cent of your stomach remains in use. 

Small intestine

The majority of nutrient absorption takes place as food travels through the small intestine. From top to bottom, food travels through the duodenum, jejunum and ileum, mixing with digestive enzymes from your pancreas, liver and gallbladder along the way. Different surgery types change the small intestine in specific ways: 

  • RNYGB: the duodenum and part of the jejunum can be bypassed, leading to less nutrient absorption. 
  • BPD-DS: food bypasses more of the small intestine, leading to even less nutrient absorption, allowing digestion but preventing some absorption.

N.B. The surgeon might recommend removing the gallbladder during weight loss surgery to prevent gallstones. You can still digest and absorb food without a gallbladder.

Large intestine

The large intestine is unaltered by weight loss surgery. Its function is store remaining food as waste which is then excreted out the rectum and anus as feces. Some fermentation of dietary fiber takes place in the large intestine. 

Vitamin and Mineral Supplements

Weight loss surgery patients need to take certain vitamin and mineral supplements to prevent deficiencies. Here are some reasons why patients are at a higher risk of vitamin and mineral deficiencies: 

Limited food intake: Meeting the body’s daily nutritional requirements without using supplements can difficult on very low-calorie diets, such as the weight loss surgery diet.

Nutrient malabsorption: Malabsorptive surgeries, such as RYGB and BPD-DS, further increase the risk of deficiencies because they reduce the absorption of vitamins and minerals.

Extra losses: Your body loses iron, vitamin B12 and folate with bleeding, which can occur during and after surgery. Rapid weight loss also increases the loss of calcium. 

Weight loss surgery patients are often required to take daily supplements of vitamins and minerals.

  • Calcium
  • Vitamin D
  • Iron
  • Vitamin B12
  • Vitamin E
  • Vitamin A
  • Folic acid
  • Zinc

N.B. Taking too many vitamins and minerals can be dangerous. Only take the required dosage as instructed by your bariatric team.


Please seek and follow the specific guidelines from your bariatric team. Guidelines vary for individual patient needs and surgeries.