Gastric bypass surgery: a before and after look at weight loss surgery

Yael Cooperman, MD - Contributor Avatar

Reviewed by Yael Cooperman, MD, Ro, 

Written by Cale Li, MD 

Yael Cooperman, MD - Contributor Avatar

Reviewed by Yael Cooperman, MD, Ro, 

Written by Cale Li, MD 

last updated: Apr 30, 2021

4 min read

Gastric bypass surgery is a common weight-loss operation to help people with obesity lose weight. Between 60-70% of all weight loss procedures performed are gastric bypass surgeries, an effective obesity treatment (Mitchell, 2020). In addition to helping people lose weight, gastric bypass surgery can improve obesity-related conditions like diabetes and high cholesterol. Here’s everything you need to know about gastric bypass surgery, the risks and benefits, and what it’s like before and after surgery. 

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How much weight do people lose after gastric bypass surgery?

Gastric bypass surgery can help some people with obesity lose excess weight when diet and lifestyle changes aren’t enough. 

You may lose roughly 30-35% of your total body weight after gastric bypass, but the exact amount varies from person to person (Wolfe, 2017). Compared to non-surgical treatments for weight loss, such as diet and exercise, people who had gastric bypass surgery lost significantly more weight (Mitchell, 2020). 

Certain factors can affect how much weight you lose after gastric bypass, including age, biological sex, diet, exercise level, and other health conditions like diabetes (Wolfe, 2017). 

Not everyone with obesity needs this type of procedure. Qualifying for weight loss surgery is based on your body mass index (BMI). BMI is a way to measure body fat using height and weight. The formula is calculated using the metric system and is equal to your weight in kilograms divided by your height in meters squared. There are plenty of online BMI calculators available.

People who qualify for gastric bypass surgery generally meet one of the following criteria (Stahl, 2020):

  • Those with a BMI equal to or greater than 40

  • Those with a BMI equal or greater than 35 who have an underlying condition like high blood pressure, type 2 diabetes, high cholesterol, asthma, acid reflux, or liver disease

  • People with diabetes and a BMI between 30 and 35 

How does gastric bypass surgery work?

Gastric bypass involves reshaping the stomach to restrict how much food it can hold. That limits the number of calories the body can absorb—a process called malabsorption. Research also suggests that cutting and reshaping the stomach might affect the hormones that control hunger (Tremaroli, 2017). 

During the procedure, part of the stomach is separated and connected to the middle of the small intestine, skipping part of the digestive tract. As a result, your body absorbs fewer calories, so you feel full after eating less (Mitchell, 2020). 

Weight loss surgery is usually performed using tools inserted into the abdomen through tiny incisions, a technique known as laparoscopic surgery.

What are the possible complications? 

Laparoscopic surgery is considered safe and, compared with open surgery, has added benefits of shorter hospital stays and a lower risk of complications (Reoch, 2011). Complications occur about 1% of the time and may include infection at the surgical site, injury to the bowel, or side effects from anesthesia (Stahl, 2020).

Gastric bypass changes how your body absorbs food and nutrients. It’s not uncommon to develop deficiencies in certain nutrients after surgery. A healthcare provider should monitor your levels after surgery to make sure you’re staying healthy. If you have low levels of any nutrients, you may need supplements like folic acid, iron, vitamin D, and vitamin B12 (Schijns, 2018). 

What does life look like after gastric bypass?

While recovering after gastric bypass surgery, a liquid diet eases the transition to your new digestive system. As you become more comfortable with a liquid diet, you can slowly adjust to eating soft solid foods. Your healthcare provider will follow your dietary progress at regular clinic visits.  

In the first year after gastric surgery, people reported a significant improvement in their quality of life. Some reported benefits included better physical functioning, more active social life, and more independence in daily activities like bathing and getting dressed (Major, 2015).

As for long-term results, studies show that gastric bypass helped people lose weight and keep it off. After one year, people generally had lost about 31% of their total body weight. At 10 years, people kept off about 28% of their original body weight (Duvoisin, 2018).

Can gastric bypass improve other medical conditions?

Some patients see improvements with other health issues after weight loss surgery. 

Diabetes is one example. One small study examined people with type 2 diabetes and a BMI over 35 and found that gastric bypass was associated with more remissions in diabetes (Wolfe, 2017). 

High blood pressure is another condition that can improve after gastric surgery. One study found that those who underwent gastric bypass saw improvements in blood pressure, needed fewer medications to treat it, had a smaller waist circumference, and had better blood sugar levels than those who didn’t have surgery (Schiavon, 2018). 

Gastric bypass surgery may also help cholesterol levels or a reduction in LDL or “bad” cholesterol. Some people with non-alcoholic fatty liver disease also saw improvements following the procedure. (Wolfe, 2017). 

And if gut bacteria is your thing, researchers have found that gastric surgery might change the way bacteria behaves, contributing to more significant fat loss (Tremaroli, 2017).

Will I have excess skin after gastric bypass surgery?

A significant source of distress for patients is whether they’ll end up with excess or sagging skin after surgery. 

Many people reported having a significant amount of excess skin following gastric bypass, particularly women (Biörserud, 2014). 

It’s difficult to predict who will experience excess skin after surgery and who won’t. The definitive way to treat excess skin is through a cosmetic procedure called body contouring. 

Body contouring removes the excess skin and tissue to improve body shape. Body contouring may also help prevent weight gain after bypass surgery. In one study, people who had gastric bypass and body contouring procedures were less likely to gain weight back years later (Balagué, 2013). 

For the right people, gastric bypass can be an effective and life-changing treatment. If you’re interested in weight loss surgery, getting a consultation with a bariatric surgeon is the first step. Gastric bypass leads to weight loss in the short and long term. While there are some potential side effects, such as nutritional deficiencies, most people notice a significant improvement in their quality of life within the first year after surgery. 


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

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How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

April 30, 2021

Written by

Cale Li, MD

Fact checked by

Yael Cooperman, MD

About the medical reviewer

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.

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