Researchers compared two popular weight loss surgeries to understand how they help people lose weight. One surgery (Roux-en-Y gastric bypass) works by making the stomach smaller AND preventing the body from absorbing all the calories you eat. The other surgery (sleeve gastrectomy) mainly just makes the stomach smaller. Scientists studied 28 patients who had one of these surgeries and measured how much food their bodies actually absorbed. They found that the bypass surgery prevented the body from absorbing more calories and fat than the sleeve surgery, but the difference was smaller than doctors expected. This helps explain why both surgeries work well for weight loss, even though they work in different ways.

The Quick Take

  • What they studied: How much food energy and fat the body actually absorbs after two different types of weight loss surgery
  • Who participated: 28 women who had weight loss surgery 1-2 years before the study. Half had one type of surgery (bypass) and half had the other type (sleeve)
  • Key finding: The bypass surgery prevented the body from absorbing about 195 calories and 9.7 grams of fat per day, while the sleeve surgery prevented absorption of about 112 calories and 3.9 grams of fat per day. The bypass surgery blocked more absorption, but the difference was smaller than doctors thought it would be
  • What it means for you: If you’re considering weight loss surgery, both types work well, but they work slightly differently. The bypass surgery blocks more calories from being absorbed, which may help some people lose more weight. However, this doesn’t mean it’s automatically better for everyone—your doctor can help you choose the right option based on your health needs

The Research Details

Researchers recruited 28 women who had undergone one of two weight loss surgeries between 1-2 years before the study. The women ate a carefully controlled diet for 6 days where researchers knew exactly how many calories and how much fat they were eating. For the last 3 days, researchers collected and analyzed the women’s stool samples using a special technology called near-infrared spectroscopy (NIRS). This technology can measure how much energy and fat passed through the body without being absorbed. The researchers also asked about exercise and bowel habits to see if these affected the results.

Understanding exactly how much food the body absorbs after different surgeries helps doctors explain why patients lose weight and helps them predict which surgery might work best for different people. The special technology (NIRS) used in this study is new and non-invasive, meaning it doesn’t require surgery or uncomfortable procedures to measure absorption.

This was a small study with only 28 patients, so the results may not apply to everyone. The study was well-controlled with a strict diet, which is good for accuracy but doesn’t reflect how people normally eat. One patient was excluded because their reported food intake seemed unrealistic, showing the researchers were careful about data quality. The study measured real absorption using scientific equipment rather than just asking patients questions, which is more reliable.

What the Results Show

The bypass surgery (RYGB) resulted in significantly more malabsorption than the sleeve surgery (SG). Patients with bypass surgery failed to absorb about 194.8 calories per day (13.2% of the calories they ate), while sleeve surgery patients failed to absorb about 111.7 calories per day (7.6% of calories). This means the bypass surgery blocked about 83 more calories from being absorbed each day. For fat specifically, bypass patients didn’t absorb 9.7 grams of fat daily (15.4% of fat eaten), while sleeve patients didn’t absorb 3.9 grams daily (6.1% of fat eaten). These differences were statistically significant, meaning they were unlikely to be due to chance. Even when researchers adjusted for how much weight each person lost, the differences remained significant.

The researchers also looked at whether physical activity and stool consistency affected the results, but these factors didn’t significantly change the findings. Interestingly, the differences in malabsorption didn’t match up perfectly with how much of the intestines were bypassed during surgery, suggesting that the surgery’s effects on the body are more complex than just blocking food from being absorbed.

Previous research suggested that bypass surgery would block much more calories from being absorbed because it physically reroutes the intestines. This study confirms that bypass does block more calories, but the actual difference (about 83 calories per day) is smaller than many doctors expected. This finding helps refine our understanding of how these surgeries actually work in real patients.

The study included only 28 women, which is a small number, so results may not apply to men or to larger groups. All participants were 1-2 years post-surgery, so we don’t know if these numbers change over time. The controlled diet used in the study doesn’t reflect how people normally eat at home. The study only measured absorption over 3 days, which may not represent typical daily absorption. Finally, one patient was excluded from analysis, which slightly reduced the sample size.

The Bottom Line

If you’re considering weight loss surgery, understand that both bypass and sleeve surgeries work well for weight loss through different mechanisms. The bypass surgery does block more calories from being absorbed, but this difference is modest (about 83 calories per day). Talk with your bariatric surgeon about which option is best for your specific health situation, as factors beyond calorie absorption matter for your success. (Confidence: Moderate—based on a small study)

This research is most relevant for people considering weight loss surgery and their doctors. It’s particularly useful for people trying to understand the differences between these two common procedures. This doesn’t apply to people who have already had surgery or who aren’t candidates for bariatric surgery.

The calorie-blocking effect happens immediately after surgery, but weight loss results typically appear over several months to a year. Most people in this study were measured 1-2 years after surgery when their weight loss had stabilized.

Want to Apply This Research?

  • If you’ve had weight loss surgery, track your daily calorie intake and weight weekly. This helps you see whether your body is absorbing nutrients as expected and whether your weight loss is progressing normally for your surgery type
  • Use the app to log meals and monitor how different foods affect your digestion and energy levels. This personalized tracking helps you understand your individual absorption patterns, which may differ from average numbers in research studies
  • Set monthly check-ins to review your calorie intake, weight trends, and digestive symptoms. Compare these patterns over time to see if your absorption or metabolism changes, and share this data with your doctor at regular appointments

This research describes how two weight loss surgeries affect calorie and fat absorption in a small group of women. These findings should not be used to make decisions about weight loss surgery without consulting your doctor. Individual results vary significantly, and many factors beyond calorie absorption affect weight loss success. If you’re considering bariatric surgery, discuss these findings with your bariatric surgeon who can explain how they apply to your specific situation. This article is for educational purposes and does not replace professional medical advice.