Researchers studied 67 people who had weight loss surgery to understand why some people lose more weight than others. They measured how much energy (calories) their bodies were actually absorbing from food by testing their stool samples over two years. The study found that people whose bodies absorbed less energy from food tended to lose more weight and keep it off better after surgery. This discovery could help doctors predict which patients might struggle with weight loss after surgery and develop better ways to help them succeed.

The Quick Take

  • What they studied: How much food energy people’s bodies actually absorb after weight loss surgery, and whether this affects how much weight they lose
  • Who participated: 67 adults who had either gastric bypass or sleeve gastrectomy (two common weight loss surgeries), tracked for up to 2 years after their operation
  • Key finding: People whose bodies absorbed less energy from food in the early months after surgery ended up losing more weight and keeping it off better. The connection was modest but real (p = 0.04), meaning it’s unlikely to be due to chance.
  • What it means for you: If you’re considering weight loss surgery, your body’s ability to absorb calories from food matters for your results. This finding may help doctors identify patients who might need extra support to achieve good weight loss outcomes, though more research is needed before this becomes a standard tool.

The Research Details

This was a longitudinal study, meaning researchers followed the same 67 patients over time (from before surgery through 24 months after). Patients had either gastric bypass or sleeve gastrectomy surgery. The researchers measured two main things: how many calories patients ate (using food questionnaires) and how much energy their bodies were actually absorbing (by analyzing the energy content of stool samples using a method called bomb calorimetry—essentially burning the stool to measure its energy). They took measurements at the start, then at 1, 6, 12, 18, and 24 months after surgery.

To analyze the data, researchers used advanced statistical methods including dynamic linear mixed-effects models (which account for changes over time) and random forest algorithms (a computer learning method that identifies patterns in complex data). These methods allowed them to see how energy absorption at early timepoints predicted weight loss at later timepoints.

Understanding what happens to energy absorption after weight loss surgery is important because surgery alone doesn’t guarantee success—some patients regain weight or don’t lose as much as expected. By measuring actual energy absorption rather than just assuming all calories are absorbed equally, researchers can better understand the biological mechanisms driving weight loss success. This approach is more precise than just looking at food intake alone.

This study has several strengths: it followed patients over a meaningful time period (2 years), used objective measurements (fecal energy analysis rather than just self-reported data), and employed sophisticated statistical methods. However, the sample size of 67 is relatively modest, which means results should be confirmed in larger studies. The study was observational rather than experimental, so it shows associations but cannot prove cause-and-effect. The findings are preliminary and should not yet be used clinically without further validation.

What the Results Show

The main finding was that intestinal energy absorption changed after weight loss surgery—patients’ bodies absorbed different amounts of calories before and after their procedures. More importantly, patients whose bodies absorbed less energy from food (meaning more energy left their bodies in stool) in the early months after surgery tended to have better weight loss outcomes later on. This relationship was statistically significant (p = 0.04), though the effect size was modest.

The random forest computer models showed that including information about energy absorption made predictions of weight loss outcomes significantly more accurate. In other words, knowing how much energy a patient’s body was absorbing helped doctors predict who would lose the most weight.

Interestingly, the amount of food patients reported eating (caloric intake) was not significantly related to how much energy they absorbed. This suggests that the surgery itself changes how the body processes food in ways that go beyond just eating less.

The study found that the relationship between energy absorption and weight loss was strongest at later timepoints (6-24 months after surgery), suggesting that early patterns of energy absorption can predict long-term outcomes. This timing is clinically relevant because it means measurements taken in the first few months after surgery might help identify patients who will struggle later.

This appears to be one of the first studies to directly measure intestinal energy absorption in post-bariatric surgery patients. Previous research has focused on calorie intake and metabolic rate, but this study adds a new dimension by measuring actual energy absorption. The findings align with the general understanding that weight loss surgery works through multiple mechanisms, not just restriction of food intake.

The study has several important limitations. First, the sample size of 67 is relatively small, which means results need confirmation in larger populations. Second, the study only included two types of surgery (gastric bypass and sleeve gastrectomy), so findings may not apply to other weight loss procedures. Third, the association between energy absorption and weight loss was modest, meaning other factors also play important roles. Fourth, this was an observational study, so we cannot determine whether lower energy absorption causes better weight loss or if other factors drive both. Finally, the study did not investigate why energy absorption changes after surgery or what factors influence these changes.

The Bottom Line

Based on this research (moderate confidence level): Weight loss surgery patients should work with their healthcare team to monitor their progress, as energy absorption patterns may predict long-term success. This research suggests that measuring energy absorption might eventually become a useful tool to identify patients at risk of poor outcomes, but this is not yet standard practice. Patients should focus on following their surgeon’s dietary and lifestyle recommendations regardless of these findings.

This research is most relevant to: people considering weight loss surgery, bariatric surgeons and their teams, and patients who have had weight loss surgery but aren’t seeing expected results. It’s less immediately relevant to people managing weight through diet and exercise alone, though the underlying biology may have broader applications.

If these findings lead to clinical tools, benefits would likely be seen over months rather than weeks. The study measured outcomes over 24 months, suggesting that meaningful patterns take time to develop. Patients shouldn’t expect immediate changes based on this research, but rather should view it as foundational science that may improve surgical outcomes in the future.

Want to Apply This Research?

  • Track weekly weight and stool consistency/appearance (using a simple 1-5 scale) if you’ve had weight loss surgery. While users won’t measure fecal energy directly, these proxy measures may correlate with energy absorption patterns and help identify trends over time.
  • For post-bariatric surgery patients: log meals and note any changes in digestion patterns. The app could send reminders to track these patterns consistently, helping users and their doctors identify whether changes in digestion correlate with weight loss plateaus or gains.
  • Establish a baseline measurement at 1-2 months post-surgery, then track monthly for the first year. Create alerts if weight loss slows unexpectedly or if digestive patterns change significantly, prompting users to discuss findings with their surgical team. This long-term monitoring approach aligns with the study’s 24-month observation period.

This research is preliminary and should not be used to make clinical decisions without consulting your healthcare provider. The study involved only 67 patients and has not yet been validated in larger populations. If you are considering weight loss surgery or have had weight loss surgery, discuss these findings with your bariatric surgeon or physician. This information is educational and does not replace professional medical advice. Do not attempt to measure or interpret your own energy absorption without medical guidance.