Gram Research analysis of a 2026 study of 46 patients found that one anastomosis gastric bypass (OAGB) and Roux-en-Y gastric bypass produced similar weight loss one year after surgery, with about 63% and 58% of excess weight lost. OAGB patients reported fewer diarrhea and indigestion symptoms, but both groups showed high rates of vitamin D deficiency.
Gram Research analysis of a study published in 2026 compared two weight-loss surgeries and how well patients’ nutrition held up one year later. Researchers looked at 16 people who had one anastomosis gastric bypass (OAGB) and 30 who had Roux-en-Y gastric bypass (RYGB). Both groups lost similar amounts of weight. Protein intake met recommendations in both groups, but fiber fell short. Vitamin D deficiency was common, found in 82% of RYGB patients and 92% of OAGB patients. OAGB patients reported fewer diarrhea and indigestion symptoms, a difference that was statistically significant. Because the study was small, reviewed past medical records, and did not randomly assign patients to each surgery, these results are best viewed as early clues. Anyone considering bariatric surgery should discuss nutrition monitoring with their care team.
Key Statistics
A 2026 retrospective study of 46 gastric bypass patients found that Roux-en-Y gastric bypass patients lost 29.55% of their total body weight one year after surgery, compared with 26.32% for those who had one anastomosis gastric bypass, a difference that was not statistically significant.
In the same 2026 study, vitamin D deficiency affected 92% of one anastomosis gastric bypass patients and 82% of Roux-en-Y gastric bypass patients one year after surgery.
A 2026 study of 46 patients found that one anastomosis gastric bypass patients reported significantly lower diarrhea and indigestion scores than Roux-en-Y gastric bypass patients one year after surgery.
The Quick Take
- What they studied: The researchers wanted to know whether a newer, simpler weight-loss surgery called one anastomosis gastric bypass (OAGB) leads to nutrition problems or digestive symptoms that differ from the more established Roux-en-Y gastric bypass (RYGB), one year after surgery.
- Who participated: The study included 46 adults who had weight-loss surgery at one clinic. Sixteen had OAGB and 30 had RYGB. Their records were reviewed after one year of follow-up under a team of doctors, dietitians, and other specialists.
- Key finding: Both surgeries led to similar weight loss, with about 26% to 30% of total body weight lost and about 58% to 63% of excess weight lost. Vitamin D deficiency was common in both groups. OAGB patients reported fewer diarrhea and indigestion symptoms, and this difference was statistically significant, meaning it was unlikely to be due to chance alone.
- What it means for you: If you are considering weight-loss surgery, this study suggests OAGB may be a reasonable option from a nutrition standpoint during the first year. However, both surgeries can lead to low vitamin D, and iron levels need watching. Regular blood tests and follow-up with your care team are important with either procedure.
The Research Details
This was a retrospective study, which means the researchers looked back at existing medical records instead of enrolling people and following them forward in real time. They compared patients who had one of two types of gastric bypass surgery, both of which make the stomach smaller and reroute part of the digestive tract to help people eat less and absorb fewer calories.
The team measured several things one year after surgery. These included how much weight people lost, results from blood tests (such as vitamin D, iron, and a hormone that controls calcium called parathyroid hormone), what people ate, how well they tolerated food, and how often they had digestive symptoms like diarrhea, indigestion, and nausea.
To make the two groups fairer to compare, the researchers used statistical methods to account for factors that could affect the results, such as age, sex, and each person’s starting health measurements. This helps reduce the chance that differences came from those other factors rather than the surgery itself.
Comparing two surgeries side by side shows whether a newer option performs at least as well as an established one. Looking at nutrition, not just weight loss, matters because long-term health after bariatric surgery depends on getting enough vitamins, minerals, and protein. Measuring symptoms and food tolerance also tells us how people actually experience life after surgery, which affects whether they can stick with their plan.
Adjusting for age, sex, and starting values makes the comparison more trustworthy, though it cannot remove every hidden difference between the two groups.
This study was published in a peer-reviewed gastroenterology journal, which means outside experts reviewed it before publication. It used standard measures of weight loss and blood testing, and patients were followed under a structured team approach, which supports consistent care.
However, the sample was small, with only 16 people in the OAGB group. The study was also retrospective and not randomized, so the two groups may differ in ways the researchers could not fully measure. Readers should treat the findings as useful but preliminary, not as a final verdict on either surgery.
What the Results Show
Weight loss was similar between the two surgeries. RYGB patients lost an average of about 29.55% of their total body weight, compared with about 26.32% for OAGB patients. For excess weight, which is the amount above a healthy weight, RYGB patients lost about 63.36% and OAGB patients lost about 58.29%. These differences were not statistically significant, meaning the two surgeries appeared to produce comparable results.
Vitamin D deficiency was very common in both groups. It affected about 82% of RYGB patients and about 92% of OAGB patients one year after surgery. Vitamin D helps the body absorb calcium and keep bones strong, so this is an important finding even though the difference between the groups was not large.
Blood levels of parathyroid hormone, which rises when the body is low on calcium or vitamin D, were high in 75% of OAGB patients compared with 50% of RYGB patients. The researchers noted this difference was not statistically significant, so it could be due to chance. Iron levels were numerically lower in the OAGB group, and their transferrin saturation, a measure of how well iron is carried in the blood, was also reduced. No patient in either group developed anemia, which is a condition of having too few healthy red blood cells.
Protein intake met recommended levels in both groups. Fiber intake, however, was below the minimum recommended amount in both groups, which is a common concern after gastric bypass surgery because the digestive system absorbs and processes food differently.
OAGB patients reported significantly lower scores for diarrhea, indigestion, and their total digestive symptom score compared with RYGB patients. This suggests that people who had OAGB may have experienced fewer bothersome digestive problems during the first year. Food tolerance, meaning how well people handled different foods, was described as satisfactory and similar in both groups. This matters because being able to eat a varied diet supports long-term adherence and nutrition.
The authors note that evidence about the nutritional results of OAGB is still limited. This study adds one more comparison to a growing body of research on newer gastric bypass variations. Its main contribution is that it compared nutrition, digestive symptoms, and weight loss together in one group of patients over one year, under the same follow-up program.
The finding that vitamin D deficiency is common after gastric bypass is consistent with the general understanding that these surgeries can reduce absorption of certain nutrients. Readers should view this study as one piece of a larger picture rather than a final answer.
The study was small, with 46 patients total and only 16 in the OAGB group. Small groups make it harder to detect real differences and make results more sensitive to chance. Some differences, such as the higher parathyroid hormone levels in the OAGB group, did not reach statistical significance, so they cannot be confirmed from this data.
Because the study looked back at existing records, the researchers could not control how patients were assigned to each surgery. Patients who chose OAGB may have differed from those who chose RYGB in ways that affected the results, even after statistical adjustments.
The follow-up lasted only one year. Nutritional problems after bariatric surgery can develop or change over many years, so these findings may not reflect long-term outcomes. Finally, the study was conducted at a single center, so results may not apply equally to all clinics, patient populations, or surgical teams.
The Bottom Line
Moderate confidence: Based on this one-year study, OAGB appears to be nutritionally comparable to RYGB in the short term, with similar weight loss and fewer digestive symptoms. Because the study was small and retrospective, this should be confirmed by larger, longer studies before OAGB is considered superior.
High confidence: Vitamin D deficiency was common after both surgeries, so vitamin D levels should be checked and managed regularly. Iron status should also be monitored, as iron levels trended lower in the OAGB group.
High confidence: Fiber intake fell short in both groups, so dietary fiber should be a focus of nutrition counseling after either surgery.
This research is most relevant to people who are considering or have already had gastric bypass surgery, and to the doctors, dietitians, and care teams who guide them. It may also interest people who are comparing bariatric surgery options.
This study is less directly relevant to people who have not considered weight-loss surgery, and it should not be used to make decisions about surgery on its own. Anyone with a history of kidney, bone, or blood conditions should discuss these findings with a qualified physician before making choices.
The study measured results at one year after surgery. Most people who have gastric bypass see the majority of their weight loss within the first 12 to 18 months, though individual timelines vary. Nutritional changes, such as vitamin D or iron levels, can take months to show up on blood tests and often require ongoing supplementation and monitoring rather than a one-time fix. Long-term results beyond one year are not addressed by this study, so expectations should be discussed with a care team for the years ahead.
Frequently Asked Questions
Is one anastomosis gastric bypass as good as Roux-en-Y gastric bypass for nutrition?
A one-year study of 46 patients found the two surgeries produced comparable nutritional outcomes, with similar weight loss and similar food tolerance. Both groups met protein recommendations, but fiber intake fell below the minimum in each group. Researchers recommend continued monitoring of iron and vitamin D.
What vitamin deficiency is most common after gastric bypass surgery?
Vitamin D deficiency was the most common problem in a 2026 study of 46 gastric bypass patients, affecting 82% of Roux-en-Y patients and 92% of one anastomosis gastric bypass patients at one year. Regular blood tests and supplementation guided by a doctor are typically recommended.
Does one anastomosis gastric bypass cause fewer digestive problems?
A 2026 study found that patients who had one anastomosis gastric bypass reported significantly lower scores for diarrhea and indigestion than Roux-en-Y patients after one year. The study was small, so researchers suggest these results be confirmed in larger studies before drawing firm conclusions.
Why is fiber intake a concern after gastric bypass?
Research reviewed by Gram found that fiber intake fell below minimum recommendations in both surgery groups one year after surgery. Low fiber can make constipation and nutrient gaps more likely, so care teams often make fiber a focus of nutrition counseling.
Want to Apply This Research?
- Log daily fiber grams and total protein grams in the app, and set a weekly review to compare totals against the goals set by your care team. Also record any diarrhea or indigestion episodes each day on a simple 1 to 5 scale so trends are easy to see over time.
- Add one high-fiber food to at least two meals each day, such as beans, lentils, vegetables, oats, or chia seeds, and log it in the app. Pair protein with each meal to help meet daily targets, and take any vitamin D or iron supplement your care team has prescribed at the same time each day with a reminder.
- Schedule blood tests for vitamin D, iron, and parathyroid hormone as directed by your care team, and enter the results into the app to track changes over time. Review fiber, protein, symptom, and supplement logs monthly, and bring the summary to each follow-up visit. Over the long term, use the app to watch for patterns in symptoms or nutrient levels so that concerns can be raised early.
This article summarizes a small, one-year retrospective study and is for general education only. It is not medical advice. Weight-loss surgery and its nutritional needs are highly individual, so decisions about surgery, supplements, diet, and blood testing should be made with a qualified doctor, bariatric surgeon, or registered dietitian who knows your health history.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.