Effects of Gastric Bypass Surgery and Calcium Metabolism and the Skeleton: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Effects of Gastric Bypass Surgery and Calcium Metabolism and the Skeleton (NCT01330914), a registered phase not stated study. The primary outcome is Change in Intestinal Calcium Absorption.

2. Clinical Question

The study rationale focuses on the possibility that gastric bypass could affect bone health through changes in calcium metabolism. The registry describes concern about skeletal effects in the context of obesity, weight loss, and bone biology. It also lists areal bone mineral density at the femoral neck and trabecular number at the tibia as secondary outcomes. These measures address different aspects of the skeleton from intestinal calcium absorption. The registry record does not report their results or establish that changes in calcium absorption correspond to changes in either skeletal measure.

3. Trial Design

The registry record describes a cohort of obese men and women undergoing gastric bypass surgery, with a single listed group named “Gastric Bypass Surgery Patients.” It does not describe a randomized comparison. Allocation is not stated, and no separate control group is listed. The design should therefore be read as a before-and-after assessment in a surgical cohort, rather than as a like-for-like comparison between treatment groups. The registry record does not give a more specific design label.

Enrollment was 55. This is the recorded enrollment, not necessarily the number contributing data to every outcome. For the primary endpoint, the registry defines the population as cohort participants assessed before surgery and at 6 months afterward. The number meeting that definition is not reported in the registry record. The record lists three outcome measures: change in intestinal calcium absorption as primary, with areal bone mineral density at the femoral neck and trabecular number at the tibia as secondary outcomes. It lists no analyses.

The study is marked completed. Its recorded start was 2011-07, and its primary completion was 2014-04. These dates and the completed status do not establish that results were posted or that every enrolled participant completed follow-up. The registry record does not report the recruitment process, eligibility criteria, participant characteristics beyond the broad cohort description, follow-up completeness, or a statistical analysis plan.

The sample-size assumptions described in the record are explicitly planning illustrations based on an observed effect, not the trial’s own design. They assume two-sided significance level 0.05, equal allocation between two arms, and a normal approximation. Those assumptions do not describe the single listed surgery group and before-and-after endpoint. The record does not report a sample-size justification for this study.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Participants from the cohort who underwent assessment of fractional calcium absorption preoperatively and 6 months postoperatively

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Change in Intestinal Calcium Absorption, measured over 6 months (between baseline and 6 months).

The primary endpoint is change in intestinal calcium absorption, expressed as the percentage of ingested calcium that is absorbed. Fractional calcium absorption is assessed using a dual stable isotope method. The registry defines the six-month change as the mean difference in percentage absorption between the preoperative and 6-month postoperative assessments. The specified summary is a mean, with standard deviation as the dispersion measure.

This endpoint is used to assess intestinal calcium uptake, a component of calcium metabolism that is relevant to the study’s stated interest in the skeleton after gastric bypass. It is a physiological measure, not a direct measurement of bone density, bone structure, osteoporosis, or fracture. The secondary outcomes—areal bone mineral density at the femoral neck and trabecular number at the tibia—refer to skeletal measures, but their results are not reported in the registry record.

The endpoint definition alone does not indicate whether absorption increased or decreased, how large any change was, or how precisely it was estimated. The registry record does not report the mean change, its standard deviation, a confidence interval, or an analysis result. It also does not report the number of participants contributing to the endpoint.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The endpoint is a continuous measure: fractional calcium absorption is expressed as a percentage, and the primary outcome is the change between two assessments. The registry specifies a mean and standard deviation as the summary statistics. Those choices describe the type of summary intended, but the registry record does not report the resulting mean, standard deviation, or any inferential analysis.

General statistical context. A mean summarizes the central value of a continuous outcome across participants. A standard deviation describes how spread out individual values are around that mean. It is not the same as a standard error: the standard deviation describes variability among observations, while a standard error describes uncertainty in an estimated statistic. Neither statistic is reported as an outcome value in the registry record. The standard deviation of change also cannot be inferred from separate baseline and follow-up summaries without information about how paired values vary together.

For a before-and-after endpoint, each participant’s change is based on their own baseline and follow-up measurements. A group mean change would summarize those participant-level changes. The registry’s endpoint definition corresponds to that type of comparison, but it does not state the statistical test, model, or method used to estimate the mean change. The fact that the endpoint has two time points does not establish whether the analysis used a paired test, a regression model, or another approach; the analysis method is not reported in the registry record.

General statistical context. A confidence interval communicates the precision of an estimate under the method used to construct it. A narrower interval indicates greater precision than a wider interval, while the interval’s location conveys which values are compatible with the data and model. A confidence interval does not guarantee that the true value lies within its limits, and it does not address biases that arise from study design or missing data. No confidence interval for the primary endpoint is reported in the registry record.

A p-value, when used, summarizes how unusual data at least as inconsistent with a null hypothesis would be under the model and assumptions used. It does not measure the probability that the null hypothesis is true, and it does not describe the size or clinical importance of an effect. The registry record lists no analyses, so no p-value or testing procedure is reported for the primary or secondary outcomes.

The study has one listed surgery group, whereas the planning assumptions described in the registry record refer to equal allocation between two arms and a normal approximation. They also specify a two-sided significance level of 0.05 and an effect to detect expressed as a fraction of an observed effect. The record explicitly characterizes this material as a planning illustration, not as the trial’s own design. The assumptions should not be mistaken for a description of the study’s actual analysis. No resulting participant-per-arm estimates are reported in the registry record. Such planning estimates, when available, are not observed power and do not tell readers whether the study found an effect.

General statistical context. Sample-size planning depends on assumptions about the outcome, effect size, variability, statistical test, significance level, desired power, and allocation. If those assumptions do not match the study design or the analysis, the resulting illustration may not describe the study’s operating characteristics. In this record, the two-arm allocation assumption differs from the listed one-group cohort. The registry record does not report the study’s own sample-size justification or the assumptions used to choose enrollment.

The outcome is measured as a percentage, but the record does not report its distribution or any diagnostic checks. General statistical context: continuous outcomes can sometimes depart from the assumptions of a proposed model, and the appropriateness of a mean-based analysis can depend on the distribution and sampling structure. Over-dispersion is a concern mainly for outcomes whose variability exceeds that expected under a particular count or probability model; it is not a result that can be inferred from this record. No distributional assessment or alternative analysis is reported.

The registry lists three outcome measures, with one identified as primary and two as secondary. The analysis strategy for those measures is not reported. General statistical context: when many outcomes or hypotheses are tested, the chance of at least one chance finding can rise if no multiplicity strategy is used. Whether that concern was addressed in this study cannot be determined from the registry record. No adjustment method, hierarchy of secondary analyses, or prespecified testing procedure is reported.

Finally, a before-and-after design can estimate change among participants with measurements at both time points, but the estimate may be affected if follow-up availability is related to participant characteristics or outcomes. The registry record does not report the number assessed at each time, missing-data methods, or whether the analysis included all enrolled participants, only participants with paired assessments, or another analysis population. These omissions limit assessment of both the analysis and its precision.

10. Confidence Intervals for Each Arm

No two-arm comparison is shown for this trial, but each arm's mean can be given with a t-based 95% confidence interval computed from the posted mean, spread and number of participants.

R code and output

arm <- c("Gastric Bypass Surgery Patients")
n   <- c(33)
m   <- c(-25.8)
sp  <- c(15.3)
sd  <- sp      # posted standard deviations
for (i in seq_along(n)) {
  se <- sd[i] / sqrt(n[i]); h <- qt(0.975, n[i] - 1) * se
  cat(sprintf("CARM|%s|%d|%.3f|%.3f|%.3f|%.3f\n", arm[i], n[i], m[i], sd[i], m[i] - h, m[i] + h))
}

CARM  Gastric Bypass Surgery Patients  33  -25.800  15.300  -31.225  -20.375

11. Multiplicity, Missing Data and Analysis Populations

The registry record lists three outcome measures: change in intestinal calcium absorption as the primary outcome, plus femoral-neck areal bone mineral density and tibial trabecular number as secondary outcomes. It does not report an analysis plan specifying how results across these outcomes were handled. No multiplicity adjustment, testing hierarchy, or other strategy for interpreting multiple outcome analyses is reported. Since the record lists no analyses, it also does not show whether any statistical tests were conducted.

The primary endpoint population consists of cohort participants assessed before surgery and at 6 months afterward. The number with both assessments is not reported in the registry record. Enrollment was 55, but the number enrolled is not necessarily the denominator for this endpoint. The record does not report assessment counts at either time point, reasons for missing measurements, or methods for handling missing data.

The analysis population is not reported in the registry record. It is therefore unclear whether any analysis included all enrolled participants, only those with complete paired measurements, or another subset. General statistical context: missing outcome data can affect precision, and it can also bias an estimate when the availability of measurements is related to participant characteristics or outcomes. The extent or direction of any such effect cannot be assessed without information about missingness and the analysis approach.

The record’s sample-size assumptions are described as planning illustrations rather than the trial’s own design. No study-specific sample-size justification or interim-analysis plan is reported. The registry record does not establish whether analyses were prespecified or whether any outcomes were added or changed after study initiation.

12. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 1.82% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: nephrolithiasis (1.8%).

The registry record lists 55 people at risk in the gastric bypass surgery group for both serious and other adverse events. It reports 1 person affected by a serious adverse event and 3 people affected by other adverse events. The top listed serious event term is nephrolithiasis, reported as 1.8%. These are the registry’s event summaries; they do not establish that an event was caused by surgery.

The record does not report the duration or method of adverse-event surveillance, event severity beyond the serious/other classification, or details about how events were collected. It does not report a death count; deaths are not reported in the registry record, which should not be interpreted as a reported count of zero. The serious and other event summaries should be read as separate reported categories; the record does not clarify whether the people represented in them overlap.

These safety figures alone do not permit comparisons with a control group, since the registry lists no separate control group. They also do not provide a complete account of event timing, recurrence, or individual clinical circumstances. The percentages and counts are descriptive registry entries, not estimates of comparative risk.

13. What the Result Does, and Does Not, Mean

The record establishes that the study asked whether intestinal calcium absorption changed between the preoperative assessment and 6 months after gastric bypass in a cohort of obese men and women undergoing surgery. It also identifies skeletal measures as secondary outcomes. However, the registry record does not report the primary outcome result or analyses for any of the three outcomes. It therefore cannot show whether absorption changed, by how much, or how precisely any change was estimated.

The one-group before-and-after structure matters for interpretation. A measured change in the cohort would describe a time-associated difference, not necessarily an effect attributable only to surgery. Without a separate control group or a reported method addressing other sources of change, the registry record does not support a causal comparison between surgery and no surgery.

The endpoint is a measure of calcium absorption, not a direct clinical outcome such as fracture, and it is distinct from the listed bone density and trabecular measures. The record does not report results that connect these outcomes. It also does not report a threshold for a clinically important change, so statistical and clinical importance cannot be assessed from the registry entry.

General statistical context: the size of an estimated effect and its uncertainty are both important. A small estimate with a narrow interval and a larger estimate with a wide interval can convey different evidence, even if a test gives the same significance classification. In this case, neither an estimate nor an interval is reported. The completed status and enrollment count do not fill that gap.

The adverse-event entries provide descriptive counts for the listed surgery group, but do not establish comparative safety or causation. The planning assumptions are illustrations and should not be treated as observed power or as evidence about the outcome. Overall, the registry record describes the question and some study features, but it does not provide the results needed to judge the magnitude, precision, or clinical meaning of the primary endpoint.

14. Why This Trial Matters Statistically

Statistically, this study illustrates why an outcome definition and an outcome result are different kinds of information. The registry specifies fractional calcium absorption, its measurement method, the before-and-after timeframe, and the intended summary as a mean change with a standard deviation. Those details identify what would be estimated. They do not show what was observed. In this record, no analyses or primary outcome estimate are reported.

The cohort structure also shapes what the primary comparison can mean. The endpoint compares assessments within participants over time, while the registry lists no separate control group. A change over time can be described, but a design without a comparator cannot by itself separate a surgery-associated change from other explanations. Making that distinction clear is important when interpreting physiological outcomes.

The record also names two secondary outcomes, which broadens the scientific questions beyond calcium absorption. Without reported results or a multiplicity plan, readers cannot assess whether those outcomes were analyzed, how findings across outcomes were interpreted, or whether the measures moved together. Similarly, enrollment does not reveal the endpoint denominator; the number with both required measurements is not reported.

Finally, the planning assumptions are explicitly illustrative and refer to a two-arm allocation, unlike the single listed surgery group. This highlights why sample-size calculations must be interpreted in relation to the design and analysis they assume. The record does not report a study-specific justification. These features make the entry useful for identifying the measurement and design questions, while leaving the central empirical result and its precision unavailable in the registry record.

15. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The principal limitation for interpreting this trial from the registry record is that the primary outcome value and analyses are not reported. The number of participants with both baseline and 6-month calcium absorption measurements is also not reported, so the endpoint denominator and resulting precision cannot be assessed. Although enrollment was 55, it cannot be assumed that every enrolled participant contributed to each outcome.

The listed cohort has no separate control group, and allocation is not stated. The record therefore does not describe a randomized or like-for-like comparison. A before-and-after change would not by itself establish that surgery caused the change. The registry record also does not provide participant characteristics beyond the broad description of obese men and women undergoing gastric bypass, limiting assessment of who the results would represent.

Follow-up for the primary endpoint is specified at 6 months, but later outcome assessments are not described in the provided registry entries. The record does not report outcome-specific follow-up completeness, missing-data methods, analysis populations, or reasons for missing measurements. It also does not report a statistical analysis plan, multiplicity strategy, interim analyses, or a study-specific sample-size justification.

Safety interpretation is limited by the lack of reported surveillance details and a comparator. Deaths are not reported in the registry record. The listed adverse-event counts do not establish causation or comparative risk. These limitations concern what the registry entry reports; they do not establish that unreported methods or analyses were absent from the study itself.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Effects of Gastric Bypass Surgery and Calcium Metabolism and the Skeleton — NCT01330914.
• Registry record last updated 2019-03-15; status last verified 2019-02.
• Results first posted 2016-01-07.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

19. Record Summary

NCT01330914, titled “Effects of Gastric Bypass Surgery and Calcium Metabolism and the Skeleton,” is marked completed. The recorded start was 2011-07, primary completion was 2014-04, and enrollment was 55. The registry describes obese men and women undergoing gastric bypass surgery in one listed group.

The primary outcome is mean change in intestinal calcium absorption from baseline to 6 months, measured by a dual stable isotope method and expressed as the percentage of ingested calcium absorbed. Secondary outcomes are areal bone mineral density at the femoral neck and trabecular number at the tibia. The registry record lists no analyses and does not report outcome results, endpoint denominators, or a statistical analysis plan.

The adverse-event entry reports 1 person with a serious event and 3 with other events among 55 at risk; nephrolithiasis is the top listed serious event term at 1.8%. The record does not list a separate control group or report deaths.