Gastric pH and Anthocyanin Absorption: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Gastric pH and Anthocyanin Absorption (NCT01122160), a randomized na study. The primary outcome is Gastric pH.

2. Clinical Question

3. Trial Design

This was a randomized, triple-masked, crossover study. The registry lists an enrollment of 13 and describes the study as completed. The sponsor was the USDA Beltsville Human Nutrition Research Center. The listed study start was 2010-05, and primary completion was 2010-12. The registry lists the condition as anthocyanin metabolism.

The two interventions were omeprazole and placebo, each taken with blackberries and strawberries. The experimental arm description specifies a 20.6 mg omeprazole tablet. In a crossover design, participants receive more than one intervention in a planned sequence, allowing comparisons within participants in principle. However, the treatment sequences, period structure, washout interval and handling of period or sequence effects are not reported in the registry record. It is therefore not possible to establish from the record how those design features were implemented or addressed in the analysis.

The primary outcome listing is a continuous measure: gastric pH, summarized by a mean and standard deviation. Its timeframe is 1 hour prior to gastric emptying on Day 7 of the given intervention. The posted results give arm-level summaries for 11 participants in the experimental arm and 11 in the placebo arm. The record does not explain why those summary counts differ from the enrollment figure, and it does not state whether any participants were excluded from the outcome analysis or why.

The record lists one outcome measure and no posted secondary outcomes. It does not report a phase designation beyond NA. The number of participants, treatment sequence, original sample-size rationale and detailed analysis plan are not reported in the registry record. Those omissions limit reconstruction of the intended design and analysis from the posted information alone.

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.

Before assignment: Discrepancy between actual enrollment and number of participants in participant flow module is due to enrolled participants no longer meeting exclusion criteria (i.e., inability to swallow capsule).

First Intervention (7 Days)

Washout (7-14 Days)

Second Intervention (7 Days)

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 Gastric pH, measured over 1 hour prior to gastric emptying on Day 7 of the given intervention.

The primary endpoint is gastric pH, a continuous measure of acidity in the stomach. The registry specifies that the measurement was taken 1 hour prior to gastric emptying on Day 7 of the given intervention. It reports the mean and standard deviation for each treatment arm. The observed mean was 0.96 in the experimental arm and 2.31 in the placebo arm.

Gastric pH is relevant to the stated research question because the study description asks whether altering stomach pH with omeprazole affects anthocyanin absorption from berries. As a general statistical and physiological distinction, a pH measurement describes the acidity environment; it is not itself a measurement of how much anthocyanin entered the body. A change in the endpoint can therefore speak to the measured gastric condition but cannot, by itself, establish an absorption result.

The registry does not define additional endpoint details such as the measurement instrument, repeated measurement schedule, or how observations were summarized across the crossover periods. It also does not post a numerical anthocyanin absorption endpoint. The result should consequently be interpreted as a comparison of gastric pH summaries at the specified time, rather than as a direct estimate of anthocyanin absorption.

7. Results Posted in the Registry

• Experimental: mean 0.96 versus 2.31 on control; difference -1.35 (95% CI -1.78 to -0.92), standardized effect d = -2.61; z-test p = <0.001.

8. Statistical Interpretation

9. Statistical Methods Explained

The registry posts arm means and standard deviations, a mean difference, an interval, and an unadjusted p-value for gastric pH. It labels the outcome as a continuous measure summarized by its mean. No detailed statistical analysis plan or description of the original analysis method is reported in the registry record. The explanations below distinguish the general meaning of these statistics from unreported trial-specific procedures.

What does a mean describe?

As a general statistical concept, a mean is the arithmetic average of the observations included in a group summary. It gives a central value on the scale being measured. Here, the scale is gastric pH, and the posted means are 0.96 for the experimental arm and 2.31 for the placebo arm. A mean does not show the full distribution: different sets of individual observations can share the same mean while differing in their spread, shape or unusual values.

The standard deviation is a descriptive measure of how dispersed observations are around the group mean. The posted standard deviations are 0.37 and 0.63, respectively. They describe variability among the measurements, not the precision of the between-arm contrast by themselves. General interpretation of standard deviations also depends on the data distribution, which cannot be inspected from arm-level summaries alone.

How should the mean difference be read?

A mean difference expresses the contrast between two group means on the original outcome scale. The reported contrast is experimental minus placebo, with an estimate of -1.35. A negative value indicates that the experimental-arm mean is lower than the placebo-arm mean. The estimate is in pH units, not a percentage or a standardized unit. It directly describes the reported difference between the means; it does not describe a difference in anthocyanin absorption.

General statistical interpretation of a treatment contrast depends on how the observations were collected and analyzed. With a crossover design, measurements from the same participant under different interventions can be related. An analysis that accounts for this relationship may differ in its uncertainty from an analysis that treats observations as independent. The registry does not report whether the displayed contrast was calculated from paired observations, how treatment periods were modeled, or how sequence and period effects were handled. The arm summaries alone cannot recover those details.

What does the interval communicate?

As a general concept, a confidence interval represents a range of values compatible with the observed data under a specified statistical procedure and its assumptions. It communicates uncertainty in an estimate more directly than a point estimate alone. The posted interval for the mean difference is entirely below zero, so the reported range does not include zero. That is consistent with the direction of the posted contrast and the unadjusted p-value below 0.001.

The confidence level and exact interval procedure are not reported in the registry record. The interval should therefore be described as the reported interval, without assigning it an unreported confidence level or assuming a specific model. In general, interval width depends on variability, the amount of information in the data and the analysis method. A narrow interval indicates greater precision under the procedure used; a wider interval indicates greater uncertainty. The displayed interval remains conditional on the underlying analysis and design assumptions.

How should the p-value be understood?

The unadjusted p-value is reported as <0.001. In general, a p-value quantifies how compatible the observed data are with a null model that specifies no treatment contrast, when the model assumptions hold. It is not the probability that the null model is true, nor is it a measure of the probability that the result occurred by chance in an everyday sense. A p-value also does not measure the size or practical importance of an effect. The estimated mean difference and its interval provide separate information about size and precision.

The term unadjusted indicates that the posted p-value is not presented as adjusted for multiple comparisons. The registry lists one outcome measure and zero posted analyses, and does not describe a multiplicity strategy or the statistical test used for this contrast. It is not possible to determine whether this p-value reflects the trial’s original prespecified analysis, nor what role it played in the study’s conclusions.

What is known about the crossover analysis?

General crossover methods recognize that each participant may contribute observations under more than one condition. This can make within-person comparisons informative because stable individual characteristics are shared across treatment periods. At the same time, crossover inference can be affected by period effects, treatment sequence, carryover and missing measurements. Those concerns are reasons the analysis needs to reflect the design; they are not evidence that any particular problem occurred in this trial.

The registry confirms randomized allocation, triple masking and a crossover model, but does not report the sequence allocation, washout, period-specific outcomes, paired-difference summaries or model specification. There is also no posted analysis-population definition. As a result, readers cannot verify from the registry whether the analysis used the crossover structure as intended or assess how sensitive the result is to alternative approaches.

Finally, the arm-level summaries are limited to gastric pH at the specified time. They cannot establish how pH changed over time, whether the pH response differed among participants, or whether the intervention affected anthocyanin absorption. Those would require outcomes or analyses not reported in the registry record.

10. Confidence Intervals and Precision

The reported mean difference is -1.35 pH units, and the interval associated with that estimate lies entirely below zero. Its lower limit is -1.782, and the upper limit also remains below zero. This means that the full reported interval favors a lower mean pH in the experimental arm than in the placebo arm, under the analysis used to produce it. The registry does not report the interval’s confidence level or construction method, so those should not be inferred.

Precision is about the range of values that remain compatible with the data and the analysis, rather than whether the estimate is important in another context. The interval gives more information than the point estimate alone: it indicates uncertainty around the contrast and whether zero is within the reported range. However, an interval does not account for every source of uncertainty. It does not resolve questions about the crossover analysis, the difference between enrollment and reported arm counts, the unreported handling of missing data, or the absence of an anthocyanin absorption result.

The arm-level standard deviations were 0.37 for the experimental arm and 0.63 for the placebo arm. These describe the observed spread of gastric pH measurements in the summaries, whereas the interval describes uncertainty in the estimated contrast. The record reports 11 participants in each arm summary. With only these group summaries, it is not possible to inspect individual values, evaluate distributional shape, or determine whether a small number of observations had strong influence on the result.

The p-value below 0.001 is consistent with the reported interval excluding zero, but the p-value and interval answer different questions. Neither establishes the direct effect on anthocyanin absorption, which is not reported as an outcome.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The planning figures are participants per arm, not total events. The illustration assumes a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. It uses the observed mean difference as its reference effect and considers planning for 100%, 75% and 50% of that observed effect. For the 100% planning scenario, the figures are 3 participants per arm for 80% power and 4 participants per arm for 90% power. For 75% of the observed effect, they are 5 and 6 participants per arm. For 50% of the observed effect, they are 10 and 13 participants per arm.

In plain language, the 80% power column gives the illustrative number per arm that would yield an 80% chance of detecting the specified effect under the stated planning assumptions, if that effect and the assumptions were correct. The 90% power column describes the corresponding illustration for a 90% chance. These are prospective planning calculations, not observed power and not the original trial’s sample-size justification. The registry does not report the assumptions used to choose the trial’s enrollment.

Smaller effects require more participants in these illustrations because they are harder to distinguish from sampling variability. The values depend on the assumed effect size and calculation method; they should not be read as a reconstruction of the trial’s design. The illustration does not incorporate unreported crossover features, missingness, or other design details. It is not evidence that the trial was adequately or inadequately sized for its original objectives.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = -1.350; pooled standard deviation σ = 0.517.
2. n = 2 × (1.96 + 0.842)² × 0.517² / -1.350² = 2.3, rounded up to 3 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
delta <- -1.35                      # observed difference in means
sigma <- 0.51662365412358                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(1122160)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 3 (80% power), 4 (90% power)
 75% of observed effect: n per arm = 5 (80% power), 6 (90% power)
 50% of observed effect: n per arm = 10 (80% power), 13 (90% power)
power.t.test: 3.6 
Simulated power at n = 3 per arm: 0.673 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Multiplicity, Missing Data and Analysis Populations

The registry lists one outcome measure, gastric pH, and no posted secondary outcomes. It reports an unadjusted p-value for the mean difference. No plan for controlling the risk of false-positive findings across multiple outcomes, time points or analyses is reported in the registry record. Because the registry does not describe the complete analysis plan, it is not possible to determine whether other analyses were planned or conducted but not posted.

Missing-data methods are not reported in the registry record. The record gives an enrollment of 13 and outcome summaries with 11 participants in each arm, but does not explain the difference, identify missing observations, or state how missingness was handled. It does not say whether participants with incomplete crossover data contributed to one arm summary, were excluded from a contrast, or were managed by another method. These possibilities should not be assumed.

Analysis-population definitions are also not reported. The registry does not state whether results were based on all randomized participants, participants who completed both treatment periods, or another group. Nor does it report whether deviations from assigned treatment, incomplete measurements or other eligibility conditions affected inclusion. This makes it difficult to assess the relation between enrollment and the people represented in the posted estimate.

The crossover structure adds an important analysis detail: methods may need to account for repeated measurements within participants and treatment periods. The record does not describe a paired analysis, model specification, sequence or period adjustment, or a procedure for addressing possible carryover. These are unreported methods, not evidence that any particular analysis was or was not performed. The available record supports interpretation of the posted summary contrast, while leaving multiplicity, missingness and analysis-population questions unresolved.

14. 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 0% reporting threshold.

The adverse-event listing reports zero serious adverse events among 11 participants at risk in the experimental group and zero among 11 at risk in the placebo group. It also reports zero other adverse events among 11 at risk in each group. These are counts as recorded for the groups, not a demonstration that no adverse events could occur or that the groups are equivalent in safety.

The registry does not list serious adverse-event terms. Death counts are not reported in the registry record. The record does not provide event definitions, observation duration, severity details beyond the serious-event category, or a narrative of how adverse events were collected. It is therefore not possible to use these figures to characterize the full safety experience or compare event rates beyond the posted counts.

With the reported group sizes, zero recorded events means no events were listed in those group summaries. It does not establish the absence of risk, and it should not be interpreted as proof of safety. These figures are descriptive and do not alter the distinction between the reported gastric pH endpoint and the unreported anthocyanin absorption outcome.

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

The posted result establishes that the mean gastric pH differed between the experimental and placebo arms in the reported summaries. The experimental-arm mean was 0.96, compared with 2.31 in the placebo arm, and the reported experimental-minus-placebo difference was -1.35. The interval remains below zero, and the unadjusted p-value is <0.001. Together, these statistics describe a clear separation in the measured endpoint under the analysis used for the posted comparison.

The result does not establish how much anthocyanin was absorbed. The registry’s stated research purpose concerns anthocyanin absorption from strawberries and blackberries, but the posted primary endpoint is gastric pH and no absorption result is reported. A difference in the acidity measure cannot be substituted for a measured absorption outcome. The record also does not establish that the pH difference caused a change in absorption or identify a mechanism linking the two.

The estimate is specific to the trial’s participants, interventions and measurement timing as recorded. The summaries concern gastric pH 1 hour prior to gastric emptying on Day 7 of the given intervention. The arm-level counts are 11 in each group, while total enrollment is listed as 13; the record does not explain the discrepancy. Participant characteristics, crossover sequences, treatment periods and the original analysis details are not reported in the registry record.

Consequently, the findings should be read as a statistical description of gastric pH, not as a broad conclusion about berry intake, anthocyanin metabolism or outcomes in other populations. The p-value does not measure clinical or practical importance, and the interval does not resolve missing reporting about analysis methods. The zero adverse-event counts likewise describe the posted data only; death counts are not reported.

16. Why This Trial Matters Statistically

This trial illustrates the importance of distinguishing a study’s motivating question from the outcome for which numerical results are posted. The registry describes a question about whether changing stomach pH alters anthocyanin absorption, yet the primary result presented is gastric pH. That result can characterize the measured gastric condition, but it cannot answer the absorption question without an absorption outcome and its analysis.

It also demonstrates why the design and the analysis need to be read together. The record identifies a randomized crossover trial, a structure that can support within-participant comparisons. But the posted arm means and mean difference do not show how the crossover data were modeled or whether the statistical analysis accounted for repeated measurements, periods and sequences. Such details matter for interpreting uncertainty; they are not recoverable from the means alone.

The contrast, interval and p-value illustrate complementary statistical information. The estimate gives the direction and size in pH units, the interval conveys uncertainty around that estimate, and the p-value describes compatibility with a null model under assumptions. None of these alone indicates the result’s importance for absorption or establishes a causal mechanism.

Finally, the difference between listed enrollment and outcome summary counts, together with the absence of reported missing-data and analysis-population methods, shows why transparent reporting is valuable. The posted summaries support a focused interpretation of gastric pH, while the registry record leaves consequential questions about participants and methods unanswered.

17. 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.

Several limitations specific to this record constrain interpretation. First, the registry lists 13 enrolled participants, but the gastric pH summaries include 11 in each arm. It does not explain the count difference, describe participant flow, or say whether all randomized participants contributed data. This prevents a clear account of the analysis population from the posted record.

Second, the crossover design is identified, but details needed to understand its implementation and analysis are absent. Treatment sequences, periods, washout, paired outcome summaries, and handling of period, sequence or carryover effects are not reported in the registry record. The arm-level summaries do not show how within-participant dependence was treated.

Third, only gastric pH is posted as an outcome, with no absorption outcome or secondary results. Although the study description identifies anthocyanin absorption as a research objective, no numerical absorption findings are available in the record. The pH result therefore cannot be interpreted as a direct absorption result.

Further limitations include the absence of participant characteristics, detailed measurement methods, a stated analysis plan, missing-data methods, multiplicity procedures and analysis-population definitions. The confidence-interval level and calculation method are not stated. The record lists no serious or other adverse events in the group summaries, but provides no event terms and does not report death counts. These omissions limit what can be concluded about the study beyond the posted gastric pH comparison.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Gastric pH and Anthocyanin Absorption — NCT01122160.
• Registry record last updated 2013-08-23; status last verified 2013-07.
• Results first posted 2013-08-23.
• 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.

21. Record Summary

The completed trial was randomized, triple-masked and crossover, with enrollment of 13. It compared omeprazole, listed as a 20.6 mg tablet taken with blackberries and strawberries, against placebo with the same berries. The primary outcome was mean gastric pH, measured 1 hour prior to gastric emptying on Day 7 of the given intervention.

The posted mean was 0.96 in the experimental arm and 2.31 in the placebo arm, with a reported mean difference of -1.35 and an unadjusted p-value of <0.001. The reported arm summaries include 11 participants each. The record does not explain the enrollment-to-summary count difference, report anthocyanin absorption results, or describe key crossover analysis details. Zero serious and other adverse events were listed in each arm; death counts are not reported in the registry record.