Meat Protein and Calcium: Do They Interact Synergistically or Antagonistically?: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Meat Protein and Calcium: Do They Interact Synergistically or Antagonistically? (NCT00700804), a randomized na study. The primary outcome is Calcium Absorption.

2. Clinical Question

The central question is whether the relationship between dietary protein and calcium absorption changes according to the calcium level of the diet. The study’s stated primary objective was to determine whether meat interacts positively with calcium to improve calcium retention. Its secondary objective concerned whether an interaction between dietary protein and calcium affects biomarkers of bone metabolism. The registry record identifies serum insulin-like growth factor 1 (IGF-1) as a secondary outcome.

The primary outcome was the percentage of Calcium-47 absorbed. This is a continuous measure, summarized in the registry as a mean with standard deviation, and the stated timeframe was 17 weeks. The record says that the statistical analysis included only participants who completed both protein diet periods. It does not report how many participants met that criterion.

3. Trial Design

This was a randomized, unmasked, factorial study listed as completed. Enrollment was 34. The registry record lists the study start as 2003-06 and primary completion as 2004-07. The sponsor was the USDA Grand Forks Human Nutrition Research Center. The phase is listed as NA.

The factorial structure is relevant because the study considered two dietary factors: protein level and calcium level. In general, a factorial design can examine each factor and whether the effect associated with one factor varies across the levels of another. The registry lists high protein and low protein diets as behavioral interventions, and identifies high calcium and low calcium diet groups. It does not report the detailed diet composition, the assignment sequence, or how participants were distributed across the diet combinations.

The primary outcome was calcium absorption over a stated timeframe of 17 weeks. The record describes a measurement procedure based on Calcium-47 retention monitored for 28 days. The primary statistical population was restricted to participants who completed both protein diet periods. The number enrolled is therefore not necessarily the number contributing to the primary analysis; the number analyzed and reasons for any exclusions are not reported in the registry record.

The record identifies serum IGF-1 as a secondary outcome and lists two outcome measures. It does not give a complete account of all planned comparisons, the statistical model, adjustment for baseline values, or the method used to estimate any protein-by-calcium interaction. Randomization and masking details beyond the labels randomized and none are not reported in the registry record. The record also does not state whether participants or investigators knew the diet assignments during outcome measurement.

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: Subjects were assigned to either the High or Low Calcium diets. The subjects then consumed the high and low protein diets in random order, i.e. half of the subjects consumed high protein, then low protein while the other half consumed low protein, then the high protein diet. Drop-outs were replaced.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Only participants which completed both protein diet periods were included in the statistical analysis

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 Calcium Absorption, measured over 17 weeks.

The primary endpoint was the percentage of Calcium-47 absorbed, a continuous measure intended to describe calcium absorption. The registry record says Calcium-47 retention was monitored for 28 days using whole body scintillation counting. The percentage absorbed was estimated from the y-intercept of the linear portion of a semilogarithmic retention plot of percent Calcium-47 retained over time.

This measurement approach uses the observed retention pattern to estimate the absorbed fraction, rather than treating a single retention reading as the endpoint. In general, a continuous absorption measure can help compare average values between diet conditions and can be used to assess whether the association with protein level differs by calcium level. The registry specifies that the outcome is summarized as a mean with standard deviation. The stated endpoint timeframe was 17 weeks.

The record does not report the numerical mean, standard deviation, group-specific estimates, or an interaction estimate. It also does not report how the measurement was handled when a participant did not complete both protein diet periods. As a result, the endpoint definition and measurement process can be described, but the magnitude and precision of the observed outcome cannot be assessed from the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry record identifies a randomized factorial design, a continuous primary outcome, and a completer-based primary analysis population. It does not report a detailed statistical analysis plan or numerical estimates. The concepts below explain how such a design and outcome are generally interpreted; they do not describe unreported procedures as though they were confirmed for this study.

What does a factorial design contribute?

In general, a factorial design studies more than one intervention factor within the same trial structure. Here, the factors named in the registry are protein level and calcium level. Looking at both factors allows a study to ask about their separate associations with an outcome and about whether those associations depend on each other. The latter is an interaction question. If the difference between high and low protein varies across high and low calcium conditions, then a single overall protein contrast may not fully describe the pattern.

Interaction is not the same as showing that one diet combination has the largest observed mean. A formal interaction analysis compares the pattern of contrasts across the factor combinations, using a specified statistical model and scale. Interaction can appear different depending on whether a model considers additive differences or another scale. The scale, model terms, and test used for this study are not reported in the registry record. Without these details and numerical results, the interaction cannot be reconstructed or interpreted.

How should a mean and standard deviation be read?

The primary outcome is continuous and is described as a mean with standard deviation. In general, the mean represents the average outcome in a group, while the standard deviation describes variation among individual measurements around that average. The standard deviation is not the same as the uncertainty in the estimated mean. A standard error or confidence interval is commonly used to express uncertainty about an estimate, but neither is reported for the primary outcome in the registry record.

For an interaction question, group means may help show the pattern across combinations, but a direct interaction estimate and its uncertainty are needed to assess how compatible the data are with different interaction sizes. Separate comparisons within each calcium condition do not automatically answer whether those comparisons differ from one another. The exact estimates, model, and inferential procedure are not reported in the registry record.

What does randomization and lack of masking imply?

Random allocation is intended, in general, to reduce systematic differences between assigned groups at the start of a study. It does not guarantee that groups will be identical, particularly when enrollment is limited, and it cannot by itself address missing outcome data or errors in measurement. The record labels allocation as randomized and masking as none. Details about the randomization procedure and who knew assignments are not reported in the registry record.

In an unmasked study, awareness of diet assignment may affect behavior, reporting, or measurement in some settings. Whether that concern applies to this outcome depends on how the diets and outcome assessment were conducted. The registry describes whole body scintillation counting for the Calcium-47 retention assessment, but does not report whether outcome assessors were masked or how procedures were standardized.

How should planning calculations be understood?

The stated planning illustration uses a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. It describes effects to detect as fractions of the observed effect. These assumptions describe a planning exercise based on the observed effect, not the trial’s original design justification. The registry record does not report the trial’s own sample-size justification.

In general, a sample-size calculation estimates the number of participants needed under particular assumptions about the effect, variability, significance level, and desired power. Changing those assumptions changes the planning result. A calculation based on an observed effect is especially dependent on that estimate, which may itself be imprecise. Such planning figures are not observed power and do not show the probability that a treatment worked. No numerical planning values are reported in the registry record.

Over-dispersion is a general concern when a model assumes less variability than the data contain. If the variability is larger than assumed, standard errors may be too small and intervals may be too narrow. The primary outcome is continuous and the registry describes a standard deviation, but it does not report evidence of over-dispersion or a model that would require such an adjustment. No claim about over-dispersion in this study can be made from the registry record.

What information is needed to judge the evidence?

Interpretation requires estimates for the primary endpoint, their uncertainty, and a clearly described analysis of the protein-by-calcium interaction. The record also needs to clarify the number of participants analyzed, how missing measurements were handled, and whether the analysis followed a prespecified plan. Those details are not reported in the registry record. General statistical principles explain what these elements mean, but cannot replace the absent study-specific results.

10. Multiplicity, Missing Data and Analysis Populations

The registry record identifies calcium absorption as the primary outcome and serum IGF-1 as a secondary outcome, and lists two outcome measures. It does not report a complete list of planned hypotheses or comparisons, nor does it describe a multiplicity adjustment. When several outcomes, time points, or contrasts are examined, the chance of at least one apparently notable result can increase. In general, multiplicity procedures account for that feature; whether one was planned or used is not reported in the registry record.

The record’s analysis note indicates that the primary outcome has several categories or time points and was not reduced to one comparison. It therefore does not present a single primary comparison that can be described from the registry information. The specific categories or time points, the associated estimates, and any method for combining or selecting them are not reported in the registry record.

For the primary analysis, the record states that only participants who completed both protein diet periods were included. This defines a restricted analysis population, but does not give the number of participants analyzed or how many failed to complete both periods. The reasons for noncompletion and the handling of missing measurements are not reported in the registry record. The use of imputation, sensitivity analyses, or other missing-data methods is also not reported.

In general, excluding participants with incomplete data can affect how well an analysis represents the randomized group if completion is related to participant characteristics or outcomes. That is a methodological possibility, not evidence that a particular bias occurred in this study. The registry record does not report enough information to assess the effect of the completer restriction or to compare it with an analysis of all randomized participants.

11. 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 4% reporting threshold.

The registry record lists serious adverse events and other adverse events separately for the high calcium and low calcium diet groups. In each group, 17 participants were reported at risk for serious adverse events and 17 at risk for other adverse events. The recorded number affected was 0 for serious adverse events and 0 for other adverse events in each group.

These figures describe what is recorded for the listed groups. They do not establish that no adverse events occurred outside the categories or reporting period represented in the record, and they do not provide information about event severity beyond the serious-versus-other classification. The registry record does not report deaths. It also does not provide additional event descriptions, follow-up details, or a narrative about safety monitoring.

Zero recorded events should be read as an observed count, not as proof that the risk of an event is absent. With no event counts beyond those listed, a comparative safety estimate cannot be assessed from the registry record.

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

The registry record establishes that a randomized factorial study examined protein and calcium diet conditions in relation to calcium absorption, with serum IGF-1 listed as a secondary outcome. It also explains how the primary endpoint was estimated and states that the primary analysis included only participants who completed both protein diet periods. These details clarify the scientific question and the planned measurement, but they do not reveal the study’s outcome pattern.

The record does not report numerical results for calcium absorption or IGF-1, so it cannot show whether the measured averages differed between diet conditions, whether any interaction was estimated, or how precise the estimates were. It therefore cannot support a claim that an interaction was present or absent. The absence of numerical results in the registry record is not itself evidence for either conclusion.

The study’s title and objective refer to meat protein, but the listed interventions are high protein and low protein diets. The diet composition is not reported in the registry record. Consequently, the record does not establish whether any difference could be attributed specifically to meat protein rather than to other aspects of the diets. The calcium amounts and the details of the diet combinations are also not reported.

In general, an outcome estimate describes what was observed in the participants analyzed; a confidence interval describes the uncertainty around that estimate. Neither the estimates nor their uncertainty are reported for the primary endpoint. Enrollment of 34 participants does not by itself establish the precision or generalizability of an effect, especially when the analysis population is defined by completion of both protein diet periods and its size is not reported.

The primary outcome concerns an absorption measure, while the stated secondary objective concerns biomarkers of bone metabolism. The registry record does not report results linking these outcomes or any broader clinical outcomes. No conclusion about health effects or clinical importance can be drawn from the information available in the record.

13. Why This Trial Matters Statistically

This study raises a useful statistical question: whether the association between one dietary factor and a continuous outcome depends on a second dietary factor. That is the purpose of examining an interaction in a factorial design. A design can be well suited to that question, but interpretation depends on presenting the interaction estimate and its uncertainty rather than relying only on separate group averages.

The endpoint also illustrates the importance of understanding how a biological measure is constructed. Calcium absorption was estimated from a pattern of Calcium-47 retention over 28 days, with the percentage absorbed derived from a specified portion of a semilogarithmic retention plot. Knowing this measurement approach helps distinguish the measured endpoint from a direct clinical outcome and highlights that the estimate depends on the defined measurement procedure.

The registry record’s restriction to participants completing both protein diet periods is another important statistical feature. An analysis population can differ from the randomized population when some participants do not contribute complete measurements. Without the number analyzed, reasons for noncompletion, and missing-data methods, readers cannot judge how that restriction affects interpretation.

Finally, a question framed around synergy or antagonism requires estimates that show the joint pattern across the factors. The record describes the factorial question but does not report the numerical outcome or interaction results. This distinction matters: a clearly stated study question and a defined endpoint are not substitutes for estimates, uncertainty intervals, and analysis details. The available record therefore supports methodological discussion while leaving the trial’s empirical findings unresolved.

14. 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 most consequential limitation of the registry record for interpretation is that it does not report numerical results for the primary endpoint. Group means, standard deviations, interaction estimates, confidence intervals, and p-values are not reported. The direction, size, and precision of the observed pattern therefore cannot be evaluated.

Enrollment was 34, but the primary analysis was limited to participants who completed both protein diet periods. The number included in that analysis, the amount of missing data, and the reasons for incomplete participation are not reported in the registry record. Missing-data procedures and sensitivity analyses are also not reported, so the effect of the completer restriction cannot be assessed.

The intervention labels identify high and low protein diets and high and low calcium diets, but the diet composition and nutrient amounts are not reported. In particular, the record does not explain how the high and low protein diets operationalized the study’s focus on meat protein. This limits interpretation of what dietary contrast the study actually tested and whether any finding could be specific to meat.

The record does not describe the statistical model, the interaction scale, or how the multiple categories or time points for the primary outcome were addressed. It also does not report a multiplicity strategy, a detailed randomization procedure, or who knew the assignments during outcome assessment. These gaps prevent a full appraisal of analysis choices and potential sources of uncertainty.

Finally, the registry record identifies a secondary biomarker outcome but does not report its results or any broader clinical outcomes. The scope of inference is therefore limited to the stated study question and measurement definitions; conclusions about clinical significance are not supported by the record.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Meat Protein and Calcium: Do They Interact Synergistically or Antagonistically? — NCT00700804.
• Registry record last updated 2018-07-02; status last verified 2018-05.
• Results first posted 2009-06-02.
• 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.

18. Record Summary

The registry describes a completed, randomized, unmasked factorial study with enrollment of 34. The listed behavioral interventions were high and low protein diets, and the diet groups included high and low calcium conditions. The primary outcome was the percentage of Calcium-47 absorbed, measured using a retention procedure over 28 days and reported as a mean with standard deviation. The stated endpoint timeframe was 17 weeks.

Only participants completing both protein diet periods were included in the primary analysis, but the number analyzed is not reported in the registry record. Serum IGF-1 was listed as a secondary outcome. The record does not report numerical primary or secondary outcome results, interaction estimates, uncertainty intervals, or p-values. Diet composition, detailed analysis methods, and missing-data procedures are also not reported.

The record identifies the study question and measurement approach, but does not permit assessment of the direction, magnitude, or precision of the findings.