Acute effects of almond milk vs. almond milk supplemented with almond protein powder or whey protein on plasma amino acid profiles and hydration status in postmenopausal women: a randomised crossover study protocol.

Background: Postmenopausal women experience hormonal changes that negatively impact muscle mass, hydration, and metabolic health. Optimising protein intake during this life stage can mitigate musculoskeletal function declines, yet many consume inadequate protein. As interest grows in choosing sustainable, plant-based alternatives over animal protein, almond protein powder (APP), a by-product of almond oil extraction, presents a novel option for consumers. Differing from a protein isolate, APP retains fat, fibre, and micronutrients, which may influence its potential for promoting muscle protein synthesis and hydration. However, little is known about APP's acute physiological effects, or perceived responses such as fullness/satiety, taste or gastrointestinal (GI) symptoms. This study aims to investigate the postprandial amino acid, glycaemic, insulinemic, hydration, and perceived responses to APP in postmenopausal women, comparing it to a whey protein supplement. Findings will inform tailored supplementation strategies for an underrepresented and nutritionally vulnerable population in the future.

Methods: This protocol paper details a randomised, crossover study of 14 postmenopausal women, assessing the acute amino acid profile and hydration responses in postmenopausal women following ingestion of APP as compared to whey protein, water or almond milk. Venous blood, urine and questionnaires are to be collected during each session for 180 min post-ingestion. Primary outcomes include aminoacidemia (total amino acids, as well as all 20 individual primary amino acids) and hydration markers (urine volume excreted, body mass, plasma volume, urine specific gravity, urine colour, plasma osmolality, fluid retention). Secondary outcomes include perceived appetite, thirst, gastrointestinal symptoms, and supplement tolerability and acceptability. Between-treatment comparisons will be made using linear mixed models with a fixed effect of treatment and a random intercept for participant.

Discussion: The study strengths include a focus on postmenopausal women, an underrepresented group in clinical and nutrition research who are susceptible to problems with muscle function and hydration. The use of a randomised crossover design to reduce inter-individual variability strengthens this study further. A combination of objective measures and subjective responses offers insights into both physiological effects and consumer acceptability of APP versus whey protein. Hydration assessment is particularly relevant given the postmenopausal population can be at increased risk of fluid imbalance. Acute measurement of responses to APP limit the transferability to longer-term supplementation, and there are some inherent macronutrient discrepancies between treatments which may influence glycaemic and insulinemic responses to supplementation.

Trial registration: DUHREC: 2024/HE000669, ANZCTR: ACTRN12625000127404p.