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Dietary Standardisation in a Nutrient plus Exercise Intervention: Derivation, Implementation, and Evaluation

Marta Kozior*, Philip M Jakeman, Catherine Norton

Department of Physical Education and Sport Sciences, University of Limerick, Limerick, Ireland

Abstract: The objective was to devise and implement a dietary standardisation protocol for use in nutrient plus exercise intervention studies, and to report participant adherence to, and acceptability of same. The context was a nutrient plus exercise intervention study undertaken by resistance-trained men (18 to 35y). Participants’ habitual, seven-day, weighed dietary intake records informed the dietary standardisation protocol. Participants received a six-day meal plan that was prescribed relative to their body mass and provided 35 kcal∙kg⁻¹∙d⁻¹ comprising 2.0 g∙kg⁻¹∙d⁻¹ protein (including 0.33 g∙kg⁻¹ of dietary protein supplement), 4.5 g∙kg⁻¹∙d⁻¹ carbohydrate, and 1.0 g∙kg-1∙d⁻¹ fat. Apportioning of total protein intake was evenly distributed across six eating occasions (EOs), at three-hour intervals during waking hours. Median (25th-75th percentile) participant adherence to the prescribed meal plan was 100 (99-100)% for energy, carbohydrate and fat intakes, 100 (100–100)% for protein intake and frequency of EOs consumed, and 97 (93–100)% for distribution of EOs consumed. On study completion, 10 participants (45%) expressed that the standardised diet was easy to follow. Ten participants (45%) indicated activities of daily living as the primary challenge to adherence. The offered dietary standardisation (i.e. a protocol introduced 72 hours before the intervention study and a standardised diet) was effective in minimising the variability of dietary intake among participants undertaking a nutrient plus exercise intervention. The dietary standardisation resulted in high participant adherence and was well accepted.

Keywords: dietary control, standardised diet, sports nutrition, research methodology, dietary intake, exercise nutrition

Received June 05, 2022; Revised July 13, 2022; Accepted July 19, 2022

Citation: Marta Kozior, Philip M Jakeman, Catherine Norton, Dietary Standardisation in a Nutrient plus Exercise Intervention: Derivation, Implementation, and Evaluation. Journal of Food and Nutrition Research, 10 (7). 488-495. January 2022. doi:10.12691/jfnr-10-7-7

1. Introduction

Habitual dietary patterns vary from day to day within, and between, individuals [1,2,3,4,5]. In nutrient plus exercise interventions, these observed variations in eating patterns, an independent variable, have the potential to impact the interpretation of the outcome [6,7,8]. In their commentary in 2008, Braun and Brooks [9] highlighted the critical importance of controlling dietary energy status when undertaking studies of the effect of exercise on metabolism. More recently, Close et al. [10] re-iterated the need for carefully devised standardisation of dietary intake tailored to the measured outcome. Ideally, a dietary standardisation protocol should, where possible, consider individuals’ habitual dietary practices and preferences in advance of an intervention [10,11,12,13]. Such consideration could contribute to greater adherence to the study protocol by research participants [14]. Furthermore, reporting adherence to dietary standardisation could allow researchers to evaluate the efficacy of the dietary standardisation protocols and assess the need for their refinement in future research. However, there is a reported lack of appreciation for the effect of participants’ habitual dietary practices in research [4]. To compound the matter, the practice of dietary standardisation varies widely in nutrient plus exercise research studies. Moreover, the level of participant adherence to the introduced standardisation has not always been reported [4]. Dietary standardisation, as defined by Jeacocke and Burke [4], refers to “all methods of minimising pre-existing differences in the dietary intake or nutritional status of participants”. The same authors suggest that control of this independent variable would minimise the variability of inter and intra-participant dietary practice, and increase study validity, reliability and sensitivity of performance measurements [4]. Dietary standardisation protocols may introduce opportunities to regulate energy, nutrient, and non-nutrient intakes in short- and long-term research studies. For example, dietary standardisation may normalize the intake of certain ingredients in advance of research studies, which may enhance (e.g. caffeine) or decrease (e.g. alcohol) exercise performance. Dietary standardisation may be implemented hours or days before the conducted research. Among the most frequently used methods of dietary standardisation are dietary replication, i.e. repetition of participants’ diet for each study trial [10], and dietary prescription, i.e. the provision of a standardised diet designed to meet the targeted nutrient intake [4]. To date, a standardised diet has been deemed more effective than replication [15].

This paper aims to report the derivation, implementation, adherence, and acceptance of a dietary standardisation protocol used in a nutrient plus exercise intervention study. The principal construct of the dietary component of the study was to enact a dietary standardisation protocol to investigate the effect of supplementary dietary protein on muscle protein synthesis, post resistance exercise, over six days. Comprehensive details on the nutrient plus exercise intervention in which the aforementioned work was applied are described elsewhere [16,17].

2. Methods

2.1. Ethics

This study conformed to the standards set by the Declaration of Helsinki and was approved by the local research ethics committee (2016_12_09 EHS). The research study was re-registered at clinicaltrials.gov (NCT03297151). Participants were familiarised with the study protocol and provided written consent.

2.2. Participants

Eligible participants were male, aged 18 to 35 years; resistance-trained, defined as 0.5-years continuous resistance training ≥3 h·wk⁻¹; injury and illness free; and lactose tolerant, necessary due to the dairy (whey protein supplement) derived nutrient intervention employed in the nutrient plus exercise intervention in which this current work has context.

2.3. Development of the Standardised Diet

Table 1 depicts the timeline for each step in the derivation and implementation of the dietary standardisation.

As outlined, seven-day weighed dietary intakes were recorded by all participants at least four weeks in advance of the intervention commencement date. This allowed researchers to collect, analyse (Nutritics v5 Research Edition, Dublin) and report habitual nutrient intakes among participants. These habitually consumed foods, beverages, and reported food preferences were used to inform the composition of each EO of the standardised diet. A single EO was characterised as energy-containing (>0 kcal) food items, non-alcoholic and alcoholic beverages as well as dietary supplements consumed within 30 minutes [18]. The optimisation of habitual nutrient quality and quantity intake, timing, frequency and distribution per EO [i.e. main meals (breakfasts, lunches, and dinners) and snacks (afternoon and pre-sleep)] was compliant with recommendations for nutrient intake peri-resistance training [8,19], focusing on protein intake specific to the context of this current work. The standardised meal plan was prescribed per unit of body mass and designed to provide 35 kcal∙kg⁻¹∙d-1 energy, 2.0 g∙kg⁻¹∙d-1 protein, 4.5 g∙kg⁻¹∙d-1 carbohydrate, and 1.0 g∙kg⁻¹∙d⁻¹ fat (Table 2).

Table 1. Schematic representation of tasks, timelines, and task owners in a collaborative design and implementation of a standardised diet approach in a nutrient plus exercise intervention study

Table 2. Daily and per eating occasion energy and macronutrient intake prescription within the six-day meal plan

The menu met or exceeded the Dietary Reference Intakes for micronutrients [20,21] for the study population, except for vitamin D for which 50% of the daily Recommended Dietary Allowance was achieved. To attain an optimal protein feeding pattern [6], a meal or snack was prescribed every three hours.

In accordance with participants’ preferences, main meals (except for breakfast on Day 1) and snacks could be swapped between days. This flexibility was possible because all main meals and all snacks provided the same quantity of energy and macronutrients, and all EOs were balanced for protein (Table 2). An illustrative representation of the meal plan, composed of 30 different EOs, is provided in Figure 1. The meal plan accommodated for food allergies and intolerances, and where required alternate EO options that retained the prescribed energy and macronutrient composition were provided.

2.4. Production and Delivery of Standardised Diet

A catering company external to the primary research team was employed to provide individual meal plans based on recipes prepared by a registered dietitian. Chefs prepared main meals and snacks for which energy and macronutrient intakes were prescribed relative to participants’ body mass (Tanita MC, 180-MA, Tanita United Kingdom Ltd). Recipes provided specifications including the brand names of products, quantities of ingredients (including salt and seasonings), and cooking methods. Disposable food containers were used for daily delivery. Each EO was labelled with a participant identification code, name of EO, and prescribed date and time of EO consumption. All EOs were checked for completeness before distribution to participants.

Figure 1. Visual representation of a six-day standardised meal plan for participants. (Note. ¹ The energy and macronutrient composition of breakfast on Day 1 differed from the rest of the breakfasts due to the basal biopsy occurring after breakfast. Breakfast on Day 1 provided 5 kcal·kg⁻¹ composed of 0.05 g·kg-1 of protein, 0.8 g·kg⁻¹ of carbohydrate, and 0.2 g·kg⁻¹ of fat.)

2.5. Overview of the Implementation of the Dietary Standardisation Protocol

Following consultation to ensure dietary preferences were accommodated, the dietary standardisation protocol was explained in detail to participants. The dietary standardisation protocol began 72h before the intervention study. During this time participants were asked to refrain from alcohol, caffeine, and dietary supplement intake, and to continue this throughout the six-day intervention (i.e. nine days in total). The prescribed intake for the intervention comprised six EOs per day, three main meals, two snacks, and the dietary protein supplement, prescribed per unit body mass. All EOs were distributed evenly across each day within waking hours. On Day 1, participants consumed a low-protein (≤ 5 g∙EO⁻¹, i.e. 0.05 g·kg⁻¹), plant-based breakfast, and protein-based supplement at the same time point (Davies et al., 2019). Therefore, participants consumed five, not six, EOs on Day 1 [16]. A list of low-protein (< 1.5 g∙EO⁻¹), plant-based snacks (98–104 kcal∙EO⁻¹, 24–26 g∙EO⁻¹ of carbohydrate) were provided as alternative food and beverages options if participants craved savoury or sweetened food or beverages between the planned EOs.

2.6. Evaluation of Participant Adherence to Dietary Standardisation

On Day 1 of the six-day experimental trial, participants confirmed in writing their adherence to the protocol introduced 72-hour before the intervention. A checklist with information regarding adherence to the six-day dietary standardisation protocol was provided and reviewed daily with participants. This daily checklist allowed participants to report deviations from the dietary standardisation protocol. Participants were asked to report food and beverage intakes from the snack list, other EOs consumed but not provided within the research study, weighed food leftovers, or any changes made to prescribed frequency, timing, and distribution of EOs. Weighing scales (DYMO M2®, USA) were provided to all participants for the duration of the intervention for weighing leftovers or additional food items and fluids consumed. Food containers were returned with leftovers before EOs for the next day were provided. This offered an additional opportunity to monitor participants’ adherence to the prescribed meal plan and to calculate the actual quantity of energy and macronutrient intakes as well as the frequency and distribution of EOs.

2.7. Assessment of Participant Acceptability of Dietary Standardisation

Following the study completion, participants were asked to provide feedback regarding the acceptability of the standardised diet via an online digital survey platform (Survey Monkey®, www.surveymonkey.com). A single-choice question was asked to assess participants’ ease or difficulty in following the dietary standardisation, and the rationale for each response was ascertained using a multiple-choice question and a free text response box. Participants were asked about the benefit of a snack list in adhering to a prescribed protocol. Single-choice questions were also provided to assess participants’ willingness to modify habitual, peri-resistance training nutrient intakes.

2.8. Statistical Analysis

Statistical analyses were carried out with the Statistical Package for Social Sciences version 25.0 (SPSS Inc., Chicago IL, USA) and MS Excel 2016. Data were assessed for normality using the Shapiro-Wilk test and presented as mean (standard deviation, SD) or median (25th–75th percentiles), as appropriate. The differences between participants’ habitual dietary intake pattern and the standardised meal plan (after accounting for the weighed leftovers after consumption as well as additional food items and fluids consumed by participants) were examined by a one-sample t-test (P < 0.05). Cohen’s effect size (d) was reported. Additionally, participants’ acceptability of dietary standardisation was expressed as a percentage (%).

3. Results

3.1. Participants

This paper reports the outcome of the dietary standardisation protocol of 23 participants [median age 22.6 (20.4–25.0) years, mean body mass 79.7 (12.6) kg] who were recruited and completed the nine-day dietary standardisation protocol.

3.2. Habitual Dietary Intake

Twenty-two participants returned complete seven-day weighed habitual intake records that were included in the analysis. One record was not completed due to a change in one participant’s personal circumstances. Average habitual daily energy intake was 36 (6) kcal·kg⁻¹·d⁻¹, and average habitual daily macronutrient intakes were 1.9 (0.5) g·kg⁻¹·d⁻¹ for protein, 3.6 (1.0) g·kg⁻¹·d⁻¹ for carbohydrate, and 1.4 (0.3) g·kg⁻¹·d⁻¹ for fat. Average habitual energy and protein intake per EO were 8 (1) kcal·kg⁻¹·EO⁻¹ and 0.4 (0.1) g·kg⁻¹·EO⁻¹, respectively. Average habitual daily frequency of EOs was 5 (1), distributed every 03:03 (00:29) h:min. In comparison to the standardised meal plan (Table 2), there was no statistically significant difference for daily habitual energy [36 (6) vs. 34 kcal·kg⁻¹·d⁻¹, P=0.15, d=0.3], protein [1.9 (0.5) vs. 1.9 g·kg⁻¹·d⁻¹, P=0.68, d=0.1] or distribution of EO [03:03 (00:29) vs. 03:00 h:min, P=0.61, d=0.1]. There was a statistically significant difference between habitual intake per EO vs. standardised meal plan for energy [8 (1) vs. 5.7 kcal·kg⁻¹·EO⁻¹, P>0.001, d=1.3], protein [0.4 (0.1) vs. 0.32 g·kg⁻¹·EO⁻¹, P=0.001, d=0.8], and number of EO [5 (1) vs. 6 (P>0.001, d=-1.3].

Figure 2. Rationale for evaluating the six-day meal programme as either easy or difficult to follow by participants (N = 22)

3.3. Adherence to the Standardised Dietary Protocol

All participants (N=23) reported adherence to the conditions relating to alcohol, caffeine, and dietary supplement consumption. Measured by a weighed return of food leftovers and checklist, adherence to the six-day meal plan was (consumption as prescribed) 100 (99–100)%, 100 (100–100)%, 100 (99–100)%, and 100 (99–100)% for energy, protein, carbohydrate and fat, respectively. Participant adherence to the prescribed timing interval between EOs was 97 (93–100)%. This resulted in an EO every 03:04 (03:04–03:05) h:min, during waking hours, over six days. Adherence to the recommended frequency of EOs was 100 (100–100)% for all participants.

3.4. Acceptability of Dietary Standardisation

All participants (N = 23) completed an online survey on the acceptability of the standardised diet. In response to questioning whether participants had an interest in modifying their habitual dietary intake patterns, a majority responded in the affirmative. Specifically, 87% (N = 20) would modify or consider modifying, the quantity of energy and nutrient intakes of EOs; 83% (N = 19) the timing of EOs peri-resistance training; and 78% (N = 18) the distribution of EOs throughout the day.

Further analysis revealed that 45% (N = 10) of participants rated the prescribed meal plan as easy to follow and comply with. The most frequently cited reason for the ease of use was a similarity to their habitual dietary pattern (for detail see Figure 2).

In addition, 83% (N = 19) of individuals reported that the snack list was a useful addition to the proposed meal plan, which further facilitated adherence. The most frequently cited difficulty in maintaining compliance was participant engagement in activities of daily living and divergence of the time pattern of consumed EOs from habitual practices (Figure 2). Participants could provide more than one reason as to why the meal plan was easy or difficult to follow. One participant reported that the meal plan was both easy and difficult to follow, so this answer was excluded from the analysis. Despite an acknowledgement from 55% (N = 12) of participants that the standardised diet was difficult to follow, overall, participants demonstrated adherence to all components of the implemented meal plan.

4. Discussion

The complexity of a dietary standardisation approach is often overlooked by researchers and may not be specified when reporting nutritional intervention studies [4]. The aim of this research was to design and implement a dietary standardisation protocol and examine participants’ adherence and acceptability to the proposed approach. Habitual weekly dietary data were collected and analysed to construct and implement a standardised dietary intake, per day and EO, for use in a six-day nutrient plus exercise intervention. The six-day meal plan was provided in a ready-to-consume format to minimise the variability in quality, quantity, frequency, timing, and distribution of energy and nutrient intakes, within- and between- participants during the intervention. Daily and per EO energy and macronutrient prescription was made relative to participants’ body mass, considered their food preferences, and was based on the scientific evidence to support the adaptive outcome of resistance exercise, the context of the standardisation protocol. Dietary standardisation was particularly important in this six-day intervention because the dietary supplement under investigation contributed only 17% to the prescribed total daily protein intake. Nevertheless, the consumption of the dietary supplement influenced the frequency of EOs as well as energy and protein intake per EO in comparison to participants’ habitual practices.

Dietary standardisation is one of nine variables used to assess the translational potential of nutrition research [10]. According to Close et al. [10] the highest score for “Dietary and exercise controls” is awarded for “Dietary provision provided by researchers, exercise control, supported by relevant objective data and representative of real-world context”. Therefore, dietary standardisation should be considered as a matter of best practice in the design of nutrient plus exercise intervention. To be comprehensive, dietary standardisation requires engagement and close collaboration between participants, registered dietitians, sports scientists, and chefs. In this instance, participants were familiar with the purpose of the study and the importance of their engagement. Personalisation of the proposed meal plan and daily, in-person communication with participants resulted in high adherence. Provision of a high degree of detail was required by dedicated chefs to prepare each meal to participants’ specific characteristics (i.e. body mass, food allergies, and preferences). Furthermore, the implementation and management of dietary standardisation within this study necessitated daily supervision by a registered dietitian.

This research offers a bespoke dietary standardisation approach among a cohort of resistance-trained male individuals that is both evidence-based and informed by participants’ habits and preferences. Since it has been shown that quality, quantity, timing, and distribution of protein intake per EO may alter muscle protein synthesis [6,8,19,22,23,24], energy and macronutrient composition per EO was standardised relative to body mass together with timing and distribution of EOs. In literature, dietary standardisation protocols have been proposed in nutrient plus exercise research investigating the effect of carbohydrate periodisation on performance in endurance-trained individuals and elite endurance athletes [11,25,26,27]. Similar to this research, the reported dietary standardisation in literature is based on participants’ food preferences and is related to participants’ body mass [25,26,27] or lean tissue mass [11,12]. In studies by Marquet et al. [26] and Louis et al. [25], the dietary standardisation protocol introduced the habituation phase before the one-week intervention and participants were given precise dietary guidelines of food allowances to meet daily and per EO recommended carbohydrate intake. This protocol resulted in no statistically significant difference in daily carbohydrate intake (g·kg⁻¹·d⁻¹) between groups. Despite a change in protein intake within groups, there was no difference in protein intake between groups [25,26]. These research studies [25,26] demonstrate that dietary prescription and instruction provided to research participants might be a successful method in achieving a targeted daily carbohydrate intake and distribution. In the research study by Burke et al. [11], participants were assigned to one of three versions of isocaloric diets with different macronutrient ratios, to test the efficacy of a low-carbohydrate/high-fat diet on fat oxidation and performance during training in elite race walkers. Meal plans were individualised, prescribed per kilogram of lean tissue mass, and served within two grams of accuracy. Snacks were provided throughout each day and adherence to the prescribed diet was checked daily. As intended, there was no difference between daily energy (kJ·kg⁻¹·d⁻¹) and protein (g·kg⁻¹·d⁻¹) intake between groups; for further details refer to Mirtschin et al. [12], who reported the dietary standardisation protocol implemented in the research study by Burke et al. [11].

Therefore, dietary standardisation discussed in this research and other studies [11,25,26] demonstrates that prescription of energy and macronutrient intakes, and food and beverage provision according to an individual’s food preferences is an effective approach in achieving high participant compliance among trained individuals as well as elite athletes participating in research.

To the authors’ knowledge, this is the first intervention study where quality (source), quantity, timing, distribution, and frequency of energy and nutrient intakes per single EO were standardised and EOs were provided for six days. Additionally, EOs reflected the habitual dietary food preferences of research participants. The authors recognise that the cost of high-quality dietary control, daily collaborations with external food providers, and adherence to a study protocol by free-living individuals could be limitations of this approach over time. Besides this, the weighed seven-day habitual dietary intake record might be perceived as overly burdensome by participants when considering their training programmes and activities of daily living. On the other hand, the authors perceive that the acceptability of and adherence to the standardised diet by participants was increased by the familiarity of the EOs provided, since it was just a modification to their reported habitual intakes. However, a dietary standardisation protocol that is based on habitual food choices necessitates additional time before the study to deliver a diet plan tailored for individuals’ needs. Despite high adherence to the meal plans provided, 55% (N = 12) of participants reported that the prescribed dietary programme was difficult to follow. Activities of daily living were identified as the main challenge (29%, 10 of 35 responses) in following the dietary programme for six days. Other researchers have also found that a busy lifestyle may be a barrier in following recommended nutrient intakes among athletes [28].

In conclusion, this research study provides detailed information regarding derivation, delivery, and assessment of a well-controlled and rigorous dietary standardisation for short-term intervention studies (~7 days) and complies with best practice guidelines [4]. Additionally, this paper describes participants’ adherence to provided EOs and participants’ reflections on the delivered dietary programme. The six-day dietary standardisation was strengthened by the consideration of habitual food choices of resistance-trained individuals and was optimised by nutrient intakes based on dietary requirements peri-resistance exercise. The high adherence to the dietary standardisation limited the influence of the variability of nutrient intake patterns such as quality, quantity, timing, distribution, and frequency within and between the studied groups. The control of dietary intakes for research purposes allowed researchers to measure the dietary protein supplement’ efficacy on the rate of muscle protein synthesis, and to develop a better understanding of nutrient plus exercise intervention [16,17].

The authors propose that the implemented dietary standardisation may be an effective approach in minimising the inter- and intra-participant variability of dietary practices within nutrient plus exercise intervention studies. The application of a peri-exercise dietary standardisation in research design requires expertise and understanding of participants’ dietary intake and training patterns. The dietary standardisation here is based on the collaboration of registered dietitians with chefs and sports scientists, and might prove cost-effective if it is well planned and executed.

Further work should investigate adherence to the peri-exercise nutrition standardised dietary intake in different designs of nutrient plus exercise intervention studies in trained populations and athletes. Moreover, further research should examine adherence to dietary replication as opposed to a standardised diet prior to or during a research study. High adherence to dietary replication could provide a more affordable alternative than the standardised diet provided in this study.

Acknowledgements

The source of funding was Food Health Ireland (FHI) TC20130001. The authors acknowledge Dr Robert W Davies for participant recruitment and Zest! Ltd. (Ireland) as well as their chefs for the provision of meals according to the study requirements.

M.K., P.M.J., C.N. designed the dietary standardisation protocol. M.K. developed the meal plan and implemented the dietary standardisation protocol. M.K. collected and analysed data. M.K. wrote the manuscript. All authors reviewed, edited, and approved the final version of the manuscript submitted for publication.

The authors declare no conflict of interest. The funders had no role in the study design, collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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