EAL

NAP: Training (2014)

Citation:

Hamzah S, Higgins S, Abraham T, Taylor P, Vizbaraite D, Malkova D. The effect of glycaemic index of high carbohydrate diets consumed over five days on exercise energy metabolism and running capacity in males. J Sports Sci. 2009; 27(14): 1,545–1,554.

Study Design:

Randomized Crossover Trial

Class:

A - Click here for explanation of classification scheme.

Quality Rating:

NEUTRAL: See Quality Criteria Checklist below.

Research Purpose:

To investigate the impact of consuming a high carbohydrate diet with either a low or high glycemic index for five days on energy substrate utilization during running conducted in a fasted state
To determine if the glycemic index of high carbohydrate diets has an impact on exercise capacity.

Inclusion Criteria:

Involved in regular endurance training for at least four years
Not a vegetarian or a smoker
No diagnosed cardiovascular or metabolic disease
Not consuming medication or drugs known to influence lipid or carbohydrate metabolism.

Exclusion Criteria:

Not stated.

Description of Study Protocol:

Design

Nine males participated in this randomized, crossover trial. The subjects performed three treadmill runs to exhaustion at 65% VO_2max, separated by a washout period of at least 11 days:
- After following their habitual diet (control)
- After five days on a high-carbohydrate, high-glycemic index diet (HC-HGI)
- After five days on high-carbohydrate, low-glycemic index diet (HC-LGI).
Each treadmill run was done after a 12-hour overnight fast. Participants rested in a seated position for 10 minutes before a blood sample and expired air sample were collected, then had a five-minute warm-up of continuous running at 60% VO_2max and five minutes of stretching. Following the warm-up and stretch, participants ran to exhaustion at 65% VO_2max. Participants were allowed to drink water during the trials and were cooled by an electric fan, and strong verbal encouragement was given throughout the runs. Participants signaled when they could only complete two more minutes of running; then an expired air sample, heart rate and a blood sample were immediately measured.
Every 15 minutes during each run, blood samples and expired air samples were collected, and heart rate (HR) and rating of perceived exertion (RPE) were monitored. For five days prior to the first treadmill run, subjects recorded all planned and structured exercise, and were asked to replicate this before their second and third runs.

Intervention

For each five-day prescribed diet period (HC-HGI and HC-LGI), subjects were provided with all the food and menus describing the exact amounts of each food required to consume and when during the trials, along with digital food scales and written instructions explaining how to cook and prepare prescribed foods. Any deviations from these instructions were recorded by the participants. For the control diet, subjects completed a five-day weighed food diary using the digital scales provided and were asked to follow their usual diet during this period.
Subjects followed each of the following three prescribed diets for five days prior to a treadmill run to exhaustion:
- Control diet: The subjects' habitual diet and was recorded on a five-day food diary
- HC-HGI: Prescribed and with provided food and menus
- HC-LGI: Prescribed and with provided food and menus.
The main sources of carbohydrate in the HC-HGI diet were refined breakfast cereals, white rice, wholemeal bread, instant mashed potatoes, biscuits and Lucozade
The HC-HGI diet provided 511±11g CHO, 7.4±0.2g CHO per kg per day, GI of 71±1, glycemic load (GL) of 391±9, 71% energy from CHO, 15% energy from fat and 14% energy from PRO
The HC-LGI diet provided 508±12g CHO, 7.3 ±0.2g CHO per kg per day, GI of 36±0, GL of 216 ±6, 70% energy from CHO, 15% energy from fat and 15% energy from PRO
- The main sources of carbohydrate in the HC-LGI diet were All Bran cereal, porridge, pasta, basmati rice, rye bread, oatcakes and apple juice.
Control diet provided 349±27g CHO, 4.6±0.5g CHO per kg per day, GI of 56±1, GL of 171,±17, 53% energy from CHO, 26% energy from fat, and 18 % energy from PRO.

Statistical Analysis

Responses during the exercise period were compared by a two-factor (trial x time) repeated measures ANOVA, with Tukey post hoc tests used to locate the differences
Differences between baseline values and those at the point of exhaustion, and between-dietary profiles were compared by one-factor (trial) ANOVA.

Data Collection Summary:

Timing of Measurements

Before each running trial, a baseline blood sample and expired air sample was collected. During the trial, subjects ran to exhaustion; the last minute of every 15-minute period of the trial an expired air sample and blood sample were collected and HR and RPE were monitored. When a subject signaled he could only manage a further two minutes, expired air and blood samples were collected and HR was recorded.

Dependent Variables

Fat oxidation: Measured by expired air samples
Carbohydrate oxidation: Measured by expired air samples
Plasma insulin: Measured by blood samples
Glucose: Measured by blood samples
Non-esterified fatty acids: Measured by blood samples
Glycerol: Measured by blood samples
Time to exhaustion: Measured in running trial.

Independent Variables

Subjects followed each of the following three diets for five days prior to each running trial:

Control (habitual diet)
HC-HGI
HC-LGI.

Control Variables

Subjects recorded all physical activity for five days prior to the first treadmill run; this activity was repeated before runs two and three
For two days prior to each running trial, subjects were asked to limit activity to activities of daily living and slow walking or cycling for transport over short distances, as well as to avoid alcohol consumption
All fluid intake was recorded for 24 hours prior to the first treadmill run; this was replicated before runs two and three
Subjects were instructed to consume approximately one liter of water the night before and about 500ml in the morning before each running trial
All running trials were performed under similar experimental and environmental conditions.

Description of Actual Data Sample:

Initial N: Nine males
Attrition (final N): Nine.
Age: 23.9±4.3 years
Other relevant demographics: Mean VO_2maxwas 59.8ml per kg^-1per minute^-1 (SD±4.3).

Anthropometrics

Weight: 71.6±8.2kg
Height: 1.79±0.07m
Body mass index: 22.3±2.0kg per m^-2.

Location

Glasgow, United Kingdom.

Summary of Results:

Key Findings

The extent to which high-carbohydrate diets consumed for five days reduce the rate of fat oxidation during running in the fasted state is not influenced by the glycemic index of the diet
Concentrations of plasma glucose, insulin, non-esterified fatty acids and glycerol measured in the fasted state before running, during the run to exhaustion and at the point of exhaustion were not significantly different between the HC-HGI and HC-LGI trials
At exhaustion, plasma glucose concentration in the HC-LGI trial tended (P=0.06) to be higher than in the control trial, but glucose concentration was similar between the HC-HGI and HC-LGI trials
The rates of fat and carbohydrate oxidation in the fasted state before running, during the run to exhaustion and at the point of exhaustion were not significantly different between the HC-HGI and HC-LGI trials
During the run to exhaustion, the rate of fat oxidation in both experimental trials was significantly lower (P<0.05), and the rate of carbohydrate oxidation higher (P<0.05) than in the control trial
At the point of exhaustion, compared with the control trial the rate of fat oxidation was lower (P<0.05) and the rate of carbohydrate oxidation higher (P<0.05) only in the HC-LGI trial
There were no differences in HR, RPE or oxygen consumption during the run to exhaustion and at the point of exhaustion between trials
Time to exhaustion and distance covered by subjects were not significantly different between trials.

Author Conclusion:

The extent to which a high-carbohydrate diet consumed for five days reduces the rate of fat oxidation and increases the rate of carbohydrate oxidation during subsequent running in the fasted state is not influenced by the glycemic index. The findings suggest that when high-carbohydrate diets are consumed for three to five days leading up to an endurance event, consideration of the glycemic index is not necessary.

Funding Source:

Other:

Not specified.

Reviewer Comments:

Quality Criteria Checklist: Primary Research
Relevance Questions
	1.	Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies)	Yes
	2.	Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about?	Yes
	3.	Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice?	Yes
	4.	Is the intervention or procedure feasible? (NA for some epidemiological studies)	Yes

Validity Questions
1.	Was the research question clearly stated?		Yes
	1.1.	Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified?	Yes
	1.2.	Was (were) the outcome(s) [dependent variable(s)] clearly indicated?	Yes
	1.3.	Were the target population and setting specified?	Yes
2.	Was the selection of study subjects/patients free from bias?		???
	2.1.	Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study?	Yes
	2.2.	Were criteria applied equally to all study groups?	???
	2.3.	Were health, demographics, and other characteristics of subjects described?	Yes
	2.4.	Were the subjects/patients a representative sample of the relevant population?	???
3.	Were study groups comparable?		Yes
	3.1.	Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT)	N/A
	3.2.	Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline?	Yes
	3.3.	Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.)	Yes
	3.4.	If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis?	N/A
	3.5.	If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.)	N/A
	3.6.	If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")?	N/A
4.	Was method of handling withdrawals described?		Yes
	4.1.	Were follow-up methods described and the same for all groups?	Yes
	4.2.	Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.)	Yes
	4.3.	Were all enrolled subjects/patients (in the original sample) accounted for?	Yes
	4.4.	Were reasons for withdrawals similar across groups?	Yes
	4.5.	If diagnostic test, was decision to perform reference test not dependent on results of test under study?	N/A
5.	Was blinding used to prevent introduction of bias?		N/A
	5.1.	In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?	N/A
	5.2.	Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.)	N/A
	5.3.	In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded?	N/A
	5.4.	In case control study, was case definition explicit and case ascertainment not influenced by exposure status?	N/A
	5.5.	In diagnostic study, were test results blinded to patient history and other test results?	N/A
6.	Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described?		Yes
	6.1.	In RCT or other intervention trial, were protocols described for all regimens studied?	Yes
	6.2.	In observational study, were interventions, study settings, and clinicians/provider described?	N/A
	6.3.	Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect?	Yes
	6.4.	Was the amount of exposure and, if relevant, subject/patient compliance measured?	Yes
	6.5.	Were co-interventions (e.g., ancillary treatments, other therapies) described?	Yes
	6.6.	Were extra or unplanned treatments described?	N/A
	6.7.	Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups?	Yes
	6.8.	In diagnostic study, were details of test administration and replication sufficient?	N/A
7.	Were outcomes clearly defined and the measurements valid and reliable?		Yes
	7.1.	Were primary and secondary endpoints described and relevant to the question?	Yes
	7.2.	Were nutrition measures appropriate to question and outcomes of concern?	Yes
	7.3.	Was the period of follow-up long enough for important outcome(s) to occur?	Yes
	7.4.	Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures?	Yes
	7.5.	Was the measurement of effect at an appropriate level of precision?	Yes
	7.6.	Were other factors accounted for (measured) that could affect outcomes?	Yes
	7.7.	Were the measurements conducted consistently across groups?	Yes
8.	Was the statistical analysis appropriate for the study design and type of outcome indicators?		Yes
	8.1.	Were statistical analyses adequately described and the results reported appropriately?	Yes
	8.2.	Were correct statistical tests used and assumptions of test not violated?	Yes
	8.3.	Were statistics reported with levels of significance and/or confidence intervals?	Yes
	8.4.	Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)?	Yes
	8.5.	Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)?	Yes
	8.6.	Was clinical significance as well as statistical significance reported?	Yes
	8.7.	If negative findings, was a power calculation reported to address type 2 error?	No
9.	Are conclusions supported by results with biases and limitations taken into consideration?		Yes
	9.1.	Is there a discussion of findings?	Yes
	9.2.	Are biases and study limitations identified and discussed?	Yes
10.	Is bias due to study's funding or sponsorship unlikely?		???
	10.1.	Were sources of funding and investigators' affiliations described?	No
	10.2.	Was the study free from apparent conflict of interest?	???