Nutrient Timing in Connection to Evening Exercise

The aim of this study is to evaluate how pre- or post-evening exercise carbohydrate (CHO) ingestion influences glucose metabolism and substrate oxidation (fat/CHO) during exercise and after exercise in athletes during the nocturnal period and the morning after during an oral glucose tolerance test (OGTT).

The primary objective is to assess glucose regulation during a 120-minute OGTT the following day.

The secondary objective is to investigate substrate metabolism (CHO and fat oxidation) during a 120-minute OGTT the following day.

Ten healthy, well-trained male endurance cyclists/triathletes, aged 19-45 years, are recruited via local contacts from the Gothenburg region, Sweden. The study is performed in accordance with the Declaration of Helsinki 2008 and is approved by the Swedish Ethical Review Authority (Dnr: 2022-03056-01). All participants gave their written informed consent before inclusion.

The study has a double-blind, randomized, placebo-controlled, crossover design.

After baseline assessment and determination of maximal oxygen uptake (VO2 max), participants undergo a familiarization cycle exercise trial. This trial serves a dual purpose: first, to establish the appropriate amount of CHO to be incorporated into the pre- or post-drink for subsequent cycle experimental trials, and second, to familiarize participants with the equipment and study protocol.

Three days prior to the cycle experimental trial, subcutaneous insertion of a glucose sensor for continuous glucose monitoring (CGM) takes place. Participants are randomly assigned to consume either a CHO drink or a placebo (PLA) drink (containing no energy) two hours prior to the cycle experimental trial that starts at 19:30. During the exercise trial, participants engage in submaximal cycling exercise at 70% of their maximal workload (Wmax), followed by a 25-minute time trial (TT). The total duration of the exercise session is approximately 90 minutes. Workload during the TT is individualized based on each participant's maximal work capacity (Wmax). During exercise, capillary glucose levels, heart rate, and rate of perceived exertion (RPE; Borg category scale 6-20) are measured. Additionally, respiratory gases are collected during 5-minute periods at every 15-minute interval to assess substrate utilization.

Immediately after completing the TT, participants switch to the opposite drink condition: those who initially received the CHO drink consume the PLA drink, and vice versa. The amount of CHO in the CHO drink consumed by each participant is precisely matched to the amount of CHO that the participant oxidizes during the exercise trial.

After the ingestion of the CHO- or placebo drink, participants fast overnight. CGM technology is used to measure interstitial glucose values during the night.

At 08:00, participants consume a 300mL solution containing 75g glucose for a 120-min OGTT. Capillary blood samples are collected at baseline and every 15 minutes during the OGTT. Respiratory gases are collected to measure substrate oxidation. The same protocol is reproduced the following week, with the reverse order of the test drink intake.

Three days preceding both control and experimental trials, participants are provided with a standardized diet consisting of pre-packaged meals that match their daily energy and macronutrient needs to prevent energy deficiency.

Inclusion criteria

• Females and males
• 18-50 years
• Endurance fitness level of ≥40 mL O2/kg/min.
• Signed informed consent

Exclusion criteria

• Medical treatment and/or nutritional supplements that may affect glucose tolerance, substrate oxidation and performance capacity