The Metabolic Effects of β-hydroxybutyrate on Working Skeletal Muscle

The goal of this clinical trial is to test ketone bodies in healthy elderly and young individuals. The main question it aims to answer are:

• Do ketone bodies improve skeletal muscle function?

Participants will ingest a ketone monoester and skeletal muscle function will then be evaluated by:

* Special magnetic imaging techniques
* Intravenous infusion of tracer-marked nutrients
* Performance tests on a ergometer bike and in a dynamometer

Researchers will compare the outcomes between within the young and elderly groups and between the young and the elderly group to investigate if age has an effect on the outcomes.

BACKGROUND With ageing, skeletal muscles metabolism changes and muscle function declines. This may lead to muscle weakness and increased risk of developing metabolic diseases. Ketone bodies, namely 3-hydroxybutyrate (3-OHB), is an energy substrate that may change the metabolism and improve efficiency of skeletal muscles in a setting of ageing.

OBJECTIVE The study aims to investigate the effects of beta-hydroxybutyrate ingested as a monoester on skeletal muscle function and metabolism during muscle work in young and elderly individuals.

DESIGN Healthy young (20-25 years) and elderly (65-85 years) untrained males will be paired based on age corrected VO2-max. Participants will be evaluated in a double blinded cross-over design on two study days: One day with ketone ester ingestion (D-beta-hydroxybutyrate/D-1,3-butanediol; KetoneAid Pro KE4), one day with ingestion of a volume and calorie and taste matched placebo (lipid emulsion). Blood ketone levels will be kept elevated through a sipping protocol. During both conditions a low glucose dose will be continuously infused to block physiological ketogenesis.

The order of the study days will be randomized and interspaced by at least 4 weeks.

On experimental days, participants meet fasted to perform voluntary contractions with tibialis anterior muscles in a MR compatible dynamometer while oxidative capacity, ATP generation, intramuscular pH, fatiguability and work efficiency is evaluated through 31P-MR spectroscopy.

After, participants will bike at a fixed intensity (\~45% of Wmax) while lipid fluxes and glucose oxidation rates are measured by palmitate- and glucose tracer infusions and carbamide-corrected indirect calorimetry over 60 min. This was initially intended for both young and elderly individuals, but only involves the young group, as the elderly group had difficulties completing 60 min cycling.

Lastly on the study days, participants will perform a cycling test as a measure of performance starting at 70 % of Wmax for 5 min thereafter increasing by 10 % of Wmax every 1 min until exhaustion. Muscle biopsies are obtained before (both groups) and just following fixed intensity cycle work (only young group). Adipose tissue biopsies are collected at the beginning of the experimental day (both groups) before ketone/placebo ingestion and after the constant load cycle work (only young group). Blood samples are performed throughout the day to assess substrate levels, hormones and for proteomics analysis.

Before each study day, participant's activity level is measured for 7 days by accelerometry (ActiGraph). Participants will log their diet 3 days prior to each experimental day. Participants are asked to ingest a similar diet 2 days before both experimental days and to withstand from strenuous exercise 2 days prior to both experimental days. Participants are asked not to make any significant changes to their lifestyle while taking part in the study.

Eligibility Criteria:
Inclusion Criteria:

* Male 20-25 years old (n = 12) OR male 65-85 years old (n = 12)
* BMI range: 19 to 27
* Stable weight (\< 5% change over last 6 months)
* Less than 3 x 60 min of structured exercise per week.

Exclusion Criteria:

* Medication that affect energy metabolism.
* Non-MR-compatible metals or electric devices in the body.
* Anaemia or bleeding disorders.
* Heart, lung or other disease that affects the subjects ability to exercise.
* Smoking.
* Drug abuse.
* Lack of compliance.
* Known allergy towards local anaesthetics.
* Any condition that the principal investigator considers unsuitable for the subject's ability to complete the study.