Can Supplemental Leucine Offset Disuse-induced Muscle Atrophy?

It is well established that skeletal muscle mass progressively declines with healthy ageing, a phenomenon commonly termed sarcopenia. Overtime, this leads to a loss in functional independence and provides an increased risk of developing a co-morbidity. Recent estimates suggest that 30% of those aged 75-84 years suffer from sarcopenia. As the 85-and-over population are rapidly expanding, sarcopenia places a considerable socio-economic burden to healthcare services. From a physiological perspective, the maintenance of muscle mass is dependent on a fine balance between muscle protein synthesis (MPS) and breakdown (MPB) rates. In old age, the MPS response to nutrition is impaired (termed 'anabolic resistance'), leaving muscle in a chronic catabolic state and ultimately underpinning the progression of sarcopenia.

Age-associated muscle anabolic resistance may stem from an accumulation of periods of inactivity/disuse, which are strongly associated with a decline in muscle mass (atrophy) in both the young and old. In fact, the associated loss of muscle mass is the most rapid at the onset of muscle disuse, with significant declines in muscle mass occurring after just 5 days of disuse. This is of particular importance since the average length of hospitalization in older individuals in 5-6 days. Disuse-induced muscle deconditioning is attributed to declines in MPS in response to nutrition, and potentially an elevated MPB. Therefore, an accumulation of these periods of disuse over a lifetime may lead to an acceleration in the trajectory of sarcopenia that is commonly seen with ageing. Consequently, nutritional strategies to maximize healthy musculoskeletal ageing should focus on alleviating muscle metabolic dysregulation during short-term disuse.

One approach to counteract muscle deterioration during short-term disuse is to increase dietary protein consumption. In fact, adequate protein intake is a necessity for muscle to remain in a positive net protein balance, with the importance of this nutritional intervention increasing during periods of disuse. However, this approach is not always feasible as activity levels are more limited during periods of disuse and individuals are thus less likely to consume adequate nutritional intake. A more feasible approach may be to enhance the anabolic potency of sub-optimal protein doses through supplementation with the amino acid, leucine. Leucine is unique in its ability to increase the protein balance more so than any other amino acid through stimulation of MPS and suppression of MPB. Leucine provides a strong anti-catabolic effect during hind limb immobilization in rats, which has yet to be examined in humans. Therefore, it is clear that leucine represents a viable strategy to offset the dysregulation of MPS and MPB and preserve musculoskeletal health during short-term disuse in both young and old individuals and thus warrants further investigation.

Participants:

Twenty-four healthy (non-obese, non-diabetic, non-smokers) males aged 18-35 years will be recruited to participate in the study. The current study will be a placebo controlled double-blind study with two intervention groups, in which, participants will be randomly assigned to receive supplemental leucine (LEU; n=12) or placebo (PLA; n=12) during 7 days of unilateral leg immobilization. Participants will be active, but athletically untrained (i.e. exercise ≤3x/week). All study procedures will be clearly explained and participants will provide written informed consent prior to obtaining baseline measures.

Enrolment:

Participants will be invited to the University of Birmingham's School of Sport, Exercise and Rehabilitation Sciences. Upon arrival, a researcher involved in the trial will explain the study design and interventions to the participant. Participants will be given the opportunity to ask any questions related to the study and will be given time to decide whether or not to take part. If the participant is happy to take part the participant will be asked to sign a consent form and fill in a health questionnaire. When the participant has left, the participant will be categorized at random to the leucine or placebo group. Participants will be asked to fill in a three-day diet diary to assess habitual dietary intake.

Preliminary assessments:

Following explanation of the study and the acquisition of informed consent, participants will report to the laboratory at 0800-0900h having fasted for 10 hours and refrained from vigorous exercise and alcohol for 24 hours previously. Participants baseline muscle function will be acquired through measures of maximal isometric and isokinetic strength of the knee extensor and flexor muscles using an isokinetic dynamometer. Following strength testing the investigators will provide individuals with a three-day diet diary and activity monitor. Three days following the baseline strength measures, participants will once again report to the laboratory at 0800-0900h following a 10h overnight fast. Participants will be weighed on a digital scale to the nearest 0.1kg in light clothing. Dual x-ray absorptiometry (DXA) and ultrasound scans will be conducted to determine the composition of the thigh (fat and fat free mass).

Immobilization:

At 0800-0900h on the morning following preliminary assessments, participants will undergo 7 days of unilateral leg immobilization. A single leg will be randomly selected (counterbalanced left/right) and placed in a full-leg knee brace. Participants will ambulate using crutches and perform daily ankle exercises to minimize the risk of deep vein thrombosis, as indicated by a qualified physiotherapist. Participants will also be permitted to remove the knee brace during overnight sleep. A trained physiotherapist will instruct the participant on the safe use of crutches (i.e climbing stairs). During each main meal in the immobilization period, participants will consume 5 g of a powdered LEU or PLA supplement with 250-300 ml of water, which is within the safe limits. Dietary intake will be controlled throughout the immobilization period according to the individuals total calorie intake (derived from standardized equations), with a macro-nutrient composition of 55% carbohydrate, 30% fat and 15% protein. The total quantity of protein ingestion will equate to 1.0g/kg/day. Participant's activity levels will be monitored throughout the 7 day period through a wrist-worn activity monitor.

Experimental trials:

On the morning marking the end of the 7 d immobilization period participants will report to the laboratory at 0600-0700h following an overnight fast. Ultrasound and DXA assessments will be repeated, immediately after which a catheter will be inserted into a forearm vein of both arms for i) frequent blood sampling at -150, -90, 0, 20, 40, 60, 80, 120, 180, 240-minutes of the experimental trial (∼100mL in total) and ii) a continuous infusion of a stable isotope amino acid tracer (L- [ring] 13C6 phenylalanine). Participants will remain in a supine position throughout the trial. After 150 min of infusion, a muscle biopsy will be obtained from the quadriceps muscle of immobilized and non-immobilized legs under local anaesthetic. Participants will then consume a 20g milk protein beverage. Further biopsies will be obtained from both legs after 270-min and 390-mins of infusion. Thus, a total of 6 invasive muscle biopsies will be obtained during the trial (3 from each leg), with each biopsy obtained from a separate incision spaced ∼3cm apart. This two- stage stable-isotope infusion design allows efficient assessment of MPS in the transition from post-absorptive (0-120 min) to postprandial conditions (120-240 min). Leg strength will be reassessed a day following the stable isotope infusion at 0800-0900h following a 10 hr fast to avoid any interference effect of prior contraction on muscle protein turnover.

Data Analysis:

To calculate myofibrillar and mitochondrial protein synthesis, the investigators will adopt typical sophisticated mass spectrometry techniques to determine isotopic tracer enrichment in biopsy-isolated muscle proteins and plasma. Intramuscular "anabolic signals" (in the mTORC1 pathway) and "catabolic" signals will be determined via western blot and q-rtPCR analysis. High resolution respirometry will also be used to assess the mitochondrial function of biopsy-isolated skeletal muscle tissue in both the control and immobilized limbs.