A tightly supervised 12-week strength-training program helped a small group of frail and pre-frail older women become stronger, move better, and gain lean mass. Adding isolated leucine to an already protein-adequate diet did not improve the results.
That is the useful tension in this randomized, double-blind trial: the exercise-and-protein foundation produced broad changes, while more of one amino acid did not add a measurable advantage.
What the researchers tested
The study enrolled community-dwelling women older than 65 who met at least one criterion in a modified version of the Fried frailty assessment. Of 304 women screened by telephone, 24 entered the study and 19 completed it. Their average age was 77.5 years and average BMI was 25.1.
All 19 women followed the same core program for 12 weeks:
- supervised resistance training three times per week on nonconsecutive days;
- a diet adjusted to provide about 1.2 grams of protein per kilogram of body weight per day; and
- either leucine or an active placebo at each main meal.
Ten women received 2.5 grams of leucine with breakfast, lunch, and dinner, totaling 7.5 grams per day. Nine received 1.7 grams of alanine at each meal. Alanine supplied an equivalent amount of nitrogen but is not thought to stimulate muscle protein synthesis independently, making it a more informative comparison than an empty placebo.
The women mixed each dose into 80–100 mL of water or a sugar-free drink and took it at the start of the meal. A dietitian helped them make small changes to their usual food intake while keeping energy intake stable. Food recalls and diaries were used to monitor protein and calorie intake.
The training was specific and progressive
This was not an instruction to “be more active.” Each session included four machine-based exercises:
- horizontal leg press;
- chest press;
- knee extension; and
- lat pulldown.
Participants completed three sets of 8–15 repetitions per exercise. Loads were maintained at roughly 60–80% of one-repetition maximum and increased by 1–5 pounds (0.45–2.27 kg) once a participant could perform 15 repetitions with proper technique. Each set lasted more than 35 seconds to discourage the use of momentum.
Attendance averaged 90.8%, and all 19 completers adhered to at least 80% of both the exercise and supplement protocols. That level of supervision and adherence matters when interpreting the result.
Frailty scores fell sharply
Across both supplement groups, the average number of frailty criteria fell from roughly 2.6–2.7 before training to 0.7 in the alanine group and 1.2 in the leucine group. The authors summarized the combined reduction as 64%.
The category changes make that result easier to picture. Nine women moved from frail to pre-frail, four moved from pre-frail to healthy, and two moved directly from frail to healthy. Three pre-frail participants stayed in the same category.
Walking speed improved in both groups. Average four-meter gait speed rose from 1.02 to 1.20 m/s with alanine and from 0.99 to 1.17 m/s with leucine. Timed up-and-go performance also improved: from 10.3 to 9.1 seconds and from 10.6 to 8.9 seconds, respectively.
The Short Physical Performance Battery score increased from 10.0 to 11.4 in the alanine group and from 9.9 to 11.2 in the leucine group. In a five-chair-stand test, completion time fell from 13.2 to 11.1 seconds and from 13.1 to 10.6 seconds.
These were changes over time in the full trained cohort. There was no consistent advantage for leucine.
Strength improved across all four exercises
One-repetition maximum increased significantly for the leg press, chest press, knee extension, and lat pulldown.
For example, average leg-press strength rose from 75.4 to 99.8 kg in the alanine group and from 71.2 to 96.6 kg in the leucine group. Chest press increased from 24.0 to 29.5 kg and from 23.1 to 28.6 kg, respectively.
Participants also completed more chair stands and arm curls in 30 seconds and improved several flexibility and mobility tests. Again, the statistical signal was for training over time, not for an added leucine effect.
Handgrip strength was an exception to the otherwise parallel pattern. It increased in the alanine group, from 19.2 to 22.6 kg, but did not increase in the leucine group, where it changed from 22.7 to 21.7 kg. The trial was too small to treat this isolated interaction as evidence that leucine impaired grip strength.
Lean mass and muscle fibers changed
DXA measurements showed an average whole-body lean-mass gain of about 750 grams, or 2%, across the study. In the group-level table, lean mass increased from 38.1 to 38.9 kg with alanine and from 35.2 to 35.9 kg with leucine.
Body-fat percentage fell modestly, while total body weight did not change significantly. Appendicular lean mass index—the lean mass in the arms and legs relative to height—did not change significantly.
Muscle biopsies added a more direct view. The cross-sectional area of type I muscle fibers increased by 16%, and type IIa fibers increased by 28%. Type IIx fiber area did not change significantly.
The researchers also used a stable-isotope tracer to measure myofibrillar protein synthesis. Fasting, or basal, synthesis increased by 47% after the program. The protein-synthesis response after a meal did not increase further, possibly because the protein-optimized test meal was already a strong anabolic stimulus.
Why leucine may not have added anything
Leucine is an essential amino acid and an important signal for muscle protein synthesis. The trial does not show that leucine is biologically irrelevant. It shows that 2.5 grams at each meal did not add measurable benefit in this particular setting.
Every participant was already receiving two potent stimuli: progressive resistance training and about 1.2 g/kg/day of dietary protein. The study meal used for the tracer testing also provided 0.67 g of protein per kilogram of lean body mass, plus the assigned supplement. The authors suggest that the protein dose may have left little room for an extra leucine effect.
This distinction is practical. The result cannot tell us whether leucine might help an older adult whose total protein intake is low, whose meals contain little high-quality protein, or who cannot train. It only tests leucine on top of a well-supported training program and an optimized protein intake.
Important limits
This was a mechanistically detailed but very small trial: only 19 women completed it, with nine in one group and ten in the other. The small sample limits precision and makes subgroup findings fragile.
The participants were community-dwelling older women. The result may not apply to men, hospitalized or institutionalized adults, people with more severe frailty, or people with confirmed sarcopenia. Five of the 24 enrolled participants withdrew, although the reported reasons were illness, injury, moving, or difficulty adhering rather than adverse effects attributed to the intervention.
There was no non-exercise control group. The study can compare leucine with alanine, but it cannot cleanly separate the contribution of resistance training from that of raising protein intake because everyone received both.
The researchers measured muscle protein synthesis but not muscle protein breakdown, so they could not calculate net protein balance. They used DXA and bioelectrical impedance for body composition rather than MRI or the D3-creatine method, and some biopsy analyses had fewer usable samples because of technical issues.
Finally, “reversed frailty” describes movement across this study’s modified Fried categories after 12 weeks. It should not be read as proof of a permanent cure. Longer follow-up would be needed to know whether the improvements persisted.
The practical takeaway
For these frail and pre-frail older women, three well-supervised resistance sessions per week—built around four basic machine exercises and progressively heavier loads—coincided with meaningful gains in strength, mobility, lean mass, and muscle biology.
The extra leucine did not improve those outcomes when daily protein intake was already about 1.2 g/kg. The study therefore supports getting the larger pieces in place before assuming that an isolated amino-acid supplement will add more.
The trial was funded by the Montreal General Hospital Foundation. Two researchers were supported by doctoral scholarships from the Fonds de recherche du Québec – Santé. The authors reported that the funders had no role in the design, data collection, analysis, interpretation, or manuscript preparation, and they declared no competing financial or nonfinancial interests.
This article is for general educational purposes and is not medical advice. Frailty, kidney disease, and major changes to protein intake or exercise warrant individualized guidance from a qualified health professional.