The small study, with only 12 volunteers, challenges previous studies that showed no benefits, and highlights the potential benefits of amino acid supplements for this growing age group at risk of gradual decline in muscle mass, strength and function.
World population is expected to increase to 9.1bn by 2050 - up from 6.5bn in 2005; and 21 per cent of people will be over the age of 60 in 2050, compared to 10 per cent in 2000.
"Previous studies in elderly subjects have shown that provision of dietary supplements has not been effective in improving lean body mass.
However, ingestion of nutritionally balanced supplements has often been found to reduce the caloric intake of the rest of the food eaten in the day by an amount equivalent to the calories supplied in the supplement," explained the authors, led by Elisabet Borsheim from University of Texas Medical Branch.
"Therefore a dietary supplement in the elderly would be more appropriately considered as a dietary substitute."
The study is published online ahead of print in the journal Clinical Nutrition .
Borsheim and co-workers recruited seven female and five male glucose-intolerant subjects with an average age of 67 and assigned them to receive a daily amino acid supplement (22 grams) between meals for 16 weeks.
The supplement contained histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, valine, and arginine "The composition of the mixture of essential amino acids (EAA) was designed to be maximally effective in stimulating muscle protein synthesis in the elderly," explained the researchers.
After 12 weeks an increase in lean body mass (LBM) of 1.14 kg was recorded, while this decreased slightly to 0.6 kg after 16 weeks, compared to LBM measured at the start of the study (baseline).
Moreover, strength in the legs increased by an average of 22 per cent after 16 weeks, from a baseline value of 127.5 kg.
Borsheim also notes an improvement in measures of walking speed, including the usual gait speed, timed 5-step test, and timed floor-transfer test.
"Generally, inclusion of a placebo group is preferred.
However, we assumed that no improvement in lean body mass, muscle strength and physical function would have occurred over 16 weeks without intervention.
Thus, the individual pre-intervention data were used as control," stated the researchers.
"In summary, the results of the present study showed improvements of lean body mass, muscle strength and physical function in response to supplementation of the diet with EAA plus arginine in glucose intolerant elderly subjects," they added.
"We propose that over a more prolonged time, exercise will amplify the beneficial effects of EAA supplementation on lean body mass, strength, and muscle function in both healthy and insulin-resistant elderly.
This remains to be studied," they concluded.
The other researchers were affiliated with Shriners Hospitals for Children (Texas) and Ajinomoto's AminoScience Laboratories.
The study was funded by grants from the National Space Biological Research Institute (NSBRI), and Shriners, while Ajinomoto provided the amino acids.
State of the market The latest estimate from Frost and Sullivan, a consultancy, valued the European amino acids market at US$1357.7m (€1063.8m) in 2005.
The consultancy expects it to grow to around $1944.3m (€1523.4m) by 2012.
The world's two leading amino acid suppliers are Kyowa Hakko and Ajinomoto.
Source: Clinical Nutrition (Elsevier) Published online ahead of print 4 March 2008, doi: 10.1016/j.clnu.2008.01.001
"Effect of amino acid supplementation on muscle mass, strength and physical function in elderly" Authors: E. Borsheim, Q.-U.T. Bui, S. Tissier, H. Kobayashi, A.A. Ferrando, R.R. Wolfe