Reflecting higher health consciousness and an ever-rising number of fitness enthusiasts, sports drinks containing amino acids are continuing to enjoy brisk sales in Japan.

Professional athletes have long been taking amino acid supplements to boost muscle strength. But an increasing number of general consumers are ingesting them to lessen fatigue, maintain health and even lose weight.

Initially sold at health food stores, products containing amino acids are now widely available in the form of drinks, jellies, powders and pills, and can be purchased at general retailers or common vending machines. Cosmetics makers have jumped on the "amino acid fad" as well, touting them as the main ingredients in their beauty products.

Amino acids are the building blocks of protein. According to Otsuka Pharmaceuticals Co., there are 20 known amino acids, but nine that cannot be synthesized by the human body. These must be supplied externally and are called the "essential amino acids."

According to the Japan Soft Drinks Association, sports drinks were originally developed in the United States in the 1960s so football players could quickly replenish themselves with water and electrolytes, such as ionized natrium and calium, lost during heavy exercise.

In Japan, the market for sports drinks took off in 1980 when Otsuka Pharmaceuticals launched Pokari Sweat, a cloudy, light-tasting drink containing sodium and potassium ions. Despite the odd name, Pokari Sweat turned out to be a success and was followed three years later by Coca-Cola Japan's Aquarius. The two have dominated the market ever since.

Amino acid drinks were pioneered by Ajinomoto Co., which launched Amino Vital Pro in March 1995, which was followed by Meiji Dailies Corp.'s "VAAM" in September the same year. The current boom, however, was triggered much later by the launch of Amino Supli in February 2002. The drink, made by Kirin Beverage Co., soon became a hit.

Pressed by the surging popularity of these drinks, Coca-Cola Japan relaunched "Aquarius" in March this year, playing up amino acid as its key ingredients.

Industry sources estimate that the 330 billion yen sports drinks market, including the fast-growing sector for amino acid drinks, will soon account for about 10 percent of the 3.7 trillion yen soft-drinks market. If sales of amino acid drinks meet expectations and expand to 90 billion yen this year, the upstarts will surpass oolong tea and mineral water and be second only to coffee, the sources said.

The latest entrant into the booming amino acid drinks market has been Otsuka Pharmaceuticals, whose Pokari Sweat still has the largest share of the soft-drinks market. In September, Otsuka launched its own drink, called Amino-Value.

A marketing official at Otsuka claimed Amino-Value contains 2,000 mg of what are called branched-chain amino acids -- valine, leucine and isoleucine -- and 500 mg of arginine per 500-ml bottle.

BCAAs are metabolized by the muscles during exercise, the official said, suggesting that consuming them before, during or after exercise will inhibit protein breakdown, repair damaged muscle fibers, and increase strength and stamina.

"We are not a newcomer or a late-comer," another Otsuka official said. "We have just come to provide what deserves the name of an amino-acid drink in the true sense of the words."