Testosterone: Can it make you live longer?
Bombarded with adverts promising a longer, healthier life, BBC News Los Angeles correspondent Peter Bowes goes in search of eternal youth.
Dr Jeffry Life, a Las Vegas-based age management doctor, is 74. He has the body of a man half his age. In fact he has a level of muscular definition that many men never achieve.
Genial, soft spoken, bald and lean, Life, who practised family medicine for much of his career, is a poster child for the longevity business.
He was in his late 50s when he realised that as a paunchy middle-aged man, he could drop dead at any time.
"I had really got out of shape, got a lot of body fat and was heading down a disastrous course of diabetes and heart disease," he says.
Inspired by a muscle magazine that someone left in his examination room, Life decided to get in shape. He started an intensive regime of working out at the gym and within a year transformed his body.
In 1998 he entered a competition, the Body for Life contest, for people who have made dramatic changes to their physique. To his surprise he won first prize in the section for 55-year-olds and older.
"It changed my life and I felt great," he says.
But as he got older he says he noticed that he was losing ground and finding it more difficult to maintain a lean body. While in his 60s, he visited Las Vegas for a medical conference and was introduced to Cenegenics, an age management company that aspired to make its patients "look and feel years younger". He went on to become a senior partner.
Life believes that the right kind of exercise and nutrition are important - but correcting hormone deficiencies are the key to his success.
"I got my blood checked and I found that I was profoundly deficient in testosterone," he says.
He now has weekly injections of the hormone.
"The problem is that when a man's testosterone gets low they lose their incentive to go to the gym.
"Even if they want to, they dread going to the gym and exercising and it's a losing battle. Especially abdominal fat around their belly. They spiral out of control.
"A lot of men come to me who are suffering from male-menopause, known as andropause, which creeps into their lives," he adds.
Andropause, says Life, is characterised by a decline in a man's sexual function, cognitive ability, an inability to get rid of body fat and fatigue.
He adds: "When I get their testosterone up to a healthy level, it changes everything. They get re-energised, they start seeing body fat disappear and muscle growing."
Different laboratories vary in what they consider to be a normal testosterone level. According to the Facey Medical Group, a health provider California, the normal reference range is 250 to 1100ng/dl. The Mayo Clinic says testosterone levels drop, within this range, by about 1% a year after the age of 30.
Andropause is not universally accepted by the medical profession as a definable condition for middle-aged and older men, although there has been a huge increase in the number of prescriptions for testosterone in recent years. As well as injections, gels are available and are widely advertised. Last year the British Medical Journal published a study which concluded that "many men in the UK are receiving unnecessary testosterone replacement".
It is an area of medicine that is subject to much debate and often focuses on the question of what is "normal" ageing.
Life argues that the medical profession is too conservative.
"Many doctors get the numbers and then they will tell their patient, 'Joe, your levels are normal', when in fact it is within the reference range, but it's at the bottom of the range. They don't tell you you're a D-student," he says.
Sex hormones - oestrogen in woman and testosterone in men - fulfil many roles in the body. Studies have shown that they are involved in age-related changes, such as the development of dementia.
Testosterone is fantastic," says Dr Christian Pike, a neurobiologist at the University of Southern California, who specialises in the brain and Alzheimer's disease.
"It increases aspects of cognition, it protects the brain from dying, it reduces Alzheimer's disease, I mean, it's wonderful," he says.
But Dr Pike has reservations about the recent trend towards an increased use of sex hormones to reverse the effects of ageing. He says further long-term research is needed to fully understand the way testosterone affects the body.
"It makes me a tad nervous," he says.
"We know that prostate tumours respond to testosterone with incredible growth."
Pike says there is promising research focusing on what can be done to turn on the brain's own testosterone-making system. He says relatively higher levels of testosterone are associated with greater longevity and that people who live longer have better levels of the hormone.
The challenge, he says, is balancing the benefits and the risks.
"So many of the factors that we are looking at are double-edged swords."