Many middle-aged men find their waistlines expanding and their sex drive shrinking, symptoms a new study out Wednesday suggests can be traced to a hormone deficiency -- but not the one you might imagine.

Previously, a drop in testosterone production might have been suspected as the culprit, but researchers said a decline in estrogen may be part of the problem.

"This study establishes testosterone levels at which various physiological functions start to become impaired, which may help provide a rationale for determining which men should be treated with testosterone supplements," says Joel Finkelstein, of Massachusetts General Hospital.

But he said, "the biggest surprise was that some of the symptoms routinely attributed to testosterone deficiency are actually partially or almost exclusively caused by the decline in estrogens," Finkelstein said.

Traditionally, a diagnosis of male hypogonadism -- a drop in reproductive hormones sharp enough to have physical effects -- was based solely on a measurement of testosterone levels.

These diagnoses have increased dramatically in recent years -- leading to five times as many testosterone prescriptions in 2000 as in 1993.

Yet doctors understand little about the exact levels of testosterone needed to support normal function.

Testosterone production also has a direct impact on estrogen levels in men, since a portion of the male hormone is always converted into estrogen.

That makes it hard to know which hormone is needed, and in what quantity, to counteract the symptoms that appear in older men.

To find out, researchers suppressed all natural hormone production among more than 300 male test subjects, aged 20 to 50. Half of the subjects were given a daily dose of a testosterone gel or a placebo for 16 weeks. The other half were given the gel plus a drug that inhibits the conversion of testosterone to estrogen.

The results of the study suggest that testosterone regulates lean body mass and muscle strength, while estrogen impacts the accumulation of body fat.

And sexual function was impacted by both hormones: a lack of estrogen decreased desire, while a low level of testosterone impeded erectile function.

Further studies are necessary, Finkelstein said, to confirm whether the findings of this controlled study are accurate for naturally occurring hormone fluctuations.

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  • Work Out

    “We should be recommending about 30 minutes of physical activity most days of the week,” said Canfield. Exercise has many benefits beyond helping to raise testosterone, “exercise has been shown to increase sexual desire, No medicine has been shown to do that,” said Canfield. You’ll want a mix of cardio and weight-training exercises to get a full-body workout. Exercise helps both libido and testosterone in numerous ways and can improve heart health and aid weight management. Male sexuality is intimately related to heart health. In fact, difficulty achieving or keeping erections can be an early warning sign of heart disease, with a coronary artery disease event likely to occur two to five years after the onset of ED. “Anything you can do to help your heart will help your erections,” pointed out Canfield. Weight loss can help as well; “Losing weight will improve testosterone and libido. Androgen, a hormone essential to normal sexual development in men, increases at a greater rate in men who lose weight,” he said. Exercise can help you meet your weight goals and as a bonus, exercising with your sex partner in a way you both enjoy can be very stimulating.

  • Get Some Sleep

    Your body is a testosterone factory with its greatest production during a night of uninterrupted sleep. Sleeping may not be the initial reason you want to be in bed, but enhancing the quality of your sleep can also enhance those other activities. Keep your sleeping space cool, dark, and inviting for both sex and sleep. Be creative and add little touches with music, candles, and textures that allow you to set a sexy mood but also support sound sleep. Getting more sleep and exercise can help reduce stress, a factor Canfield said could contribute to lost libido and low T, although it is not well documented as of yet.

  • Take Ginseng

    Canfield said that between a half a gram and two grams of ginseng a day can help improve your sexual function and desire, without negative side effects (although you should always let your doctors know before you try a new supplement to make sure it won’t interact with other medications you’re taking). “Ginseng is an Asian food supplement that has been studied quite extensively, and research suggests it significantly improves erectile dysfunction,” said Canfield.

  • Eat a Mediterranean Diet

    “In line with weight loss, a diet that’s low in calories and fat, high in protein, and has a reduced amount of carbohydrate can improve testosterone and improve desire,” advised Canfield. The Mediterranean-style diet, which features plenty of fish, healthy fats (such as olive oil), whole grains, vegetables, fruits, and dairy, is one such diet. Changing your diet and watching portion sizes can also help with weight management.

  • Tackle Bad Habits

    Those little indulgences you secretly know are bad for you — smoking cigarettes, drinking a little too much alcohol, or even a post-dinner coffee — can add up to lost desire. Quit smoking and avoid second-hand smoke; drink moderately, if at all; and give up caffeine after midday to improve your sex life — and your sleep. The prescription for reclaiming your sex life is a lot like the prescription for overall health. You’ll be looking and feeling better than ever, in bed and out, as you work on these five strategies for improving libido.