A low sex drive and declining muscle mass are among men's primary concerns about low testosterone. But there are many reasons for a man to care about a decline in his testosterone levels.

The one he may not be considering is heart health.

"Testosterone deficiency has an unfavorable effect on cardiovascular health," says Dr. Khurram Nasir, a preventive cardiologist at Houston Methodist. "It can even be considered a risk factor for heart disease."

That's not good news for the 1 in 3 men aged 40 to 80 who suffer from low testosterone, or low T. The condition's development is made more likely by certain lifestyle factors and health conditions, including being overweight and having type 2 diabetes.

After age 30, notes Dr. Nasir, there's a steady decline in testosterone levels, about 1%-2% per year. While that's natural, he says some men experience a more exacerbated decline, resulting in early-onset testosterone deficiency.

Some of the best known symptoms of low testosterone — such as reduced libido, general fatigue, loss of muscle — are relatively straightforward. The link between the heart and low T is more complex.

Some studies have identified a cardioprotective effect of testosterone, while others report that the link between low T and heart disease isn't particularly clear. And you may recall the FDA issuing a warning about testosterone therapy several years ago, claiming that it may actually lead to heart problems.

It's enough to leave any man feeling confused about how testosterone affects the heart exactly. Fortunately, Dr. Nasir can help us sort through what's currently known about low T and testosterone therapy — and what both mean for heart health.

How low testosterone affects the heart

"Based on what we know about the association between testosterone deficiency and metabolic issues, impaired glucose metabolism, poor cholesterol levels and inflammation — all of which are themselves linked to heart disease — we can infer that testosterone deficiency is also associated with poor heart health," explains Dr. Nasir.

One of the most noticeable examples of this is erectile dysfunction (ED), the most common sexual problem men 40 and older report to their doctor. ED is often the result of poor blood circulation caused by atherosclerosis, when the arteries that supply the body with blood become narrowed due to plaque buildup. This link might be indirect, but it's why ED can actually be a warning sign of heart disease.

Many other connections between testosterone and heart health have been drawn, with studies showing that:

• Testosterone helps slow (and maybe even reverse) atherosclerosis
• Higher testosterone levels are associated with a decreased risk of heart disease in men over age 70
• Age-related declines in testosterone levels are associated with increased risk of heart disease
• Low T levels in men with congestive heart failure are associated with poorer disease outcomes
• Testosterone therapy can help lower cholesterol levels, thereby reducing the risk of atherosclerosis

"We also know that low T can be thought of as a biomarker of poor overall health," says Dr. Nasir. "And when your overall health is poor, you're more likely to have poor cardiovascular health."

More evidence is needed to clearly define the link between low T and heart disease, but most cardiologists agree that taking steps to keep testosterone levels healthy is important for promoting good heart health.

(Related: 5 All-Natural Ways to Boost Your Testosterone)

Is testosterone therapy safe for the heart?

The most common reason to consider testosterone therapy is to reduce those obviously troublesome symptoms of low T, such as ED. But there are more upsides to this therapy. Testosterone replacement can also benefit waist circumference, lean muscle mass, type 2 diabetes, metabolic syndrome and more.

But does it reduce the risk of heart disease? And what should we make of the warnings that testosterone therapy might actually negatively impact the heart?

"It's complicated," says Dr. Nasir. "Over ten years ago, a few early studies suggested that testosterone replacement therapy might increase a man's risk of experiencing a cardiovascular event, like heart attack and stroke. But that evidence was incredibly controversial."

Dr. Nasir adds that these studies either weren't designed to specifically assess such a risk or were retrospective studies that weren't well controlled. An FDA committee evaluating the studies even stated that the evidence against the therapy was insufficient.

Despite this, the agency issued a warning of prescription testosterone therapy in 2014.

"Fast forward to today — a large trial designed to solely and thoroughly assess the cardiovascular safety of testosterone replacement therapy recently found no difference in the risk of adverse cardiovascular events in men taking the therapy versus those not taking it," says Dr. Nasir. "The trial did find, however, that arrhythmia risk may be slightly higher while on testosterone replacement, which has implications for how we think about treating testosterone deficiency."

Dr. Nasir points out that only men who need the therapy should be considered for it, but it's also important to understand a man's risk of heart disease before prescribing testosterone therapy.

"We need to take the guesswork out of measuring a person's cardiovascular risk, and to do that we use the coronary artery calcium (CAC) test," adds Dr. Nasir. "This is safe, noninvasive test we can use to very accurately predict risk."

(Related: Calcium Score: What Is a CAC Test?)

The first step for men who suspect they have low testosterone is a blood test. Someone whose levels are low and who also has a low risk of heart disease likely can be prescribed testosterone therapy without much concern. But a man with low T whose cardiovascular risk is high will need to consider whether the therapy is a safe treatment option.

How to boost testosterone levels and protect your heart

Testosterone levels naturally decrease with age. That's something you can't change.

But certain lifestyle factors amplify this decline. And many of those factors are also ones that increase heart disease risk. This means that the same behaviors that help support normal testosterone levels also promote a healthier heart.

The lifestyle behaviors that can help improve both testosterone levels and heart health include:

• Eating a healthy diet
• Being physically active
• Keeping a healthy weight
• Managing blood sugar, blood pressure and cholesterol levels

"And, certainly, if you have low T or are experiencing symptoms of low T, it's important to see a cardiologist for a full evaluation of your cardiovascular health," adds Dr. Nasir. "Despite some of the unknowns, what we know for sure is that low T can be a warning sign for heart disease."