"I’m kind of worried now. I shouldn’t be."

Those were the famous last words of Taoiseach Leo Varadkar as he waited for the results of his metabolic age test, which would tell him how truly fit and healthy he was.

On last night's episode of Operation Transformation, the 40-year-old Taoiseach was among a host of famous faces who had their metabolic age tested on air, including Aengus Mac Grianna, Anna Geary and Bláthnaid Treacy. 

While many volunteers came away with metabolic ages far less than their actual ages, such as 30-year-old presenter Treacy who measured in at 15, Leo walked away with a metabolic age of 53 and a look of horror across his face. 

It's a compliment to the Taoiseach that viewers around the country were just as shocked as he was to hear his results, given how active and fit we know our Taoiseach to be. In recent years Varadkar has been vocal about his active lifestyle, avoiding meat and even bringing Canadian Prime Minister Justin Trudeau on a jaunt through the Phoenix Park during Trudeau's 2017 Dublin visit.

So how could a fit and active man living such a healthy lifestyle receive such a result? Dr. Niall Moyna joined Today With Seán O'Rourke to discuss how and why this might happen. 

Moyna himself was surprised at the result, saying "I think Leo is a guy who we know in the last few years has really started to take his health very very seriously and you could see that he’s lost some weight. He’s a very very big guy as well, you would expect [him] to carry a lot of muscle mass". 

As for the test itself, how legitimate is it? Varadkar's first reaction was to question how valid it was, but Moyna explained that the test is indeed to be trusted, adding that it is designed to test "your resting metabolism". 

"It’s basically the energy you need for all your basic vital functions. It compares your current resting metabolic rate with to what it should be for your chronological age. So, he should have a resting metabolic rate for someone who’s 40, but his is actually someone who’s much older. And we know that [metabolic age] decreases throughout life 2%-3% per decade … primarily because we lose muscle mass as we age.

"That’s why I was very surprised last night to see that his was so poor."

Elaborating, he said that "the gold standard way to measure this, the most important one, is you go into a room and you stay there and because through your metabolism you produce heat, you measure the heat and that’s the gold standard way".

"Or we can measure oxygen uptake. This is an indirect way. They took the person’s age, their gender, their body size and then they took into account the amount of fat that they had and the amount of lean tissue, and that’s where the test last night came in.

"The test last night was to measure - using bioelectrical impedance - the fat and muscle tissue. Basically, muscle doesn’t conduct very well. So, a small current is passed through the body and the muscle conducts it very well but the fat impedes it. His fat must have been much higher than the lean body mass on the test."

When compared to results for other people on the show, such as fellow 40-year-old Kathryn Thomas who walked away with a metabolic age of 25, one would wonder why the Taoiseach's age should be so much higher. Moyna explains that with the form of the test performed on Operation Transformation, there are a number of individual variables at play. 

"Now, to be fair, you should be resting, you shouldn’t exercise for 24 hours, you shouldn’t have eaten for 12 hours." 

There's also simply the question of build: "Maybe for Leo, he doesn’t realise he is carrying excess body fat. Big people tend to be able to carry it and hide it better than smaller people."

These variables notwithstanding, factors such as age, gender, and body size are key to determining your metabolic age, as is "the amount of fat and the amount of muscle tissue that [a person] has". Environmental factors such as stress and sleep are to be taken into account also.

But unlike chronological age, you can decrease metabolic age with some lifestyle changes and hard work. So here's hoping An Taoiseach doesn't lose any sleep over this. 

You can listen back to Dr. Niall Moyna on RTÉ Radio 1 in the video at the top of the page.