Yesterday, we profiled two men taking the Pho Challenge – two pounds of noodles, two pounds of meat and one hour to eat it. But ever wonder what that kind of “competative eating” does to the body?

In today’s video – where we find out if Tak and Andrew complete the challenge – Tak starts to feel some pain. He feels tingly and his face looses color.

A few years ago, I produced another extreme eating video called “Recycled Ramen” about a guy who tries to eat the spiciest ramen in the world. The title should give you a hint of what happens but if you still don’t get it, the last line of the video is “a man can only eat his puke so many times.” (To see a less extreme video of the contest, click here.)

One of the most striking features of watching these guys in action is that they’re so exhausted at the end – I mean totally spent like they’d run a marathon. Wonder why?

According to dietitian Sarah Koszyk, RD of MV Nutrition, their bodies are using so much energy to process the food that there’s not enough left to keep the other systems functioning properly. She figures that Andrew and Tak ate about 3,000 calories in an hour, more than a days worth of calories for most people.

At the same time, the body is also expending huge amounts of energy trying to protect itself, says UC Berkeley professor of Nutritional Science and Toxicology Sharon Fleming.

Fleming watched the Pho videos with a clinical eye and pointed out a few of details that we missed.

In the video, Fleming notes that when Andrew tries to cheer Tak on and Tak responds, “No, no, no.”

Towards the end, the guys can’t stand the look and smell of the Pho.

Fleming says there’s a scientific reason for this. It’s the vagus nerve trying to tell Tak to stop eating. The vagus nerve makes things smell and taste good.

“The vagus nerve was making the food taste and smell horrible,” Fleming says.

She goes on to explain that their body was saying no but their will was overriding it. This can be dangerous. Fleming notes a water-drinking contest a few years ago where the mother of three died because she overrode her body’s signals to stop.

What happens if they don't stop?

"Tell these guys to call me if they continue eating like this becasue they'll need a good weight-loss plan," says dietician Sarah Koszyk.

(Check Out: Part One - Ready for Pho Challenge? 2 Lbs. of Noodles, 2 Lbs. of Meat and 1 Hour to Eat It (VIDEO))