It’s the intensive intervals.                                       
It takes intense paddling to get to the wave line. Then you might get to rest for a minute so your heart rate comes down. Then you paddle hard to catch a wave and have a nice rest while letting mother nature do the work; unless you are shredding.

Then you have to paddle back out through the waves duck diving the foam and racing to miss the impact zone. If there is current, you have to paddle against it to maintain your position where the waves are breaking.

If you get in a good hour with five to ten waves, you have really burned up some muscle. Now the body goes to work as the Cytokene 6 calls in the inflammation and white cell patrol to clean out the dead cells. They in turn call in the Cytokene 10 patrol who come into build new stronger muscle.
The body could use some nutrition at this time to help the rebuilding process. Amino acids are the favored food du jour and so protein in the form of shakes or food helps this process.  Fueling up before you go in calls for carbohydrates.

I usually start the day with my 7 a.m. meal so that I am prepared by 8 a.m. for my first surf session. I have a bowl of apples, bananas, raisins, peanuts, peanut butter, some 10 grain cereal on the bottom and then sprinkle it with flax seed. My body really enjoys this fuel and uses it when I am in the water.
A short while after my morning session I will have some protein and more fuel for hopefully an afternoon session. I like a small portion steak, an egg, tomatoes, a slice of orange, and a slice of whole grain toast. This helps my body rebuild and prepare for more exercise.

I then fit in a small lunch about 1 or 2 p.m. of a tuna salad wrap. Ideally, my second session will be around 4 p.m. and then I have pasta for dinner.

You can create these aerobic sessions at a gym or on your own. Boxing for an hour is an excellent example. Aerobic classes will do it. Hill riding on your mountain bike or hiking up hills are real calorie burners. Intervals on a tread mill or stair climbers are good.

The beauty of surfing is that you go to exhaustion without thinking about it. You are thinking about catching waves and paddling back out and not about trying to burn muscle. You just have to stop when you are spent. When exercising in a gym, you are often thinking about how tired you are, so there is a mental disadvantage.

Depending on your determination, discipline, or mental toughness, you can take your body to exhaustion or maybe you stop way short. An hour of extreme exercise a day and a diet without sugar, flour, processed foods, fried food, nor too much alcohol, will really start stripping the fat.