Cardio in itself is not so bad. It's the chronic part, where you train at a consistently intense level and don't recover enough to allow cortisol levels to come back down, that is bad, according to Sisson. The key to good exercise appears to be allowing sufficient recovery, mixing up high-intensity and low-intensity and adding in some resistance training. Sufficient recovery seems to be the main issue for recreational cardio-addicts. Every run is a hard run, and after a while you wear yourself down.

I'm no exercise expert, but it seems to me with a bit of common sense cardio is great. Listen to your body while you exercise. If you feel drained at the start of a workout, quit (that can be a sign of overtraining). Don't obsess over weekly mileage or laps swum. On slow days, really go slow. A heart rate monitor can keep you honest there. Mix intervals in once a week or so and shorten that workout. Give the old bod a break after a tough workout, and most of all, ditch the watch and remember to have fun.

Depending on your level of training you can adapt to a keto diet without performance issues -

http://www.nutritionandmetabolism.com/content/1/1/2

Hi Alex H. I think what Gydle said about cardio is great. About complex carbohydrates: That's not a term that's used so much in the paleo world. I suppose that mainstream nutrition would describe it as any chain of three or more glucose-fructose-galactose-type molecules (anything that's C6-H12-O6). But that includes a whole lot of stuff, and doesn't really divide up the conceptual world of carbohydrate in a useful way. For one, it includes starch like potatoes, and we love potatoes, and potatoes will probably refuel you nice and good, so ...

Basically there are grains, which are to be avoided. (With the major exception of white rice, for many if not most of us.) Then there is fruit and there is starch. Basically starch is just a whole bunch of glucose molecules strung together. You'll find it in potatoes, you'll find it in sweet potatoes, you'll find it in squash, you'll find it in ... white rice. Fruit will have a mixture of glucose and fructose generally (both separately and combined into sucrose -- not that it matters much).

Fruit may not be a "complex carbohydrate" according to the definition above, but who cares. Both fruit and starch will probably be excellent sources of fuel for you as an athlete. As for refilling glycogen and how quickly that goes, I'm not an expert, but I'm guessing that once you cut grains out of your diet you'll probably be able to listen to what your body tells you. If you crave bananas with your meal after you work out you should eat them. If you take a liking to potatoes with your meat, you should eat them.

So if you mean by "complex carbohydrates" anything bigger than a disaccharide (anything bigger than sugar, basically), then yes, eating a paleo/ancestral/human diet can include complex carbohydrates, and in great quantities: starch! If you mean by "complex carbohydrates" the "heart-healthy" whole grains, then no, you won't be getting those. And if you're wondering if you'll have enough fuel for swimming, then yes, you will: both starch and fruit.

(Many people profit from eating low-carb, of course. I was one of them. But it sounds to me like you are very active and don't have glucose tolerance problems. If you do, then that's another story.)

You will most likely find difficulty, like I did in cross-country running. If you ever feel excessively weak one day, make sure you get some complex carbs in your body, because you are probobly running dangerously low on your glucose storage. The average human body can only store about 2000 calories of carbs in his/her muscles so a few intense swimming workout has the potential to drain you of that instant energy which is burned during intense cardio.

I think the paleo community frowns upon frequent bouts of intense cardio because of Dr. Harris's article. Dr. Harris explains how some studies show that marathoners and other cardio athletes have more heart scarring and heart disease rates than non-marathoners.

After long workouts, make sure you have eat some complex carbs for quick recovery. For the quickest recovery after workouts, eat complex carbos within 30 minutes of the end of your workout, because your body stores the carbs 3x faster in this time frame.

I hope I answered your question. :D

You should go to www.marksdailyapple.com and look search for chronic cardio. He discusses this a lot. I think the post from yesterday or today is a good start.

It's because chronic cardio is detrimental, but a lot of that is actually more the observation that it's not worth it. So for people who spend hours a day chugging along trying to lose weight or improve their health, we overstate the case as there are better more effective and natural ways to exercise.

Others have explained about the food situation. Paleo isn't low carb, it just doesn't use grains (ie. anything that has to be ground/refined in order to be eaten) as a staple. You shouldn't have difficulty distance swimming - endurance events often benefit from training with less reliance on glycogen (carbs) in any case once you get the hang of it.

It depends why you want to do extended cardio. If its to lose weight, improve physique/muscle, or be healthier and more metabolically fit then its does not work as well as resistance training and HIIT.

IF its cause you love that runners high or because thats the form of competition you enjoy, well then have at it! Yes you are more prone to that emaciated look and overuse injuries, but do what you love. And as peter pointed out carbs are not your only source for energy.

Here you go, read this and decide for yourself: http://www.psychologytoday.com/blog/p-nu/201103/cardio-may-cause-heart-disease-part-i