Try fasting for the day before your training, breaking your fast an hour before you train for a small meal of eggs or something similar (protein & a small amount of fat, but not too much) then another small meal straight after training. Fasting after training is a bad idea as your body needs to repair.

Keep an eye on your water in-take during the fast. The headache suggests dehydration - you may be taking on too little. Aim for 500ml to 1000ml of cool water per hour (ignore the nonsense that water should be body temperature before you drink it - it's pseudo-science), but don't quaff since a large amount of water suddenly hitting your stomach may induce the sickly feeling.

You might also find it easier to fast from sleeping - take a look at the fast-5 plan. This works best for me since I train at around 5pm, breaking my fast 30mins before training with a paleo protein shake. Fast-5 mean a daily fast of around 17hrs (inc sleep), and as long as you drink enough water and can keep yourself busy (as my job does) then you won't even notice you're fasting.

One other thing - "palaeo" means different things to different people. I personally adhere to the core principle of not eating "Grains, Beans, or Potatoes, or products thereof" as I learned it when I stumbled across the palaeolithic diet a couple of years ago. If I ever slip I feel terrible shortly after eating, and this is compounded if I'm also training. You may find that, if you follow the definition of palaeo being "non-processed foods" that restricting your intake of GBP may help.

I went with the Warrior Diet approach and it has served me well. I only eat late in the day when my metabolism is stoked and I can really enjoy my food (instead of grazing and feeling hungry all the time). Ori had it right.

Perhaps it's a matter of "too much, too soon" and your body isn't ready to go without for that long of a time?

I would suggest - and mind you, I am not an expert on IF - doing an IFast for a smaller amount of time (9 hours?) and then slowly increasing the amount of time for each successive fast. Your body would then get used to going without for longer periods of time, and you wouldn't feel so crummy.

16 hours is kind of long. That's a pro-level fast. Simply skipping a meal or two is the best way to start out.

I agree with Phillip about fasting from sleep. Did you see the new study that shows what you eat for breakfast primes your metabolism for the day?

My advice would be to start by skipping breakfast, then when you are able to do that, skip lunch. Then maybe go on to dinner.

I would say that you haven't been doing paleo long enough. I've been doing paleo for about a year and have been intermittent fasting for about a month. i've really just cut out breakfast and have without wanting to lost 10lbs. I work out in the morning and have not seen a decrease in strength or endurance. If I do a heavy workout I make an 8oz carb shake to help recover.

It might also be that you are not eating enough during your feeding window. Keep track of how many calories you eat on a non-IF day and then try to hit around the same number when you fast. IF doesn't work for everyone, but some of the people who say that it doesn't work for them are just not making up the caloric difference in their feeding window.

I do Fast 5 and love it. I break my fast at 3:30 pm which is during our tea break at work. I eat 2 hard boiled eggs and a slice of Emmanthaler cheese, then I drink a cup of tea. When I get home at 6pm I eat a regular meal with my husband. My window closes at 8:30 pm. I tend to follow the Perfect Health diet (which is based on Paleo but allows "safe starches" such as sweet potato, white rice noodles.)

Technically the Fast 5 plan allows you to eat whatever you want in your 5 hour window (and a lot of people eat crap!) but I keep it healthy with an occasional sweet or popcorn at the movies - but then I feel like hell afterwards,.. I'm going to stop that actually. The splurge isn't worth it.

@gilliebean today I broke the fast with some pulled pork but then I still felt hungry so I also had some homemade chili.

@tattooedchef I generally eat dinner no later than 5pm and eat breakfast around 7am so I guess that's really like a 12-14? hr fast daily. I wonder if just that is enough to get the benefits of ifasting? I definitely attributed the first time I got sick to too much too soon. I guess I'll try to be more methodical later. Maybe start with a 14-hr fast to see.