Cramp during run but NOT dehydrated!

Stop using the gels and load-up properly before you start.

With gels, or anything you take while on a run, you need to find one that works for you.  High5 probably isn't it.

For a half marathon the gels/bars aren't needed.  Just take water with you and sip that as you go.  There's a myth that you need to "drink loads".  You might need to drink something, but probably not loads, and it's very unlikely you need gels.  A more sensible breakfast may well be the answer.  Train smarter, too.

The cramps could be a sympton of some other underlying health issue, too.  Are you getting it on both hamstrings at the same time?

I have suffered with unexpected cramp in an important (to me) race, and gradually realised over the next year or so that it was caused by drinking too much water and coffee without sufficient salt and other electrolytes. I now carbo-load the day before or at breakfast on pasta, toast or cereal followed by some salty snack and an electrolyte drink (sometimes with extra added salt!). But having got your body-weight down for a big race you dont want to feel bloated on the day, so make that breakfast small. You dont need loads of fluid or calories for a half marathon. One small gel 5 minutes before the start is probably enough, possibly one at halfway. 

"Train smarter" means just that.  Might be more miles, more speed, more strength.  A good programme will combine long runs with fast runs and runs with fast intervals.

Both hamstrings cramping at the same time, though.  Jeez.  Have you got the right trainers?  Do you get cramps at any other time?  What about in your sleep - cramps in one's calves are not uncommon.

Breakfast seems reasonably.  My long training runs and half marathon events are usually done on a bowl of museli and a cup of tea, but I've been doing that for a while.

Carbo-loading the day before is too late.

Cramp is poorly understood, but one of the current schools of thought on the cause of exercise-related cramp is that it has nothing to do with electrolytes/hydration, but is caused a change in the way the nerves and muscles interact - probably caused by fatigue from repeated motion. Stretching is important as is warming up, cooling down and maybe doing some cross-training to ensure balanced, strong muscles. 

That's not to say nutrition and hydration isn't important - water, sodium, potassium and calcium are all important in muscle contraction.

I take on board what others are saying, including sarah bookworm's sodium, potassium, calcium comment, perhaps also magnesium and other minerals. But the more water coffee or tea you drink, the more you flush these things out of your system. A serious mountaineer guy who I met said you need one packet of salty crisps for each pint of beer! (Thats not a recommended diet!)  And whatever the experts say, some cramp IS related to lack of electrolytes and/or over-hydration. Yes, also stretching and especially warming up - Sarah's right.

@Richard File 2 - provide evidence.  I suspect you believe in detox.

So with respect to what Sarah said - perhaps more race rehearsal runs might help (that is, runs of up to 1/2 or if you are really really keen 2/3 the race distance, done at the same time of day, over the same type of terrain, in the same clothes and shoes, using the same gels and drinks at the same intervals and running at the same pace as you plan to on race day - basically as much like the race as possibly apart from the distance).

Its not so much an issue of dehydration that causes cramp, its an inbalance of water to body salts. You need to rehydrate not just drink water.

When you sweat you also lose body salts as well as water. When rehydrating you need to take on some of these body salts as well as water to get things back to normal. You can buy drinks off the shelf of make your own (orange juice to water 50/50) and sip don't swig to get the best benefits. Milk is also known to be a good rehydration drink and also a good refuelling drink but may be a bit heavy to drink during a run. Safe for when your finished and optimise it by adding chocolate powder (chocolate milk is supposed to be the best option, better then sports drinks.)

I'd also say watch your posture- its amazing how one part of your body can have such a knock-on effect but it can so bare this in mind when trying to work out whats going on.

Lack of sleep, remembering to warm up properly and add a few dynamic stretches (as oppose to static stretches which are best kept for when you've done) and stress all can affect me getting cramp. No doubt there are other issues which can also affect cramp further.