Hi, I don't know where to post this so here goes

Today I went on a ride with my girlfriend, a sunny day at 29C temperature hot but not that much. I brought a bottle of gatorade and she brought a bottle of water. We did about 30km before stopping at a convenient store to buy some drinks (we emptied the bottles). I brought a bottle of fresh water to my girlfriend; she started drinking but at some point she started to feel light headed and completly blacked out That is, she didn't see anything and she heard her voice really loud in her head. I brought her inside the store and the clerk brought a wet towel to put on her head to cool her. She felt better and then made her drink an orange juice and eat a chocolate bar. I convinced her to take a gatorade bottle instead of the water bottle afterwards. We did the rest of the way home fine, and she doesn't feel any after effects, just sore legs

She isn't the lazy type at all; she moves a lot at her work and is quite active. But this was only her fifth ride since the beginning of the summer.. I've been commuting to work so have done quite a lot more rides.

What happened?? Were these symptoms of a heat stroke? Was it lack of energy; drinking only water and not getting the electrolytes? Could it have been too tough a workout? (I personally didn't even feel tired after the ride; and she didn't seem out of breath at all! but..?). Just want to know if it happened to some of you guys and/or what resources I can check out so as to avoid this in the future

Next thing I'm buying tomorrow: 2 more water bottles and 2 more bottle supports (one more for each).

Up to that point, how much did she drink and how much had she eaten? What did she have to eat and drink before the ride?

How was she feeling before the ride ... in the day or two before the ride?

When was the last time she had her blood pressure checked and a blood work done?

You need to have a balance of carbs with your hydration. Some light electrolyte drink like Gatorade Rain would be good to have in your second bottle (I would keep pure water in the first).

Or you could just bring a snack.

I agree with Machka, your GF needs to see a Dr. Preferably one in the field of Sports-Medicine. It may be nothing or it could be related to something serious, but nobody in internet land is going to be able to make an accurate diagnosis.

I think she drank about 3/4 of a 500ml bottle of water, which isn't much (IMO), and ate nothing. She felt fine before the ride but had a little emotion ride during diner (small fight.. ); she didn't eat that much. She did sweat a lot too. She was in her last day (or a day beyond) her periods but she doesn't tend to have changed at all during her periods (ie: she felt fine). She was last checked by a heart doctor last december. They found a heart murmur but it turned out bening. She will see a doctor this week for other reasons. Though not a fitness doctor this will be a start. Thanks!

The blackout is worrisome. The rest could just be a classic case of 'bonking', but not blacking out.

I'm glad to hear she will visit a doctor.

I've come close to blacking out after donating blood, and going up the steep hill to home. In that case, it was certainly due to temporary anemia from the blood donation.

East Hill

SEE A DOCTOR NOW! If she did just get checked and found a murmer then the Doc might recommend a stress test.

Chances are it was just a hydration issue or nutrition but she really needs to see a doctor right away.

Thank goodness she did not black out while on her bike!

3/4 of a 500 ml bottle in approx. 2 hours in that heat? No, that's definitely NOT enough. And nothing to eat in those 2 hours? Well, no wonder she blacked out. What was she thinking? I suspect the lack of food and water was her problem.

But yes, I think she should be checked by a Dr. and she should let him/her know that she has begun an exercise program. I also have a heart murmur and have had it since I was 4 years old. Two years ago, tests discovered that I now have 2 damaged valves. About two weeks ago, tests discovered that I now also have an "odd" rythmn and palpitations. But I've been given the OK to keep cycling.

You might also want to do a little bit of research on "Postexercise Peril" ... essentially, when you stop exercising suddenly, your blood pressure can drop rather suddenly ... and this can pose problems if you do have some sort of cardiac "issue".
https://jama.ama-assn.org/cgi/content/abstract/251/5/630
https://www.ncbi.nlm.nih.gov/sites/en...&dopt=Abstract

She did not have enough calories for the ride. Once she burnt the calories she had, she felt light headed. It's normal. Solution is to eat a bigger meal, or if she's watching it, eat more smaller meals.

A good meal is something healthy.. Candy bars or other empty calories get burned immediately.