I had to cut my workout a couple hundred yards short last night after heat palpitations. I had eaten ~ half a bar of 90% dark chocolate (it’s low carb) before practice and it looks like the concentrated theobromine was the cause.
Guessing I’m just very sensitive to theobromine but thought I’d give others the heads up just in case since these super dark chocolates are becoming more popular with LCHF.
Funny you should mention that, I went swimming today and really pushed it, set a new PB so felt pretty whacked afterwards. About half an hour later I was sitting browsing on my phone when my Apple Watch tapped me - ‘it looks like you are sedentary at the moment but your heart rate is elevated’. Wtf? I was running a 100-110 heart rate doing nothing. Normally my resting heart rate is in the high 50s.
I usually swim fasted but today I had half a bar of 100% chocolate just before I swam. And then a cup of coffee when I got home. Weird, it’s never had that effect before and I eat a lot of chocolate. Will keep an eye on it. Heart rate is coming down slowly, in the 70s now.
Sounds very similar to what happened to me. I felt plain odd for a while after practice then suddenly back to my normal self. Looks like it takes a few hours for the theobromine to work through our systems.
Biochemical response causing a jolt in the bodies electrical system:
Why Does Chocolate Cause the Heart to Skip a Beat?
My theory is:
The Lecithin in the chocolate act as a Liposomal delivery system for the caffeine and or other stimulants. That’s what makes it so potent! Hence heart palpitations, fluttering, murmurs etc.
That’s really powerful!
also, very cool that you swim fasted also. I started playing around with swimming while fasted over the summer and found I did very well, perhaps even better than when I ate.
Are you a biochemist of some sort? All I know is I’m not going to eat any of this low carb highly concentrated chocolate prior to any athletic activity anymore based on my results & the similar experiences of others in this thread!
I get the racing heart without any athletic endeavor – just from eating dark chocolate. I always thought it was the caffeine because I’m very sensitive to that, but maybe it’s a combination for the chocolate. I don’t know why, but chocolate still has some strange psychological hold on me. I don’t even like it that much, don’t crave it, it often makes me feel horrible, and yet for some reason feel I “need” to eat it. WTF?
No, me not a chemist, I am computer scientist, just find odd things related to this from researching other things totally unrelated to the subjects (serendipity)! I am starting to find bimolecular physics more fascinating than computer sciences! 
Heart arrhythmia is quite common. Rebecca Soni has atrial fibrillation at UCLA. She had ablation. I have lone atrial
fibrillation and my cardiologist chose not to treat it. I still swim top 5 in Canada for breaststroke in masters 65-70. It is very difficult to say what triggers it, but for me, I think it is stress. I thought it was coffee, but thank God it wasn’t coffee or chocolate!
Hey Andrew,
So far since I’ve laid off the super dark low carb chocolate I haven’t had a recurrence of the palpitations so I’m just going to stay away from the stuff! 
I seem fine with coffee but I have cut back on it a tad as well,…
The first thing I did when I got arrhythmia was to cut coffee. I was so glad to find out that it had nothing to do with my problem. Dark chocolate is also good, but I think excess alcohol may be trigger. As far as I can see, arrhythmia often has multiple triggers and may be vagus nerve related. This is true for treatment too. Many things may help. Meditation, sacro-cranial massage, Chinese herbs, breathing exercises and Tai Chi seem to help, but none of the them cures the ill. I am hoping that acupuncture will be the solution that will fix my heart meridian.
Caffeine and theobromine belong to a class of chemicals called methylxanthines, which have a number of effects in the body. Other well-known methylxanthines are the broncho-dilators theophylline and aminophylline. (Pet rat owners have to know all this because rats are susceptible to respiratory problems, and a square of dark chocolate can be very helpful to a rat in respiratory distress.)
People use other broncho-dilators besides theophylline and aminophylline, but the point is that any methylxanthine is likely to cause the same problem if ingested shortly before exercise, so know your meds!
Amounts can come into play, too.
I have never wanted to eat enough chocolate that I ever had any kind of identifiable symptom I could point to. Coffee, on the other hand, I have overdone in the past.
I am not able to just drink coffee cup after cup and not feel some of the effects of the caffeine, or whatever it would be, that causes the jitters, the excess energy and heart palpitations. For me, that meant that I switched to half-caf in the mornings and decaf at nights. But I didn’t have to eliminate it. I’m glad, as I do enjoy a cup o’ Joe…
I love coffee, I love tea;
I love the java jive, and it loves me . . . . 
Except, of course, the Ink Spots, my other favorites.
If you like barbershop harmony, these boys come close (they were gold medal winners a couple of years ago):
