Salt and fat are very important on a low carb diet.
If you've being doing low carb for a while, you may have heard of people saying that they had a "bad reaction", or suffered "side effects" on the diet. One of the most obnoxious side effects constantly stated is that you'll get bad breath. The press likes this one, as if bad breath was the worst thing that you could have happen to you. Personally, I never had a problem with this, nor did any of my family members who went low carb. Maybe we were just lucky? Anyway, the side effects people encounter are probably from them either not getting enough fat, from not getting enough salt, or from going on and off the diet regularly (see my post about how your body makes enzymes and hormones based on what you've eaten recently). Or maybe a combination of all of that. So, what to do about these unpleasant "side effects"?
First and foremost, throw your fear of fat out the window. You don't want a lot of protein. You want fat, mostly saturated animal fat. It's best to avoid synthetic, man-made vegetable fat. Eating a low-carb, low-fat, high protein diet will make you sick. They call it "rabbit starvation" because rabbits are such lean meat, you'll die if that's all you have to eat.
Some things that can happen while on a low carb diet are tachycardia (racing heart). What can cause this? Low blood pressure. Eating a low carb diet lowers your blood pressure, and if you have low blood pressure already, you know that you sometimes get a fast heart beat. The mum-in-law has low blood pressure, and she sometimes gets a racing heart. She says that it's because your heart needs to get that blood pumped around your body somehow, even if it has to beat faster to do it. It's usually nothing to worry about, although I would suggest seeing a doctor just to be sure.** Something that may help this, is to increase your salt intake. Salt intake does raise your blood pressure slightly (between 1 and 6 mg/Hg) and it may be enough to avoid the side effect of low blood pressure. In fact, increasing your salt intake on the low carb diet, especially in the beginning, can prevent you from losing too much salt from losing too much water. You retain water when you eat carbohydrates (chemically speaking, water is required for their metabolism, and they're made of water, hence the 'hydrate' part). That leads to the point that if you eat a high carbohydrate meal once or twice a week, you will gain water weight, sometimes crazily so. This is not because you've eaten too much per se, it's because you're retaining water. It can take a week or two of eating low carb again to lose the water weight.
Another cause of low blood pressure may be if you're on medication for your high blood pressure, and you go on a low carb diet, well, you may need to have your doctor adjust your blood pressure medicine down to compensate.
I personally (and I know this is anecdotal) do so much better on a low carb diet when it comes to my heart. I have a hard time getting enough potassium when I eat the standard American diet, and I once wound up in the emergency room with sinus arrhythmia, my heart was stopping momentarily, literally skipping beats. Needless to say, it scared the hell out of me. The doctors told me that my potassium was low. I don't get sinus arrhythmia on the low carb diet. Probably because red meat is a great source of potassium. I also noticed that my heart doesn't beat as "hard" as it used to when I would lay down to go to sleep. I used to be able to feel it beating loudly, and it doesn't make so much noise any more. My pulse is normal, it just seems like it's working more efficiently or something. And again, this is all anecdotal, so take it with a grain of salt (pun intended).
**Please note, that if you are having problems, especially to do with your heart, see a doctor. Have blood tests, get hooked up to an EKG and make sure you don't have an underlying medical condition.