I'm sometimes dehydrated, but I haven't yet worked out the causes. If I go out for a night and have several drinks, I'm usually dehydrated for the next day or two. Caffeine will do it, too. But I'm not yet a teetotaler, and I do drink tea fairly often, so I've got several recurring sources that might be drying me out.
Is there anything else in a low-carb diet that can contribute to dehydration?
From a comment by Peter on his blog: "there is a marked release of water from the liver as glycogen is lost, but this drops straight in to the circulation. It seems to be the excretion of this excess water through the kidneys, to maintain fluid balance, which takes potassium out with it."
Glycogen is a storage form of glucose; it is how glucose is stored in the liver and in muscle cells. The glycogen stored in muscles can't be extracted to be used by the body in general; it's pretty much locked in there for the muscle alone. If you want to burn off that glycogen, you need to do exercise, and ensure you're not stuffing any more glucose in there -- which means keeping your blood sugar and insulin levels low. Liver glycogen is the result of some carb digestion; both alcohol and excess carbs will be stored as glycogen in the liver. But... this glucose storage isn't really a big deal. As long as you're not overloading the liver with either alcohol or carbs, the glycogen stores aren't a health hazard.
And when you burn off that glycogen, the water that glycogen binds to is released into the blood stream, as mentioned in the comment linked above.
This isn't dehydration; this is the loss of excess water when one first switches to a low-carb diet. If you take a cheat day (I recommend one every two weeks), you'll build up some glycogen stores that you'll need to burn off, too. Dehydration is when there is a low volume of water in the blood; the loss of this glycogen-related water is just restoration of your normal plasma levels. Dehydration is having too little water in the blood, and that's generally caused by something stripping water out.
Essential Fatty Acids (EFA)
From a page on Chris Masterjohn's cholesterol-and-health site: "Arachidonic acid is necessary for growth, proper hydration, healthy skin and hair, gut health, and fertility." Chris points out (in another page) that he's concluded that only Arachidonic Acid (AA) and DHA are the only true essential fatty acids.
Other than the importance of these EFAs in several "cascades", I wasn't able to dig up specific references to how AA and DHA affect hydration or skin and hair. They're important in cell growth, and hence important to growing children, pregnant women, bodybuilders, and people recovering from injury, and needed in lesser amounts for general repair.
EFA concentration is highest in liver and giblets, especially of grass-fed beef and organically-raised pigs and fowl. This is one important reason to cook your own foods using organic, grass-fed meats; protein is protein, but the important vitamins contained in animal fat will vary significantly based upon the diet of the animal. I aim for grass-fed and organic meats not because I care about the lifestyle of the cow, but because I care about the quality of my food.
Poly-Unsaturated Fatty Acids
From that same page on Chris Masterjohn's site: "When total PUFA intake or EPA intake is very high, however, the EPA may interfere with arachidonic acid metabolism and contribute to deficiency symtoms such as growth retardation, dehydration, flaky and scaly skin, hair loss, gastrointestinal syndromes, or infertility."
The importance of the omega-3:omega-6 ratio is something that has reached the mainstream media. However, a number of researchers in the paleo community have argued that if you're not consuming a lot of omega-6 fatty acids, then you don't need so much omega-3. If you avoid PUFAs from vegetable oils, you don't need to obsess about getting omega-3 eggs or eating the right other vegetable oils. Just stop eating fried food, fake queso from fast-food Mexican restaurants, and soybean oil-based salad dressings; and don't cook with industrial vegetable oils. Butter and coconut oil are fine for pan-frying and sauteeing, and olive oil & vinegar are good choices for topping your salad (if you don't create your own salad dressings).
Excess PUFAs have the effects listed above -- including dehydration and flaky skin. Instead of worrying about getting enough omega-3 fatty acids, avoid the omega-6 acids. (Arachidonic Acid is an omega-6, by the way, but if you're eating liver once a week, it won't be a major component of your diet, as it is for people who eat a lot of fast food or use heavy amounts of industrial vegetable oils.)
Lectins might induce diarrhea, by pulling water into the intestines; hence dehydration. Food poisoning can also lead to diarrhea and dehydration, so ... yeah, avoid that, too. It's the same basic mechanism, and why you're encouraged to drink a lot of water if you have diarrhea -- obviously you're losing a lot of water, and you need to replace it.
I wish I had some good sources for the lectin-diarrhea link; I'm hunting up some links so that I can make a post specifically about lectins.
This is one concern that recommends a paleo diet instead of just a low-carb diet: beans, often eaten in great quantities by people trying to go low-carb, have a lot of lectins. Many cases of "food poisoning" are actually cases of eating under-cooked beans. Avoid beans, and definitely skip the grains, as both contain lectins and can lead to diarrhea.
And one last quote, which I need to move some place better:
"roughage -- which includes beans -- help people stay 'regular' by causing more cell tears, which enables more mucus to escape from cells, essentially greasing the GI tract" -- PloS Biology in 2006
Saying "dehydration isn't caused by not drinking enough water" misses the point. Dehydration is caused by losing too much water and not replacing it. Dehydration is an imbalance in the blood, and is caused by digestive problems, the lack of vitamins, or overactive kidneys (such as from caffeine!). If you're losing water from one of these sources, then you need to drink more water. Don't drink water just because some dood on TV said so; drink when you're thirsty, or if you think you're losing excess water from one of the ailments listed above, drink some extra -- but if you're not thirsty, don't bother.
Why would our body require us to drink water when we're not thirsty? Sometimes I do get very thirsty, and I obey that signal and drink as much water as I can. If I'm not thirsty, I don't worry about it. When I switch from high-carb to low-carb, I'll try to drink water whenever I get cravings for food; drinking something helps distract me from the hunger. Otherwise... just when I'm thirsty.
My skin is much softer when I'm taking levothyroxine (the generic form of Synthroid, due to my hypothyroidism) and also Vitamin K2, and I usually use skin dryness as a measure of dehydration. My skin has been much softer this year, and the major differences are going paleo and taking K2, A, and D. Tea and alcohol are drying me out, so I'm trying to cut out the tea and cut down on drinking -- which, mostly, is about one night week. So I think I'm doing pretty good. I'm glad for the changes!