I should have known, but I didn't.
Most of last week I was dragging around--and I mean d r a g g i n g. I was lethargic, and spent as much time as possible sleeping. In fact, I couldn't get enough of it.
Late Friday afternoon, however, I started feeling better--and I didn't know why. And then I remembered something I'd read on Laura Dolson's low-carb blog about a nasty topic called carb crash.
Short version: Limit your carbs significantly and your body begins runs out of glycogen and turns to your fat stores (which, in turn, triggers ketosis) to power you through the day. The experience of transitioning from one energy source to the next, however, can feel (to use a scientific term) like shit: the carb crash.
I've been cutting my carbs back pretty significantly, and was doing it based only on total carb counts, and not effective carbohydrate counts (ECC, also known as net carbs), which counts only carbs that are not fiber. That's significant, particularly when you daily carb limit is under 100, much less 40 (as mine was for a while).
Here's why: A cup of raw zucchini has 4.15 grams of carbs. Of that, 1.4 grams is dietary fiber, which doesn't contain sugar. Also, it passes through the body as the human body can't digest (I mean, it's the same stuff trees are made out of). So if my carb limit was 40 grams daily, and I ate about 10 cups of zucchini, I'd actually be eating only 26 grams of carbs, not 40.
So there I was, thinking I'd contracted a fatal illness, and it was only major carb crash brought about by (inadvertently) not doing the right carb counts.
I'm now tracking only ECCs (in addition to having raised my daily carb limit to 60 grams). I feel a whole lot better since the crash (and am now in ketosis). I may, in fact, reduce my daily carbs (eventually) down to 50g of ECCs and--possibly--down to 40. For now, wiser, and more energetic, I'm staying right where I am.