Hi Toby,
You know, I'm trying to remember back in order to accurately pin-point the exact days....but both times I've done Atkins, I experienced both weakness and this aching in my legs for about 2-3 days (if I remember correctly)...and I think this occurred around the second day or so. I've read multiple others relate these same experiences on the Atkins site....they often refer to this as "the Atkins flu".
It's generally known that when people go on diets, they usually lose some significant amount of weight (on the scale) during the first week or so....and that this is generally considered to be mainly water weight. So my assumption would be that if the body does, indeed, release water weight at the very beginning of a diet....that some electrolytes, etc....would also be released (sodium, potassium, etc.).....and this temporary water/electrolyte loss could very well explain the temporary weakness and leg aches/cramps.
I take a multivitamin and a chelated calcium/magnesium supplement....but I also added a bit of potassium both times when I experienced these symptoms 2 or so days after starting Atkins (both times).......and the symptoms did ease up and disappear......but the thing is, I have no idea if it was the potassium or if it would have simply gone away by itself in a day or so, once my fluid and electrolyte balance stabilized itself.
For one thing.....I did not continue the potassium supplement...as there's some in my multivite plus there's plenty in all the veggies I eat. So I clearly did not NEED additional potassium to be continued.....or the symptoms would have returned....and they did not.
I can't remember the exact scientific explanation for why the body releases so much fluid at the very beginning......but throughout the diet, your body will fluctuate when it comes to fluid balance (which will cause the scale to show some fluctuations all along the way)......but generally speaking, once you stabilize on the diet, your fluid and electrolyte balance should also stabilize itself and you should find that this "Atkins flu"/weakness, leg aches/cramps should resolve and should not be a persistent problem.
It just seems to be a fairly common occurrence during the first week or so of starting the Atkin's diet.
deena