I'm back on keto and I found the best way, for me, to avoid the flu was to put a good amount of lo salt into a beer flagon then fill that with water and plow away on that for the first week or so while also making sure my protein intake was high.
This is key.
A large cause for the 'flu' is similar to what causes hangovers - a combination of sudden water and electrolyte loss.
In the States, sodium chloride is the most easily found electrolyte combination and the least likely to be in chronic deficit prior to water loss-related factors. Salt intake averages range from 2-3 times the recommended daily intake, while potassium and magnesium average at 50%.
Certainly add salt to food and water (and definitely soups and broths) when eating a heavily reduced carb diet or fasting outright - not too much for the latter, though. Past a certain amount and the water won't even get absorbed and will function more like a poor man's Golytely to flush right through your pipes, leaving you even more dehydrated and electrolyte-deficient than you started.
It is very helpful to add a daily multivitamin that includes a good amount of magnesium and calcium, which isn't hard to find. I'd recommend the same for potassium, but there aren't very many high-concentration tablet forms that seem available - but there is a very simple and predictably under advertised solution I ferreted out few years back.
Remember the yellow raincoat girl on the dark-blue Morton's Salt containers? Look for her cousins: Morton's Lite Salt (sky-blue bottom half, dark-blue top half - or just all sky-blue) and Morton's Salt Substitute* (all dark-blue).
Morton's Lite Salt is a very accessible source of potassium and does the job swimmingly while directly reducing your salt intake (with no difference in taste). It cuts your sodium intake from 575mg to 300mg per 1/4 teaspoon and supplements it with a potassium increase from 0mg to 360mg in the same measure. In short, one teaspoon of the regular version gives you 100% RDI for sodium and nothing else, while one teaspoon of the Lite version gives you ~52% RDI (based on 2300mg RDI) for sodium and ~30% RDI (based on 4700mg RDI) for potassium.
With two teaspoons spread through the day, you've got your sodium covered and 60% of your potassium covered, and if you're drinking as much water as you should be, you could probably afford to work in one more teaspoon throughout the day for good measure. It's still certainly safer than what people conventionally do with even more teaspoons containing twice the amount of sodium in each.
* Morton's Salt Substitute is a more dramatic option, with 0mg of sodium and 690mg of potassium per 1/4 teaspoon - or almost 60% in one teaspoon. This is great if you are watching your salt intake and want a useful low-calorie source for potassium. However, I'd personally limit my usage of this to a single-serving
supplement to the Lite Salt, as sodium deficiency and potassium overdose is an easily overlooked risk if you go into consumption autopilot and compulsively start using this in place of salt - low-carb dieting or not.
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