Well it differs in a lot of people. I know some who have are alergic and will have a bad reaction to even the slightest amount of caffeine. Then there are people like me, who drink a gallon of coffee a day and have no problems.
I vary a lot. I think it's all in how your body reacts to the caffeine. I have had four cups of coffee when I'm drowsy, and after several trips to the privy I'm still sleepy. Other times I'm jittery after one cup. Trust your body's reaction to your intake. Stop when it makes sense.
Of course you can and its easy. Water intoxication happens when you consume ridicules amounts of water in a very short time. Basically you drown out your electrolytes, get one hell of a headache, and you die.
As for the 600mg, thats where they like to draw the line in the sand.
I wasn't going to reply but you mentioned some very controversial subjects. My grams has osteoporosis and about half the docs all said caffeine is bad for it. The other half said BS. That topic hasn't been answered properly because the tests have been inconclusive. There are so many things that can contribute or take away from osteoporosis that they really haven't been able to isolate just one thing that causes it or makes it worse. They all seem to say the same thing however. Add more calcium to your diet.
Menopausal triggers include caffeine but also contains dozens of other triggers heat, alcohol, spicy food, and stress to name a few.
As for a specific number I haven't seen anyone say 400mg. Most of the stuff out there talks about moderation. One or two cups a day. I think the reason why 600mg's get picked is because it is considered to be on the side of a heavy drinker. Someone who drinks upwards of 5 - 7 cups a day. I have friends who polish off a 6 pack of Dew before the head to work. Each bottle has about 55mg of caffeine. Most of them wouldn't consider themselves a heavy caffeine drinker.
LDLO is an abbreviation for Lethal Dose Low, the minimum amount of a chemical which tests have shown will be lethal to a specified type of animal
TDLO, lowest published toxic dose, not not lethal dose!!
The data seems to indicate they killed more women than men, but the data is a bit all over the place. Interestingly a 50kg woman would need to be intravenously injected with more caffeine than in 100 cups of coffee.
The most unfavourable data for a human appears to be in the region of 320 cups of coffee, and for children at least 210 cups of coffee.
So around 12 or 13 Gallons of coffee and you might be in trouble, but remember this is the minimum amount that could be lethal, and the report is about "pure caffeine"....so thats a large hit all at once.
It's all relative to the source anyway. Most of the deaths that have occurred resulted in an OD with caffeine in a pill form. I'm pretty sure a communist government could tell us exactly how many pills it truly takes but I guess we will never really know.