Deux wrote:

I'm not doing the Atkins diet but I have a question that people here
might know the answer to.

A couple of days ago I did a lot of endurance exercise that probably
burned 4,000 kcal. Even though I had a meal high in carbs before I
surely must have burned some fat.

That's a *VAST* amount of endurance exercise. Running a marathon can
easily take around 3500 calories so less than what you burned. The fact
that you did an amount of endurance exercise at least as much as a
marathon is key here. I'll call it a marathon for most of the rest of
my response.

The next day when I urinated I could smell something I can'¼ really
describe, perhaps a chemical smell. I also smelled this when doing the
Atkins diet. Subsequent trips to the bathroom didn't produce this smell.

After doing a marathon one possibility is you smelled ammonia. Ammonia
is a toxic product of very fast protein metabolism. It's very bad to
smell ammonia in your **** because it's a sign that your blood is toxic
and this particular type of toxicity has no natural limitations. It's
probably why the famous messenger from the original battle of Marathon
died and it's why training for marathons is different in type from
training for shorter runs - Running marathons is potentially fatal for
the ill prepared. Since ammonia is not produced while low carbing and
eating a successful predator's diet that's not the result you smelled.

After running out of glycogen a well trained body will switch to burning
fat by both of the metabolic pathways. The faster of the two pathways
uses ketones. The liver cuts fatty acids into short carbon chain
ketones. Those ketones flow through the blood but also escape through
the skin and in the urine. The cells pull the ketones from the blood
and modify them to put into the Krebs cycle of energy.

Conclusion - You did so much endurance exercise you drained your body of
its entire store of glycogen or nearly so. So your body switched to a
successful predator metabolism in other words ketosis. And you
continued the exercise without pause.

I did some Googling and one suggestion was that this could indicate
ketoacidosis because of the presence of ketones.

Definitely not ketoacidosis. Ketoacidosis is an unlimited run up of
ketones from burning fat because the metabolism is broken. Ketosis from
a successful predator diet and/or from vast amounts of endurance
exercise (includes running down a deer by the exhaustion method) and/or
from starvation is a well limited run level of ketones from burning fat
at a level tuned to handle the body's elevated metabolic needs. When
not doing vast amounts of endurance exercise this is the source of the
"metabolic edge" that Dr Atkins claimed. It works but only for a
limited length of time.

My thought is, surely endurance exercise which depletes all glucose(?)
would require the production of ketones to burn fat in order to continue
providing energy. So as well as during low carb dieting and diabetic
problems, ketosis should also happen during endurance exercise?

Exactly. The length of the marathon run is chosen to guarantee that no
human can finish it on our initial glycogen reserves no matter how much
carb loading we do in advance. It's why it's potentially fatal to the
ill prepared - Because an ill prepared body can end up burning lean and
it's possible to push such a run to death.

In which case, should I -expect- to smell something weird when I urinate
or should I see the Dr. about this?

It should be expected. Some describe it as acidic, some as garlicy,
some as chemical, some as acetone which is the most obvious of its
components.

So what did you do if you didn't run a marathon? A two hundred mile
bike ride or more in under 12 hours? Spend the entire day tracking a
deer in an attempt to kill it by the exhaustion method (possible because
deer sprint away from predators but a tracking human can non-sprint
run to chase the deer thus the deer dies by the ammonia method while
the human goes into ketosis)? Spend the entire day digging an
irrigation ditch?