I'm on a cyclical low carb diet (low carb during the week, moderate-to-high carb on friday/saturday). I run 7-8 miles 3xWeek on low-carb ~50g (100g on a bad day) and a long run on the weekend with a moderate-to-high carb-up. So my weekday runs are usually about an hour and I feel great. What is the duration of your daily cardio? If you were doing 5 days a week of an hour, I'd probably up your carbs a bit. I'd eat a good portion of them after your exercise for recovery. Its good to do some workouts on high glycogen and some on low glycogen.

I do about 60 minutes if cardio every other day and usually go to about 30 net carbs in a day. I've had no problems with it.

Here's my n=1 experience. I was on a ketogenic low carb diet for months (sub 20g cho a day), and I did short cardio and heavy weights twice a week. No problem, for a long time. Then I started developing various symptoms which I explained away- cold hands and feet, constipation, dark circles under my eyes...then the weight loss slowed down.

It wasn't until I read a post on Paul Jaminet's site "Perfect Health Diet" that things clicked: low thyroid. Low carb diets (like low calorie diets) can downgrade the body's production of T3. In my case a combination of a keto diet, exercise without refueling on carbs, and a course of antibiotics destroying gut flora *seems* to have caused a glucose deficiency, in turn causing my thyroid to slow down. From what I understand (and hope) this is temporary and reversible by upping carb intake.

I stress that this is what I *think* is happening based on my symptoms, and it matches perfectly with Jaminet's theory.

Short version: I have done exercise without carb refueling, and I think it has had negative consequences for me. If you'd have asked me a month ago I'd have said yeah, sure, exercise on a keto diet, no problem- now I'd be much more cautious and recommend a higher carb level on training days. It's unlikely to hurt you, and may prevent issues further down the line. Just my experience, FWIW.