I am currently losing weight (slowly), about 5 lbs since I started. But to answer: I eat more calories on the days I'm not fasting. Let's do the math:

At my age and weight, I need ~ 1900 calories/day to maintain my weight assuming I'm sedentary. That's 13,300 calories/week.

I run 76 miles per week. That's about 7,600 calories.

So 20,900 calories a week. On two days I restrict myself to 600/day, so that's 19,700 across the other 5 days, or 3,940 calories on those days. But I've been losing about 0.5 lb/week, which is a deficit of about 1750 calories/week. So on those other 5 days I must be around 3600 calories.

3500-4000 calories a day is a lot, but not extreme, just ask any weight lifter. And I happen to really enjoy food. So it hasn't been a problem.

But, I may have to reduce my fasting to 1 day a week once I reach my goal weight.