what is a good daily calorie deficiency number?

based on....... what?

15-20% of TDEE is a decent maximum deficit, though this will vary a bit depending on your personal stats.

paulbsr wrote: »

Here's where my head is at - my body requires 2500 calories/day to grow hair, nails, drive the heart and digestive system (correct?). I exercise 2 hours/day (1200 cal). So that means if I could have eaten nothing, 3700 cal would have been sourced from fat reserves (?). However, I consume 1700 cal throughout the day. That means 2000 was contributed by stored energy. Or have I got this all wrong??

Let me correct a couple of misconceptions here.

Your BMR is the number of calories you would use up basically just by existing, i.e. if you were in a coma. Your TDEE is your BMR plus the number of calories you burn every day via normal daily activity and exercise, i.e. your total burn. There's a third number that MFP uses, your NEAT, which is in between the first two and counts normal daily activity but NOT exercise.

I don't know your stats so I can't check your numbers. But using your example, if your NEAT is 2500 and your TDEE is 3700, then yes, you would theoretically use up 3700 calories each day to keep your body going. If you eat 1700 calories per day, then 1700 of that will come from the energy from the calories you consume that day. The rest will come from fat (and to a lesser extent, muscle) stores. You can cut back how much muscle you lose by lifting weights, to ensure that most of the losses come from fat.

However, a few considerations:

• A 2000 calorie/day deficit is NOT healthy. That's a loss rate of 4lbs/week. At most, 1-2lbs/week is considered healthy. What you want to target is roughly around 15-20% of your TDEE. So if you TDEE is 3700, then you'd want to target a ~555-730 calorie deficit each day, including exercise. So that's a loss rate of 1-1.5lbs/week, max. Really it's not recommended to go above that, for a wide variety of reasons.
• Is your TDEE actually 3700? You claim to burn 1200 calories per day via exercise. That's EXTREMELY high. Where are you getting that number from? I'd be skeptical that your calorie burns are actually that high. Most gym machines, and MFP, for that matter, tend to overestimate calories burned via exercise. Let's say you're actually burning half of that or even less. That would make your real TDEE more like ~3100.
• You may have to do some trial and error with your calorie numbers to lose weight at your desired pace, since no calculator is ever going to be perfectly accurate for your body. They give you estimates so you can come up with a starting place. Try it for 4-6 weeks and see what happens. If you lose weight faster than expected, increase your calories. If you lose slower than expected, decrease your calories.
  
  
  Hope this helps!