Fitbit and negative calories

I have my MFP settings set to sedentary, and it takes me about 2,500 steps before I start to see any exercise calories.

MFP sets your projected calorie burn based on your height/weight/gender and age - and also factors in your stated activity level. Fitbit also uses your personal stats but generates your calorie burn based on your movement. The difference between what MFP expects you to burn and what Fitbit says you actually burn: is your calorie adjustment.

But how does that work during the day? Each time you sync, Fitbit sends information to MFP about your calorie burn. And that is changing each time you sync.

In reality, you do not burn the same # of calories every minute or every hour. Some hours, you are sleeping and burning BMR pretty much. Part of the day you may be exercising and burning at a high rate, and part of the day you may be doing normal daily stuff and burning at a moderate rate. Fitbit can catch all of that. MFP cannot.

So essentially, MFP assumes you have the same level of activity 24-7. I will use an example...

Based on my stats, activity level: MFP expects me to burn about 1800 daily which works out to about 75/hr. MFP cannot calculate that I will burn lower some hours and higher others. So each time I sync, MFP takes the actual calorie burn according to Fitbit, and compares that to the 75 per hour it expected. Early in the day, I will almost always be behind. Why? Because while I was sleeping I was not burning 75/hr. It was more like 50/hr which is my BMR.

Imagine this 'daily' schedule:

6am: wake up, sync. Fitbit shows 300 burned. MFP expected 450 so I'm -150
6am-7am: doing stuff around the house. At 7am my total is 375. MFP expected 525, so I'm still -150.
7am-8am: go for a run. At 8am, Fitbit shows my calorie burn total is 775. MFP expected 600 so now I'm +175.
8am-9pm: misc stuff thru my day; working, errands, etc. At 9pm Fitbit says I am at 1850. MFP expected 1575, so I'm +275.
9pm I go to bed. From 9pm-midnight, I burn BMR but MFP expected me to burn 75/hr rather than 50/hr. So when I sync tomorrow: Fitbit will show my total burn to be 2000. So the adjustment ends at +200.

That last bit, in italics, is also why your Fitbit adjustment goes down overnight. Unless you're up & moving around until midnight, your last bit of the day will not be as active as MFP expects. The adjustment is greater if your 'last sync' happens earlier in the day. SUch as if you sync at 6pm and then sit on the couch all night/go to bed early. THen the adjustment will be for 6 hours of low activity. Also, the higher your 'activity level' setting at MFP: the greater the adjustment per hour.

So what to make of it? If you have a good idea of what your end of the day burn will be, rely on that. Know your Fitbit adjustment in MFP will vary thru the day but you'll know what it should end around. If your daily burns vary, then you can either figure out the adjustment per hour OR plan to leave 150-250 Fitbit calories 'on the table' to make up for it. If you are not happy with the Fitbit adjustment, use that as motivation to move more and earn more calories.