p90x3 calorie intake

Starvation mode doesn't exist. You're likely retaining water from changing up your workout routine.

That said, 1500 net (after exercise) is the lowest amount you should be eating. What does MFP give you as a goal for 2 pounds a week?

MFP's activity level doesn't include exercise. Upping your activity level is going to do the opposite of what you want it to do.

A few things off the rip, starvation mode isn't a thing, you didn't make any mention of how closely you track your calories and if you use a food scale. It's more likely you have been eating more than you think than that you have any kind of appreciable metabolic adaptation. Any significant change in exercise routine is going to cause glycogen (fluid) retention which can artificially mask weight changes and fluctuations over a two week time can more than account for your expected loss over that period of time.

If you're following the MFP model to the letter, the activity setting is designed to account for routine activity outside of intentional exercise (job functions, daily incidental steps, etc.); you should set that requisite to your daily life absent of exercise and log your exercise accordingly. If you're truly sedentary most of the day (non-active desk job, little activity outside of exercise) but you've adjusted your activity setting to account for exercise and count the calories from your heart rate monitor, you're effectively double-dipping on your active calories which will impact your deficit.

Per a TDEE calculator I trust (tdeecalculator.net), your sedentary TDEE is around 2500-2600 calories assuming you're a male somewhere between 25 and 35 years old. Also assuming the 300-600 calories/workout is accurate the math using averages is as follows:

2550 + 450 = total calories burned = 3000 - 1000 calorie deficit (requisite for 2lbs/week, appropriate for someone with 75+ lbs to lose) = net target of 2000 calories

I'd start with your net calorie target around there, try it for 3-4 weeks and adjust as necessary. If you're not already using a food scale to verify your portions it's recommended you use one to achieve the highest accuracy of logging possible.

twiggy8459 wrote: »

I have been using the polar monitor since it seems to be regarded as one of the best monitors. Should I be using something else?

It's a fine device for measuring your heartbeats.
But heartbeats can't be converted directly to calories. It can manage reasonable estimates for an average person doing steady state cardio if they happen to have an average exercise HR.