Will I burn fewer calories at the MAF heart rate than if I went higher?
Yes, but it means we will train our bodies to burn more fat calories over time.
In other words, not if we really think about it. Although half an hour of high-intensity interval training (HIIT) will burn more calories in that span of time than MAF training, you are burning primarily sugar, of which your body can only store some 1000-2000 calories at best. But your body has hundreds of thousands of stored fat calories (even for someone with less than 10% body fat). It’s also important to note that training at the MAF heart rate creates very little wear and tear. Ultimately, this means that you can train for much longer, burn many more calories, and on top of that be reasonably confident that you won’t get injured.
That’s why burning fat for fuel is much better for losing weight, as well as for maintaining your new weight.