Cardio Heart Rate to Burn Fat and Keep Muscle

If you use heart rate zones to track your exercise intensity, here's good news: No amount of cardio is going to directly reduce your muscle. After all, the heart is a muscle too. But if you're not eating appropriately, too much cardio may indirectly create a calorie deficit that costs you muscle.

To understand the optimal heart rate for weight loss without also losing muscle, you do need to understand target heart rate zones — but first, dig in for a quick recap of how your body develops new muscle and, also, what might cause it to lose muscle.

Muscle growth isn't magic. Instead, it's the direct result of microscopic damage inside the muscle caused by resistance-training activities. Your muscle growth can also result from metabolic fatigue — basically, working your muscles until they temporarily run out of the fuel they need to contract — although as the American Council on Exercise notes, scientists still aren't entirely sure which, if either, of these mechanisms plays a greater role in prompting muscular hypertrophy.

Once that damage is done, your body responds by rebuilding your muscle fibers during the post-workout rest period. This is when your muscles get both bigger and stronger. However, this can only happen if you're taking in enough protein for your body to have the amino acids it needs to create that new, improved muscle mass.

According to a position statement released in a June 2017 issue of the Journal of the International Society of Sports Nutrition, a daily protein intake of 1.4 grams to 2.0 grams per kilogram of body weight is enough for most exercisers to maintain or build muscle mass. That number is based on the assumption that you're eating enough calories to maintain your body weight, perhaps even with a bit of excess to encourage muscle growth.

However, if you're trying to lose fat, you need to be working with a calorie deficit. In other words, you'll be burning more calories than you take in, so that your body is forced to make up the difference by using your stored body fat as fuel. In that case, the ISSN notes that an increased daily protein intake may be necessary to maintain muscle mass, and recommends a range of 2.3 to 3.1 g per kg of body weight.

Here's a quick recap of what will help you build muscle — in a perfect world, these would be your dos:

1. Eating enough calories to maintain your body weight, or a slight excess
2. Adequate protein intake, paired with an overall healthy diet
3. Regular resistance-training exercises

Here are the behaviors that encourage your body to lose muscle in an unhealthy way. These are your clear don'ts:

1. A drastic cut in calorie intake
2. Inadequate nutrition (especially a lack of protein, but other nutrients matter too)
3. No resistance-training stimulus

If you want to lose body fat, you do need to burn more calories than you take in — which means you're going to invalidate "do" number 1. But you can make up for that by tackling the increased protein intake recommended by the ISSN, being diligent in strength-training all your major muscle groups at least twice a week, and maintaining a slow and steady rate of weight loss so that your body doesn't have to break down your muscles to keep its essential processes going.

Need a target to aim for? The Centers for Disease Control and Prevention recommends losing 1 to 2 pounds per week as the ideal weight loss rate for keeping the weight off. That means establishing an overall caloric deficit of 3,500 to 7,000 calories per week, or 500 to 1,000 calories per day, while meeting all the other conditions from your list of "dos."

Read more: How to Gain Muscle Mass at Home Fast

If you noticed, heart rate zones weren't mentioned in either the "dos" or the "don'ts." That's because there is no compilation of scientific proof that your body must be worked at a specific heart rate zone to preserve muscle. There is, however, something that used to be dubbed the "fat burning" heart rate zone, and it's only a small leap from that terminology to thinking that hitting that heart rate target is the key to burning fat but not muscle.

That very understandable — yet mistaken — leap of logic is part of why you don't see the term "fat burning zone" bandied about so frequently nowadays. As long as you're operating at a caloric deficit, increased physical activity at any intensity will help you burn excess body fat. What used to be called the fat burning zone is now more objectively referred to as the target heart rate for moderate-intensity exercise.

There's a concrete definition for what that means, of course. According to the American Heart Association, working out at roughly 50 to 70 percent of your maximum heart rate qualifies as moderate-intensity exercise, while vigorous intensity means between about 70 to 85 percent of your maximum heart rate. You can estimate your maximum heart rate with a simple chart that's also offered by the AHA, or using one of several formulas explained by the American Council on Exercise.

What does that have to do with your quest for the best cardio to lose body fat and keep muscle? In terms of heart rate, not much, except that for most healthy individuals, the higher heart rate zones correspond with greater exercise intensity — which, in turn, means more calories burned.

And that brings you right back to your list of dos and don'ts: If you want to retain muscle mass at the same time as you lose body fat, you need to make sure your nutrition — and in particular, your protein intake — is consistently on point. You also need to aim for that modest, sustainable weight loss rate of no more than 1 to 2 pounds per week. And finally, keep up your strength-training workouts, both for their health benefits and for their muscle-building stimulus.

It might be tempting to aim for faster weight loss, but that makes it harder to not only maintain your muscle mass, but also to keep the weight off in the long run. And if you're diligent about preserving your lean muscle as you lose weight, there's a bonus: That extra muscle boosts your metabolism, helping you burn calories even faster.

Read more: Target Heart Rates in High-Intensity Interval Training