For today’s edition of Dear Mark, I’m going to be answering questions about Maximum Aerobic Function, or MAF. If this is your first time hearing the term, MAF refers to a method of endurance training that maximizes the function of your fat-burning aerobic system. I’ve come down hard on conventional or popular modes of endurance training in the past for being too stressful and reliant on sugar. MAF training is the opposite: low stress and reliance on body fat.
Let’s dive right in to the questions:
What is MAF training?
MAF trains your aerobic fat-burning system to be more efficient and produce greater output at the same “intensity.” It means slowing the hell down to go faster. It means the slower you go, the more fat you’re burning and the better your mitochondria are getting at utilizing fat for energy. It means training up to but not over your maximum aerobic heart rate.
MAF was coined by Phil Maffetone, who came up with an ingenious way to calculate your max aerobic heart rate: subtracting your age from 180. 180 minus your age gives you the heart rate at which you’re burning the maximum amount of fat and minimum amount of sugar.
Say you’re 30 years old. 180 minus 30 is 150. To burn the most fat possible, you maintain a heart rate equal to or lower than 150 BPM. Now, and here’s the trick: It doesn’t sound like much. It doesn’t feel like much. It probably feels way too easy. But bear with me. It works. This is where the magic happens, where you accumulate easy volume, where the “base” is built, where you begin building more fat-burning mitochondria.
The hard truth is that if jogging spikes your heart rate past your aerobic max, you’re not very good at burning fat during exercise. Even if you don’t “mind” pushing that heart rate up. Even if you “feel fine” jogging at 153 bpm. 180 minus age is where you have to be to improve fat burning. That might look like jogging, or walking, or walking uphill, or running pretty briskly, depending on where you’re starting. It’s all relative to your aerobic fitness.
It takes patience to stay at the aerobic zone, but over time, if you’re consistent, you’ll notice that you can handle a higher and higher workload at that same “easy” MAF heart rate. You’ll be going faster while still burning mostly fat—and it’ll still feel easy.
What are the benefits of cardio using MAF training?
In some parts, I’m known as the anti-cardio guy. I coined the phrase “chronic cardio,” and the entire reason I got into this Primal business is that decades of elite endurance training—marathons and triathlons—wrecked my body and drove me to develop and pursue a different, more sustainable path to health and fitness.
But I’m not anti-cardio. In fact, moving frequently at a slow pace in all its incarnations forms the foundation of my Primal Blueprint Fitness philosophy. And MAF is just about the best way to do it.
When you build your aerobic base, you don’t just get better at running (or cycling, or rowing, or swimming, or whatever it is that you’re doing). There are more benefits that aren’t as overtly noticeable:
- You get better at utilizing the fat you eat and the fat you store, paying huge dividends in other areas of your life.
- You get steadier energy levels throughout the day. There’s always that big bolus of energy hanging around, ready to be consumed and converted into ATP. And you’re very good at burning it.
- You have a lower propensity to snack. It’s easier to stick to a healthy way of eating and refrain from snacking when you can cruise along eating your own adipose tissue in between meals.
- You have more mitochondria, and the mitochondria  you have are better at burning fat.1  This is what everything comes down to. Mitochondrial dysfunction and subsequent energy overload lie at the root of many degenerative diseases. The better your mitochondria work, the more energy you can handle, and the less likely you are to suffer the negative ramifications of chronic energy overload.
This seems to confer benefits to longevity. Although we can’t establish causation, moderate exercise—jogging up to 20 miles a week at an 11 minute mile pace—offered the most protection against early mortality in one study. Running more than 20 miles a week, or running at a 7 minute mile pace, offered fewer mortality benefits.2 
Plus, having that large aerobic base helps with any physical pursuit, and not just endurance sports. A large aerobic base helps in CrossFit. A large aerobic base helps in football or martial arts or rock climbing. Whenever you can burn more fat, save more glycogen, and still get the same amount of performance, you’re winning.
When you’re aiming for MAF, how much cardio is too much?
As long as you stay in the MAF zone, it’s very hard to overdo cardio. You’re deriving your energy primarily (90/95%) from fat, a virtually inexhaustible energy source, and very little from carbohydrate. You have thousands of calories at your disposal. Your relative intensity is lower than the person who’s out there burning sugar, so your joints aren’t falling apart and your muscles aren’t getting as fatigued. You’re accumulating less stress overall.
When you start hitting intensities that elevate your heart rate beyond the 180 minus age MAF zone, your tally begins. The stress and joint damage begins to accumulate. You become more reliant on sugar compared to fat. You can still train like this, but your margin for error is a lot smaller.
If I had to put a number to it, I’d say that you shouldn’t burn more than 4000 calories a week  from cardio.
How should you eat while doing maximum aerobic function?
MAF is most effective when paired with carbohydrate restriction. It doesn’t have to be keto reset levels, although that’s a great option. Standard Primal low-carb, staying under 150 grams per day, is good enough.
When you combine MAF training with carb restriction, everything is enhanced. You build more mitochondria after a single carb-restricted MAF training session than after the same session without the carb restriction. 3 
Going low-carb while MAF training also continues the work when you’re at rest. If you burn primarily fat when endurance training but go home to a high-carb diet, you’re squandering a lot of progress.
What if I’m too slow?
One of the most common questions I receive comes from people worried they’re too slow. “I feel like I am going too slow. I can run a 7:00 minute mile no problem at race pace and a higher heart rate, but if I stay at 180 minus age, I can’t get my speed past 10 minute miles.”
You can keep doing the higher HR runs, but you’re not building a base and you may be setting yourself up for damage down the line. That means you are good at burning glucose/glycogen and have a good tolerance for discomfort, but it also means that in this current configuration, you suck at burning fat. The whole point of MAF training is to train at the highest heart rate you can handle (and highest speed) while still getting 90-95% of your energy from fat. Over time, you’ll find that as you get better fat adapted, your mile pace will come down at that same MAF heart rate. That’s the indicator that you are becoming more efficient with your burning of fat over glucose.
Track things over months, not workouts. It may take a long time to improve, but improve you will. Pro tip: if you are a well-trained runner or cyclist, you could probably add 5 to that 180-age number and be OK.
Isn’t my MAF pace way too easy?
It seems way too easy, and that’s the whole point. It’s also where people get tripped up.
You think you can handle a bit more, so you push the HR up. I mean, running at an easy pace couldn’t possibly make you faster.
Over time, you’ll find that as you get better fat adapted, your mile pace will come down at that same MAF heart rate. That’s the indicator that you are becoming more efficient with your burning of fat over glucose.
Just be sure you are always able to carry on a conversation and not get winded as the “guard-rail.”
Folks, that’s MAF training. If you want more details and a specific plan of attack, check out my book Primal Endurance .
If you have any more questions, ask down below! Thanks for reading, everyone.