When most people think of fat loss, the first thing that comes to mind is endless hours of walking on a treadmill. Articles in popular magazines have advocated aerobic training in the so-called, “fat burning zone” over and over again, making it seem impossible to lose fat any other way.
Although there is some truth to the notion that aerobic exercise burns fat, a deeper understanding of energy metabolism, energy balance and adaptations to exercise can help clear things up.
1. Thermodynamics and Energy Balance
First things first, the primary criterion for fat loss is a sustained energy imbalance between energy intake from food and energy expenditure. Since the human body operates on the same laws of thermodynamics as does the rest of the universe, the only way to burn fat is to expend more energy than we take in from food. 
As you know, or are about to find out, energy intake comes from:
• protein (4 kcal per 1 gram)
• carbohydrates (4 kcal per 1 gram)
• fats (9 kcal per 1 gram)
• and alcohol (7 kcal per 1 gram). 
Energy expenditure includes:
• our basal metabolic rate (BMR), that is, the amount of energy required to maintain the body’s main functions,
• the thermic effect of food (TEF), that is, the calories expended in processing the food we eat,
• the thermic effect of activity (TEA), that is, the energy expended during physical activity in the form of exercise
• and non-exercise activity thermogenesis (NEAT), that is, the energy burned from activities other than formal exercise. 
2. Weight Loss vs Fat Loss
So you’re now thinking: “Great, I’ll just eat fewer calories than I burn, lay back and get shredded!” Well… hold on just a minute. If an individual’s energy intake is consistently less than their energy expenditure, they will be losing body mass. This, however, does not mean that they will only be losing fat, as the body can also break down proteins from muscle tissue to provide energy. 
Thankfully, there’s a couple of things we can do to minimize lean body mass (LBM) loss. These include:
• Providing a strength training stimulus so that your body is “convinced” that you need to keep your muscle tissue (see “use it or lose it principle”). 
• Eating enough calories and protein so that you give your body the nutrients it needs to recover from training and maintain muscular strength and size. 
Now, since this is a “is cardio essential for fat loss?” article and not a “how to maintain LBM during a cut” article, I am not going to go into detail here. I am just going to, quickly, give some general training and nutrition recommendations.
3. Resistance Training for Lean Body Mass Retention
For resistance training, these are some general guidelines taken from a recent study by Eric Helms and his colleagues :
• Exercises: emphasis on multi-joint with some single joint exercises as needed
• Frequency: 2+ times per week for each muscle group
• Load : 70-80% of 1RM for 6-12 repetitions
• Volume: 40-70 repetitions per muscle group per session
• Rest periods: 1-3 minutes
• Tempo: 1-2 seconds concentric, 2-3 seconds eccentric
It’s important to note that the above are only general recommendations and are aimed for trainees preparing to take part in natural bodybuilding competitions. This means that you may need to change some things depending on your training age, individual goals, past or present injuries, training volume tolerance, personal preferences and so on.
As the almighty Sir Alan Aragon once wrote, “Honoring personal preference is one of the most powerful, yet underrated, tactics for achieving optimal health and body composition.” 
More importantly, if you are a complete novice, I would advise that you refrain from designing your own training routine altogether. Either hire a good trainer to do it for you, or follow one of the many excellent novice routines out there. John and Russell’s TNF Novice Routine, Bryan Haycock’s HST and Allpro’s Simple Beginner Routine are good ones that pop up in my mind.
4. Nutrition for Lean Body Mass Retention
With regards to nutrition, eat in a small to moderate energy deficit, aiming to lose 0.5-1% of your body weight per week, with 2.3-3.1g of protein per kg of lean body mass, 15-30% of total calories from fats, and filling the remainder of your calories with carbohydrates .
Don’t forget to include a few servings of fruits and vegetables every day, sufficient dietary fiber (10-15g per 1000 calories consumed) and plenty of water.
The above numbers can, of course, be adapted to individual preferences and depending upon how your body responds. As the almighty Sir Alan Aragon once wrote, “Honoring personal preference is one of the most powerful, yet underrated, tactics for achieving optimal health and body composition.” 
5. What About Cardio, Then?
Well, since I’ve bored you to death by going over the basics of fat loss and how you can minimize muscle loss during your cut (you’re welcome), let’s talk about cardio. As with most things in the fitness industry, there are two (extremist) opposing camps on the subject of cardio.
The “no cardio at all” camp and the “at least an hour of cardio per day” camp. And as is usually the case, both of them are wrong, with the truth being somewhere in the middle. Let’s see why, then, shall we?
As it turns out, aerobic exercise is a valuable tool in your health and fitness journey for three main reasons:
• It is critical for good cardiorespiratory health. After all, efficiently taking in oxygen to the lungs and blood and pumping it to the rest of the body (including the brain) is a pretty big deal (if you enjoy staying alive, that is).
• It results in a number of adaptations that increase the muscles’ ability to use fat as a source of energy. These adaptations include an increase in the oxygen-carrying capacity of the blood, increased capillarization around muscles, higher levels of enzymes involved in energy production, and an increase in the mitochondrial density of slow twitch muscle fibers.
• It can burn quite a few calories, helping to increase energy expenditure, which makes it easier to achieve a calorie deficit without the need to vastly
reduce energy intake. 
“Put simply, doing cardio is good for your health, it “teaches” your body to use fat for fuel and allows you to eat more while still losing weight.”
6. Short Duration, High Intensity vs Long Duration, Low Intensity
So you’re now thinking “great! I’ll start doing some cardio, then!” However, that fitness author you really like said that you should only be doing High-Intensity Interval Training (HIIT) for 20 minutes three times per week, while a fitness expert on Facebook said that you should always train in the “fat burning zone” for an hour a day. Who should you believe? Neither of them.
Here’s the deal. On the one hand, the “fat burning zone” is a myth in that the percentage of substrate utilization during exercise is not what dictates beneficial long-term body composition changes.  In the grand scheme of things, how much fat you burn during exercise is not what matters.
What does, however, the matter is maintaining an energy deficit in the long-term (have I stressed this enough yet?). So worrying about how much body fat you’ll burn during your cardio session is exactly what we mean by saying that you’re “missing the forest for the trees”.
On the other hand, we have HIIT, and while many trainers out there are quick to prescribe it to everyone who wants to lean out, you should know that HIIT is not easy to recover from and is hard as hell to perform.
Yes, it’s a time efficient way to do your cardio (a short HIIT session can provide similar adaptations to a long, low-intensity cardio session) and may result in less interference with your strength training routine , but 3 sessions of true HIIT per week on top of 4 days of lifting weights, an energy-restricted diet and very low levels of body fat may be overdoing it. So if you do decide to give HIIT a go, start conservatively with one session per week and see how it goes before adding more sessions.
7. Fasted Cardio
And while you are trying to “digest” all this information I’m shoving down your throat, you remember that the shredded dude in your gym once told you that the best way to do your cardio is first thing in the morning, while you’ve still fasted from the night before.
Why? Because the levels of glucose in your bloodstream are low and your body will need to look for energy in your body fat stores, so you’ll end up burning more fat during your cardio session.
If you are just trying to lean down and you find HIIT to be too taxing on your body or you just don’t like doing it, good ol’ low-moderate intensity cardio will serve you just fine.
And that makes sense. Or not. Because you still remember what you read just a minute ago in an awesome article on the internet, which said that “…the percentage of substrate utilization during exercise is not what dictates beneficial long-term body composition changes.” And this is why fasted cardio doesn’t seem to provide any benefit over fed cardio (unless you consider the possibility of muscle loss a benefit, that is). 
8. So, Which Type and How Much?
Let’s move on to the recommendations, then. Since we know that both short-duration higher intensity and long-duration lower intensity aerobic exercise can result in similar adaptations , this means that you can do your cardio at varying intensities and durations, depending on individual training status, personal preferences and goals.
If you are just trying to lean down and you find HIIT to be too taxing on your body or you just don’t like doing it, good ol’ low-moderate intensity cardio will serve you just fine. You will, of course, need to devote more time to doing cardio to get the same benefits as you would with a fraction of the time doing HIIT, but you’ll just have to live with that.
If on the other hand, you find lower intensity, longer duration cardio to be boring and you’d rather just do HIIT for a few minutes and get it over with (provided you can recover from doing it), then great! Remember, personal preference is key. Doing something you like (or hate less) means you are much more likely to stick with it.
How much of it should you be doing, though? Helms and his colleagues recommend that you do enough to maintain sufficient rates of fat loss, but not so much that it interferes with your recovery from strength training . On the other hand, the ACSM recommends at least 30 minutes of moderate intensity cardio five times per week in order to develop and maintain cardiorespiratory fitness. 
We should note that the recommendations by Helms et al are aimed towards those preparing to compete in natural bodybuilding contests and are looking to get extremely lean while preserving muscle mass, while ACSM’s guidelines are for the general population who have their cardiovascular health as a priority (supposedly).
Personally, I like Lyle McDonald’s recommendations and would prefer to see everyone, whether trying to lose fat or build muscle, do at least 20-30 minutes of low-moderate intensity cardio three times per week. This should help maintain a basic level of cardiorespiratory health, burn a few calories and help keep some of the adaptations that increase the body’s ability to use fat as fuel, while not being so much that it interferes with their recovery from strength training. 
So there you have it. Although cardio is not “essential” in order to lose fat, (in fact, such a statement would inherently suggest a lack of understanding of the basic concept of energy balance) it can surely be a useful tool in your weight loss journey.
By helping you to maintain good cardiorespiratory health, “teaching” your body how to use fat for energy and contributing to increased energy output, dismissing it altogether (as some coaches do) is just plain silly, as is mindlessly doing hours of it every day.
So combine your choice of aerobic exercise at the correct intensity and duration with a good nutritional strategy and a solid training regimen designed to maintain strength and LBM while losing fat, and you are on your way to shredzville!