Fasted Training: Pros and Cons
Should you do fasted training? The answer is, it depends!
Fasted training refers to training on an empty stomach before your first meal of the day. This also applies to training in a low-carb state, such as when you are training on the keto diet (very low carb). For the purpose of this article, I am going to refer to “fasted” as meaning low-carb, and “fed” as meaning carb-fed.
There are pros and cons of training in a fasted / low-carb state. It depends on your goals and what you want to get out of your workout.
The Pros of Fasted Training
The main reason for training on an empty stomach is to teach your body to burn fat for fuel.
The body requires glycogen to fuel the muscles. The body most easily converts carbohydrates to glycogen. This means that carbs are the body’s preferred source of fuel. However in the absence of carbohydrates, the body can break down body fat to create glucose.
By working out on an empty stomach, therefore, the body will first use up the stores of glycogen, and then it will start to break down body fat for energy. This is why fasted training may be beneficial for weight loss.
The Cons of Fasted Training
The main reason against fasted training is that it may hinder your performance.
The process of breaking down fat (“fat oxidation”) requires greater amounts of oxygen. Exercising obviously also requires oxygen. When training in a fasted state, some of the oxygen that would normally go to your muscles will now have to go to breaking down fat. In other words, your performance may decline because fasted training costs you oxygen.
For this reason, the “fat burning” zone is usually low to medium intensity. So, while long, slow workouts may encourage fat burning, they may not challenge you enough to help you get faster or stronger.
It depends on your goals
If your goals is weight loss, then fasted training may help with fat loss, especially if you have a lot of weight to lose.
If your goal is performance, then you may want to train in a carb-fed state, especially if your goal is to preserve or build muscle.
Roberts & Kravitz (2005) recommend that any aerobic exercise session longer than 45 minutes should be supported by carb-fed nutrition to maintain blood glucose levels to preserve muscle mass and avoid muscle breakdown.
Have a plan
It does not have to be one or the other. It is good to have a plan and have different training days for different reasons. You can plan a fasted workout on less intense days to teach the body to use fat for fuel. This might be up to one third of your weekly workouts, or for a set number weeks in order to “lean out”. Train in a fed state on days when you plan more intense sessions, like doing fast or hard cardio, high intensity interval training (HIIT), or heavy lifting.
An alternative to training in a fasted state
Can you train in a fed state and still burn fat? Yes!
There is another way to look at fat burning, especially if your goal is performance.
The alternative is to train intensely for maximum Excess Post-exercise Oxygen Consumption (EPOC), otherwise known as “exercise afterburn”. The harder the better. “Intensity drives afterburn. The harder you work, the greater the EPOC”, says Dr. Len Kravtiz, PhD, University of New Mexico. “You don’t have to make your workouts longer, but make them harder. You are going to burn more later”.
EPOC is the extra oxygen that your body has to consume to bring several physiological variables back to pre-exercise levels. Knab et al (2011) demonstrated that EPOC was significantly elevated for 14 hours following 45 minutes of vigorous exercise. This corresponded to an additional 190 calories burned to bring the body back to pre-exercise levels. And here is the thing: In scientific terms they would say “fuel utilization favours fat oxidation” or, that it resulted in “enhanced post-workout fat oxidation”. What that means is, those calories burned in EPOC are 100% fat burning!
When you exercise with intensity, whether it is HIIT or hard resistance training, or resistance interval training, it is not the calories burned DUING the workout that makes the difference, but the calories burned AFTER the workout. Your body burns fat most efficiently when a lot of oxygen is available, like when you are at rest or sleeping.
In addition, over time, if you are building muscle, you will raise your basal metabolic rate (BMR) which means your body will be burning more calories at rest just to maintain itself. Your BMR is the largest percentage of daily calories requirements.
It is good to train your body to burn fat in a fasted state. It is also good to push your performance limits by training in a fed state. And when you train hard and with intensity, the EPOC will continue to burn fat after the workout, your BMR will improve so that you will be burning more calories all the time. Mix it up. Do some cardio. Do strength training. Most of all, do some movement you enjoy and be healthy.