Many people turn to fasted cardio because it sounds simple. You don't have to do more exercise or cut any calories or even take supplements. You just change the time you exercise and lose fat faster.
Well, not so fast. If you dig into the research on fasted cardio you will find mostly empty promises. Fat burning does increase when you exercise in a fasted state, but it doesn’t appear to result in an overall greater fat loss.
A 2013 study compared the effect of 6 weeks of interval training on a bike in either the fasted or fed state in overweight young women and found that subjects lost the exact same amount of body fat regardless of which group they were in. The women in both groups decreased fat mass by 0.6 kg and increased lean mass by 0.6 kg.
Another study that combined a reduced calorie, higher protein diet with 60 minutes of exercise in either a fed or fasted state found no signiﬁcant difference in fat loss after four weeks. The authors found that fasted cardio is a nice idea that doesn't pan out because it ignores the dynamic nature of the human body.
Why Fasted Cardio Doesn’t Lead To More Fat Loss?
You will burn more fat during fasted exercise, but total calorie expenditure will be lower causing the body to compensate by reducing the amount of fat used for fuel later during recovery. When losing body fat, you must look at how many total calories are burned, if the total expenditure doesn’t result in a caloric deficit then, in the long-term, the body will make adaptations and body fat will not be lost.
In one study, subjects did a 36-minute cardio workout fasted or after having breakfast. Although fat burning, as measured by respiratory exchange ratio, was greater during the workout in the fasted condition, by the 12-hour mark after training the results evened out. At the 12-hour mark, fat burning was much higher in the group that had breakfast than in the group that did not and remained signiﬁcantly higher through 24 hours.
How eating pre-workout affects the amount of calories burned.
Following both mid- and high-intensity exercise, people who ate pre-workout consistently burned more calories during the several hours following, known as the recovery period. Scientists explain this as the body’s metabolic adaptation. It senses a lack of food combined with exercise as dangerous therefore reducing total energy expenditure.
Lack of food may compromise quality of workouts.
While your average gym goer who may be walking on the treadmill for an hour won’t be affected, an athlete who has goals and progressions to adhere to, this won’t be the smartest idea. One way to help avoid the fall off from lack of energy is to supplement with caffeine pre-workout, but still would be best to consume food prior to workout.
Its negative affect on cortisol, the stress hormone.
Cortisol, a hormone that releases energy, is highest in the morning after overnight fasting. Cortisol levels will remain elevated and may even increase when breakfast is skipped. Couple that with cardio, which also increases cortisol output, and you may have highly elevated cortisol levels leading to negative effects on anxiety, fat loss, and health.
Making fasted cardio a long-term habit may lead to muscle loss because high cortisol levels causes the body to break down tissue in order to keep blood sugar steady. These short term effects may be diminished by consuming higher amounts of protein, but you are likely to lose muscle in the long-term.
It’s more often applied short term, any health benefits there?
A recent study, using a walking protocol preceded by an overnight fast, found an increase in gene signaling that is associated with better blood sugar regulation and insulin levels. In light of the diabetes epidemic and all the problems people are having with poor insulin sensitivity, this could be a game changer by improving metabolic health.
One should also consider that post-exercise your muscles are hungry for glucose, your insulin sensitivity is maximized, and nutrients are absorbed better. Assuming your workout isn’t compromised too much, an argument could be made that eating post-exercise is a smart option to feed the hungry muscles. Eating before and after is the smartest and most ideal option.
Improved metabolic ﬂexibility is a possible 2nd benefit.
It is known that the overweight population lacks the necessary enzymes to burn fat effectively. This inability to switch between using glucose and fat for energy (metabolic flexibility) makes a high-carb diet necessary for them to sustain blood sugar and energy levels. This high-carb diet has negative metabolic consequences leading to their failure to have lasting fat loss. Improved metabolic flexibility is essentially forced when training fasted as the body is required to use fat for fuel. Remember though, that total calories burned is what is most important for fat loss and eating pre-workout showed to be more effective in that regard.
Fasted cardio when cutting?
Now we are to the one spot in which fasted cardio has some validity. Fasted cardio can work well for men with body fat in the low single digits (5-6 percent) and females with body fat in the low teens (13-14 percent), especially if they have specific problem areas like the lower back or thighs.
Once people drop the majority of their total body fat, fasted cardio seems to work well on resistant or stubborn areas. Although there's no direct data to reference, it might be that when a person only has a small amount of fat lingering in hard-to-attack areas, exercising in a fasted state could spark those resistant fat cells to release stored fat so it can be burned for fuel.
It is important to understand the three steps the body undergoes when reducing body fat:
1) Mobilization of fatty acids. Mobilization is the process by which fat is released from fat cells. If insulin is high, you can't get the fat out of the cells, and it won’t be burned.
2) Transportation of fatty acids. Transportation is the movement of fatty acids within the blood stream so that they can be burned.
3) Oxidation (or burning) of fatty acids. Oxidation is the actual burning of fat and it only occurs after fatty acids are mobilized and transported to target tissues such as muscle, liver, and heart.
In very lean individuals, the body experiences adaptations to help it hold on to fat stores, particularly in the abdominal and upper thigh area. Blood flow to these regions is often reduced, decreasing mobilization and transportation of fatty acids. In this situation, fasted cardio might be worthwhile since it has been shown to increase blood flow to the abdominal and upper leg area, which could result in a favorable decrease in these stubborn fat stores.
The Bottom Line:
Fasted cardio is a tool that can be used in speciﬁc situations for a select period of time. It may be worth it in the following cases:
You are fairly lean and are trying to lose stubborn body fat. Use fasted training as a time to buckle down and give it all you've got. Don’t turn it into a long-tenn habit, because this will only lead to poor quality workouts, elevated cortisol, and muscle loss.
You’ve already established an exercise habit and want to mix things up. Trying something new is one of the best ways to kickstart fat loss results. Think of fasted training as a short-term fix (2 to 3 weeks) that may have metabolic benefits.
You use it selectively to boost insulin sensitivity and get more out of a lower calorie intake (due to increased absorption of nutrients). For example, you could train fasted on a periodized plan for a few weeks and then go back to eating pre-workout. Or you could do cardio or sprint workouts fasted, while eating prior to weight workouts. Instead of adopting fasted exercise as a iong-tenn habit, use it when you need it.
Avoid training on an empty stomach in these situations:
Your main goal is to increase muscle mass. If you're priority is muscle, you shouldn’t be doing cardio to begin with because it interferes with muscle building pathways.
You’re an athlete training for peak performance. Athletes need to take every advantage for optimal nutrition and recovery in order to elicit the greatest training adaptations.
Stress and high cortisol are a concern for you. Skipping meals and cardio are two of the worst things you can do if stress and high cortisol are an issue. Pairing them together is a disaster waiting to happen. Play it safe by choosing weight training and smart pre-workout nutrition.