III. Evidence for Fasted Cardio Increasing Fat Loss
IV. Opposing Arguments
V. Differences in Fat Utilization Between Men and Women
VI. Dr. O's Thoughts
The debate about fasted cardio is truly a lightning rod. If you aren’t familiar with this term, fasted (or morning) cardio is a low-moderate intensity cardio session before breakfast. The belief is that by doing cardio before breakfast, you can burn more body fat for energy due to depleted glycogen stores from your fast (a.k.a. 6-8 hours of sleep) and increase your metabolic rate for the day. However, there are some in the fitness community that don’t believe in this strategy. In their opinion, it doesn’t matter if you use fat or carbohydrates for energy. As long as you burn more calories than you consume in a day, you will lose fat. Furthermore, other strategies, such as high-intensity interval training (HIIT) are more effective methods for fat loss. Also, doing this exercise in a glycogen-depleted state can result in loss of muscle. It’s an interesting argument that has been ongoing for several decades. And there’s evidence on both sides of the fence. Does it work? Is it a myth? Let’s discuss both sides. This is, by no means, an exhaustive summary or review, but you’ll get the main idea.
The conversion of fat stores into energy is also referred to as lipid or fatty acid metabolism. So how does this work exactly? Well, let’s focus on our situation: fasted cardio. When you wake up in the morning, your glycogen stores are low (especially if you stopped eating carbs around 7-9p the previous night). When your available glycogen is low and energy is needed, enzymes such as hormone-sensitive lipase (HSL) or adipose triglyceride lipase are activated to convert triglycerides (stored in adipose tissue or within muscles) into free fatty acids (FFAs), which become available in plasma. This process is known as lipolysis. Through oxidation, these FFAs can then be broken down and used for energy. With that background knowledge, let’s take a look at some studies investigating how the body uses fat as an energy source in a fasted state.
In a study by Horowitz et al. (1997), six healthy men cycled for 60 min at a constant low/moderate intensity (45% VO2max) after an overnight fast or 1 hr after ingesting 0.8g/kg of glucose or fructose. After 20-30 min of exercise, both fat lipolysis and fat oxidation were higher in the fasted group than in the glucose or fructose group). This trend continued after 50-60 min of exercise. There was also a higher quantity of FFAs available in the blood in the fasted stated throughout the exercise. Statistical analysis comparing fat oxidation within each group also showed significantly greater fat oxidation following an overnight fast. But are these FFAs really coming from adipose tissue (preferred) or within muscles?
A study conducted by Punyadeera et al. (2005) may shed some light on this. Ten active male subjects participated in two trials. In each trial, the subjects cycled for 2 hours at 50% VO2max following an overnight fast. However, in one trial, subjects were orally administered with 500 mg Acipimox, a drug that prevents the release of fatty acids from adipose tissue. Acipimox was administered in two doses: 1) 90 min before exercise initiation and 2) 75 min into the exercise. Blood FFAs and fat oxidation were also measured throughout the cycling period. Within 5 minutes of exercise initiation, the concentration of available FFAs was significantly lower in the Acipimox group. This decrease was also coupled with a decline in fat oxidation.
These studies suggest that conducting low/moderate intensity exercise in a fasted stated (i.e. – carbohydrate-depleted) results in greater adipose tissue-derived lipolysis and great fat oxidation.
One thing to remember is that most people who conduct fasted cardio in the morning will have another workout later that day. To that point, studies have shown that repeating a 60 minute exercise at 50% VO2max after 1 hour of recovery results in increased plasma FFA and epinephrine levels after the second workout . Most people probably wouldn’t have another workout so soon after the initial one, but this study does show a potential alternative reason for the perceived benefit of fasted cardio over time.
Another opposing argument is that it is better to maximize fat oxidation after the exercise, rather than spending endless hours burning fat during exercise with steady, low-moderate intensity exercise…and that can be done through high-intensity interval training.
An alternative study showed that the intake of carbohydrate-rich meals before and after a high-intensity, exhaustive workout maximized the use of fat for energy after the workout, while the carbohydrates were being used to restore glycogen . Eight male endurance athletes ate a carbohydrate rich, high-GI meal about 2 hours before completing a 90 min exhaustive cycling exercise. The exercise protocol included “cycling at 75 % VO2max for 20 min, followed by alternating 2 min bouts of 90 % and 50 % of VO2max for 4–5 intervals, then decreasing intensity to 80 % and 50 % VO2max 2 min bouts for another 4–5 intervals, and finishing with 2 min 70 % and 50 % VO2max bouts until exhaustion”. Following exercise, subjects ate 3 meals at 1, 4, and 7 hours post-exercise. Each meal provided 65–70 % of energy from carbs, 20 % from fat, and 10–15 % from protein, with an average glycemic index (GI) of 60–65.
Results showed that in the first 7 hrs post-exercise, muscle glycogen and fat oxidation increased, while intramuscular triglyceride (IMTG) levels remained constant. This means that for at least 7 hours after exercise, there was an increase in fat oxidation from plasma FFAs and an increase in glycogen storage within muscles. Even with the high intake of mid-high GI carbs before and after workout, fat was still being burned. So where are these plasma FFAs coming from? They aren’t coming from within muscle because IMTG levels remained constant. So, one can assume that adipose tissue lipolysis is contributing to available FFAs.
But…hold on! These were MALE athletes. Does the same response occur in women?
Maybe so! A study by Trapp et al. (2008) compared 2 15-week exercise protocols in women: 1) HIIT – 5 minute warm up, 8 s of all-out pedaling at a set resistance, followed by 12 s of slow pedaling at 20-30 rpm (60 intervals or until failure), 5 minute cool down and 2) Steady-state – 5 min warm-up, pedaling at 60% VO2max (up to 40 minutes or until failure), 5 min cool down. HIIT resulted in greater loss of total body fat, insulin and leptin concentrations. More specifically, women from the HIIT group lost more abdominal fat and lower body fat (trunk and legs), while gaining lean lower body mass. Women in the steady state group actually gained fat on their lower body.
Let's dig into this difference between men and women a little bit. As you might expect, men and women utilize fat differently both during and after exercise. A study by Henderson et al. (2007) investigated the difference in post-recovery lipolysis and oxidation. On back to back days, men and women conducted exercise for 90 min at 45% VO2max and for 60 min at 65% VO2max. For both sexes, FFA availability increased during exercise at both intensities (more at 45% than 65%). However, FFA availability remained elevated 3 hours post-workout in men, but not women. Total lipid oxidation was elevated in both men and women 3 hours post-workout on day 1. However, oxidation levels were higher in men. Also, oxidation levels in men remained elevated on day 2, while levels in women did not. This difference in fat utilization between men and women may be a reason why strategies like fasted cardio and HIIT may still be beneficial.
I can honestly say that doing cardio before breakfast has DEFINITELY increased my metabolism throughout the day. But is that because of my fasted state, or just because I’m doing cardio early in the day? Could I still feel that increased metabolism after eating a small meal? I don’t know, I haven’t tried. But it’s possible. And sometimes, if I feel good in the morning, I’ll do HIIT. So, there’s another contributing factor.
What about my increased loss of fat from fasted cardio?? Is it because of my fasted state? Or is it because of the addition of a second workout in the evening, which could include resistance training, HIIT, and/or low intensity cardio? I can’t say. Now you see why it is so difficult to get a "yes" or "no" answer.
I’d have to place fasting cardio in the “Everybody’s body is different” category. For those who can drop fat easily, fasting cardio might not be necessary. But for others that struggle to burn fat in those stubborn areas, fasting cardio may be beneficial. Also, with the differences in fat utilization in women, it may be more beneficial for the ladies. And by integrating HIIT, both men and women can maximize fat loss by shocking the body with these strategies. Plus, there are MANY other factors that play a role in lipolysis and fat oxidation (BMI, inflammation throughout the body, cortisol, insulin, leptin, catecholamine production, growth hormone, just to name a few).
So if you have the time in the morning, give it a try! I'd still recommend doing HIIT as well! I've just found fasted cardio to be a nice way to keep the fat coming off while recovering from HIIT. Have fun!
"I don't live to eat...I eat to live!"