One of the most common exercise and nutrition questions we get is, “should I eat before I workout in the morning?”
As with most diet and exercise advice, the answer to this question is - it depends. It depends on many factors! Just do a simple google search for “fasted cardio and fat loss” and you will find many different options, conflicting information and like anything else - leave you wondering what you should do... Should you eat before a workout in the morning or not? In short, it depends on how you feel and what serves you personally.
But what does the research say? Let's unpack some of the pros and cons of fasted exercise, commonly referred to as fasted cardio, to help you decide for yourself.
To clarify - fasted cardio is doing any sort of cardio or conditioning workout in a fasted state. A fasted state is one in which you have completely digested and absorbed your last meal/snack and your insulin levels are at a low or baseline level. We know that digesting our food takes several hours so we are talking about performing fasted exercise in the morning prior to eating anything, which is how most research studies are conducted as the participants have not eaten for 8-12 hours.
Some of you may be thinking why would anyone eat before their early morning workout? OR just the opposite, how do people perform their best in the gym in a fasted state? Again, the answer is - it depends on the person, their health, their goals, and how they feel in their workouts. Personally, I see both sides and while I workout around 5:30/6 am, I don’t always eat before my workout. I go by feel and listen to my body’s cues. If I wake up starving, I have a small quick absorbing carb before I hit the gym. If I am not hungry, I don’t. Previously, I tested this for myself with 30 minutes of fasted cardio (cycling) for 2 months, and didn’t see any drastic changes.
But the big question is - Does it Help You Burn More Fat?
Some studies claim that training in a fasted state will force your body to burn more fat for energy due to increased fat lipolysis and fat oxidation. However, that is only one small part of the equation. To clarify as we dive into this - fat lipolysis is the breakdown and mobilization of fat that has been stored and usually occurs in fat adipocytes. (source) Fat oxidation is the burning of fat for fuel. Fat oxidation is the breakdown of fatty acids to be burned as energy inside the mitochondria of a cell. (source) If fatty acids are supplied to healthy mitochondria and oxygen is present, fatty acids will be broken down to carbon dioxide.
A small meta-analysis study, looking at 27 studies and 273 participants, concluded that aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state. (source)
However, does it help you burn more fat all day long or see better overall results? Not necessarily.
Another study, including 20 women in a controlled environment, analyzed the difference in fat mass and fat-free mass after four weeks of fasted or fed cardio found that body composition changes were similar regardless of fasting or feeding prior to training. (source) Research has demonstrated that fasted cardio does not increase fat burning over a 24-hour period. While your muscles adapt to using more fat when you exercise, you don’t actually lose more fat overall.
Diving deeper into the research, and even more interestingly, training in fed state impacts respiratory function and excess post-exercise oxygen consumption (EPOC). One study suggests that ingesting carbs before working out increases the post-exercise “afterburn” effect more than the fasted state. (source) That means more calories burned throughout the day, not just during your sweat session. One reason for this may be due to heightened EPOC. Another study analyzed oxygen consumption and respiratory-exchange ratios between fed and fasted athletes. Their findings indicated that oxygen consumption was higher in the group that was fed and the respiratory-exchange ratio was significantly lower, indicating greater lipid utilization. The difference was still significant 24 hours after exercise. The study concluded that when moderate endurance exercise is done to lose body fat, fasting before exercise does not enhance lipid utilization. (source)
So, yes there may be slight fat burning advantages training fasted. However, in the end, it comes down to energy balance over time for weight loss and fat loss.
One thing to be mindful of as you test this out yourself is that some studies have shown that appetite can increase following exercise performed in a fasted state rather than a fed state. So, if calories aren’t controlled this could lead to eating more calories throughout the day.
So again, research is not crystal clear, but there are some valid things to consider, and these are mainly based on personal preference as some people prefer to workout fasted.
One reason you may consider skipping breakfast, or training 4-6 hours after eating, is that eating too close to your workout can cause digestive distress for some individuals. When you start ramping up cardio while you’re still digesting food, your body diverts more blood flow and resources to your muscles instead of your digestive system. And this can sometimes cause an upset stomach or nausea. (source) Of course, it depends on what you ate, how much, and how close to your workout you consumed the food.
Outside of personal preference and digestion woes, there isn't much research indicating any other major benefits of fasted cardio.
If you are strength training or looking to increase your muscle mass, working out in a fasted state may put you at a disadvantage.
Getting a proper pre workout meal can provide you a source of fuel to power your workouts, essentially helping you train harder. (source) Additionally, including a source of protein before the gym may also help protect some of your lean mass. You can read more about pre and post workout nutrition in our recent blog HERE.
A very important consideration is energy levels and blood sugar balance. If you are low on energy, skipping food before a workout can cause you to have low blood sugar, leaving you feeling dizzy, nauseous, and weak. Not something we want to experience when we are trying to gain muscle and perform at our best!
Lauren Antonucci, registered dietitian nutritionist and certified sports dietitian, and owner of Nutrition Energy, cautions against fasted exercise and for very valid reasons.
“Can I force my body to burn a higher percentage of fat than carbs? Yes, I can. But I think that the more important question is will I see a benefit as an athlete? So far, the answer is no.” We have to consider the long term and for most people, that small change won’t translate into body fat changes and performance benefits.
Research has shown that hitting the weights without fuel can lead to the breakdown of muscle. (source) If you’re running on empty, you’ll go into a major breakdown phase after the workout. When you wake up in the morning, your body is in a catabolic state due to high levels of cortisol, the stress hormone. That means that your body is primed to break down molecules for energy. But if you’re in this state too long, it can lead to degradation of body tissue, including muscle, and a decrease in overall health. (source)
In conclusion, one of the best ways to decide where you stand regarding fasted cardio is to give it a try yourself! It depends on your personal preference, how you perform in your training, and your recovery. If you are going to perform fasted cardio, you can probably perform low to moderate intensity cardio for up to an hour, or high-intensity interval training for shorter durations, 20-30 minutes, before your muscle glycogen stores, aka energy, starts to run low. However, you may not tolerate high-intensity exercise well or feel as strong in your strength sessions in a fasted state. Give it a try and see how your body responds.
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