Study Spotlight: Fasted Cardio for Better Weight Loss?

By: Andrew Jagim

For a long time the idea of utilizing fasted cardio for better fat burning and weight loss was a common strategy among those seeking to improve body composition.  Body builders and physique competitors are notorious for choosing fasted cardio first thing in the morning to help maximize fat loss.

The rationale behind this strategy is based upon the assumption that when you are fasted, insulin levels are very low which helps you rely more on burning fat as a fuel versus carbohydrates. However, the question has always been, does acute fuel utilization or a greater reliance on fat lead to greater reductions in stored body fat over time. Well, in 2014 Schoenfeld et al. published an article that examined the benefits of doing fasted cardio compared to doing cardio in a fed state on weight loss and changes in body composition. They found that when total calories and macronutrients were controlled (the same between both groups) and both groups completed the same exercise protocol, there were no differences in weight loss or body composition over the course of 4 weeks.  In my opinion, this is kind of what I'd expect to see. In a well controlled environment, when someone is following a regimented diet created to help facilitate weight loss (a 24 hr caloric deficit) and following the same exercise program as someone else but they just happen to skip breakfast before their morning workout....you won't really see any differences long term, as the evidence is pretty clear in that weight loss is primarily determined by calories in vs. calories out and not whether or not you are exercising on an empty stomach.

But what happens when someone isn't really following a specific diet and they are just opting for fasted cardio as a way for them to help manage their weight?  In this study below, I examine maybe a more practical scenario and how fasted cardio may offer some advantages....

Recently an interesting study was published examining some of the acute differences in fuel utilization (over the course of a day) and how completing fasted cardio influenced caloric intake throughout the rest of the day when individuals were not following a regimented diet.

What did they do?

Researchers were trying to determine if there was a difference in rates of fat oxidation (fat burning) during exercise when completed in a fasted versus fed state. They were also trying to determine if fasted cardio had any kind of an impact on 24 hr. fat oxidation, food consumed (ad libitum), blood sugar levels and ratings of hunger throughout the day.  They had 20 male subjects (well trained) complete 2 experimental trials separated by one week. On one day, participants completed a fasted 60 minutes treadmill run at 60% of their VO2 Max. During the other experimental trial, the subjects were first provided with a standardized breakfast 2 hrs prior to their exercise test.  During the exercise trial and throughout the days, subjects were assessed for differences in energy expenditure, fuel utilization, blood glucose levels and hunger between each condition.  The researchers also monitored calorie intake over the next 24 hrs by evaluating the amount of food eaten at lunch (ad libitum at a lunch buffet) and provided all subjects with a bag of food to take home; instructing them to only eat items in the bag and return all uneaten food the next day for later analysis.

What did they find?

Interestingly, the researchers found that total calorie intake over the 24 hr period, and food eaten at dinner, was significantly greater in the group that ate breakfast prior to their exercise test as seen below:

They also found increased rates of fat oxidation (greater fat burning) during the exercise trial in the fasted cardio group. Hunger was significantly lower in the fed group compared to fasted, before exercise, after exercise, and before lunch however blood glucose and hunger did not appear to be associated with energy intake.

Take Home Message:

I have to be honest, I did not anticipate some of these results based upon related reports in the literature.  Based on these results, it definitely appears as though fasted cardio may offer certain benefits in terms of weight management strategies as it appears to reduce ad libitum calorie intake throughout the rest of the day, despite higher rates of hunger being reported.  So if you are someone who struggles to control portion sizes and maintain a certain calorie intake, fasted cardio may help you to manage the amount of calories eaten throughout the rest of the day. Not to mention, by doing fasted cardio you are also "skipping" a meal early in the day which also means you may be more likely to consume less calories throughout the day.  Is fasted cardio a guaranteed strategy to help burn more fat and improve body composition? In my opinion I don't really think so based upon the Schoenfeld article mentioned above but this acute study definitely shows some promise. Also, if you are like me you may not function very well in the morning without food, especially if you have to exercise. It would be interesting to see the long-term version of this ad libitum fasted cardio approach played out.  Until then, do whatever works best for you!

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