Intermittent Fasting and High Intensity Exercise

Intermittent Fasting (IF) is a popular and highly effective new method of eating that has been scientifically proven to lead to weight loss. By simply changing the way you eat—consuming food in a 2, 4, 6, or 8-hour window instead of all day long—you can encourage better fat and calorie metabolism. Best of all, it's not a calorie- or food-restrictive diet that causes you to suffer. All you have to do is fit your food intake into a shorter period, and you can eat all your favorite foods.

There are many ways to try Intermittent Fasting, and one of the IF plans includes a 5:2 plan: 5 days of normal eating, and 2 days of consuming just 25% of your daily calorie requirements. You are able to structure your week however you want, so you aren't restricted to certain fasting days. For those who have a busy, changing schedule, this fasting plan makes it easy to fit your low-calorie days on your less active days of the week.

But with the low calorie consumption comes an interesting question: will you have enough energy to work out? Specifically, will you be able to continue performing high intensity exercise on the days when you're consuming 25% of your daily energy requirements?

How High Intensity Interval Training Works

Intermittent Fasting and High Intensity Exercise

High Intensity Interval Training (HIIT) involves intervals of low-intensity, steady state exercise (walking, jogging, slow cycling, etc.) mixed with intervals of high intensity exercise (sprinting, fast-paced cycling, etc.). This alternating between low and high intensity keeps your body guessing, forcing it to burn through stored fats and glucose to have sufficient energy for the high-intensity periods. The result of a HIIT workout is a higher metabolism, and that high lasts for hours after you finish working out (rather than the short-term effect caused by steady state aerobics).

HIIT also encourages better muscle-building, as the high intensity exercise breaks down muscle tissue and produces lactic acid. The anaerobic condition promoted by the HIIT training leads to a catabolic state, and thereby helping you to increase muscle mass even while performing exercise aimed at boosting your cardiovascular function.

Will Intermittent Fasting Clash with HIIT?

There's the million dollar question, isn't it? High Intensity Interval Training burns through A LOT of calories at a time—up to 400 calories in just 15 to 20 minutes. During the intervals of full-on/high intensity, your body uses up a lot of the stored energy. The average person won't have enough energy to properly perform a HIIT workout for more than 20 to 30 minutes at a time.

So if you're burning through 400 calories in a workout but only consuming 600 calories in the entire day, surely you're going to run out of energy quickly!

The answer: yes and no!

The human body is highly adaptable. During the days when you are consuming a lot of calories, it is squirreling away energy for the low-calorie days. It stores all that energy in the form of fat, glucose, and ATP energy.

So if you hit the gym for a HIIT workout on the days when you're eating just 600 calories, the exercise will force your body to access that stored energy. It will burn through the glucose and ATP energy first, then tap into fat cells for a slower, longer-lasting energy source.

However, be warned: you won't have as much energy as you would in "normal" (regular eating) conditions. You will run out of available energy quickly, and fat-based energy isn't as efficient for HIIT exercise. You may find that your HIIT workout will be shorter (10 to 15 minutes) before you run out of steam.

Take the Day Off!

The beauty of the 5:2 diet is that you can pick and choose the days you want to fast on. No one works out 7 days a week, so you can schedule one of the fast days on your "Rest Day". Even if you do a light stretching workout, you'll have enough energy to complete the training session.

And what of the other fast day? Well, if you find yourself running out of energy midway through your workout, it may be a good indication that your body is still acclimating to the HIIT exercise, the fasting diet, or both. Give yourself time to adapt, and consider taking a day off (or low-intensity day) on your second fast day. The diet is going to be doing most of the work in terms of weight loss/fat-burning. The HIIT exercise is just the added bonus, and you can afford to rest and give your body time to adapt to the diet.

How to Rock Your HIIT Workout on Fasting Days

If you're the sort of person who wants to work out anyways, even in a low-energy state, there are a few tips that will help you get through your HIIT workout:

  • Do it first thing in the morning. This way, your energy reserves will be topped off from the previous day's eating, so you'll get through your workout as if it was a normal day. You may run out of energy later on in the day (thank God for power naps!), but at least you'll get your HIIT workout out of the way.
  • Lower the intensity. If you're doing a full-on sprint training workout, consider going a bit lighter. You can work at 85% intensity instead of 100%, and you'll still see the weight loss/fat-burning benefits.
  • Do it shortly after eating. Try to fit most of your 600 calories into one large meal, then get in your workout 60 to 90 minutes after that meal. Follow up your workout with the last of your calories to replenish lost energy. You'll have a much easier time getting through the training session and the rest of your day even on fasting days!

HIIT workouts can be a highly effective option for weight loss and fat burning. Mixed with the Intermittent Fasting diet, it could be exactly what you need to see serious results. It's all about understanding how your body works and learning to adapt to its daily demands!