exercise stretching
If you're in the habit of eating breakfast before exercising in the morning, you may want to reconsider the order in which you start your day as there are significant benefits to exercising in a fasted state.

A common belief is that you need to eat breakfast to optimize exercise performance. While there's evidence to support this stance,1 other evidence suggests you can reap important health benefits by exercising in a fasted state.

Fasted Exercise Curbs Food Intake and Improves Cognition

Research2,3 published in the August 2019 issue of The Journal of Nutrition found that skipping breakfast before exercise helps curb food intake for the remainder of the day, resulting in an overall energy deficit — in this case averaging 400 calories per day.

Earlier research,4 published in 2015, found that women who skipped breakfast and worked out on an empty stomach had better working memory in the mid-afternoon and reported less mental fatigue and tension later in the day than those who ate breakfast (in this case a cereal-based meal) before exercising.

Fasted Exercise Boosts Fat Loss

Fasted exercise has also been shown to be particularly helpful for fat loss — it essentially forces your body to shed fat. The reason for this is because your body's fat burning processes are controlled by your sympathetic nervous system (SNS), and your SNS is activated by exercise and lack of food.

The combination of fasting and exercising maximizes the impact of cellular factors and catalysts (cyclic AMP and AMP kinases) that force the breakdown of fat and glycogen for energy. A 2012 study5 confirmed that aerobic training in a fasted state lowered both total body weight and body fat percentage, while exercising in a fed state decreased body weight only.

Fasted Exercise Rejuvenates Your Muscles

Exercise and fasting together also yields acute oxidative stress which, paradoxically, benefits your muscle. A 2015 study6 in the journal Biomolecules explains:
"Since the discovery of exercise-induced oxidative stress several decades ago, evidence has accumulated that ROS [reactive oxygen species] produced during exercise also have positive effects by influencing cellular processes that lead to increased expression of antioxidants.
These molecules are particularly elevated in regularly exercising muscle to prevent the negative effects of ROS by neutralizing the free radicals. In addition, ROS also seem to be involved in the exercise-induced adaptation of the muscle phenotype."
In "The Exercise Mistake Which Makes You Age Faster," Ori Hofmekler,7 fitness expert and author of several books, including Unlock Your Muscle Gene and The 7 Principles of Stress, addresses this issue as well, explaining that acute states of oxidative stress are:
" ... essential for keeping your muscle machinery tuned. Technically, acute oxidative stress makes your muscle increasingly resilient to oxidative stress; it stimulates glutathione and SOD [superoxide dismutase, the first antioxidant mobilized by your cells for defense] production in your mitochondria along with increased muscular capacity to utilize energy, generate force and resist fatigue.
Simply put, exercise and fasting yield acute oxidative stress, which keeps your muscles' mitochondria, neuro-motors, and fibers intact. Hence, exercise and fasting help counteract all the main determinants of muscle aging."
Hofmekler also points out that, combined, exercise and fasting "trigger a mechanism that recycles and rejuvenates your brain and muscle tissues." The mechanism he refers to is the triggering of genes and growth factors such as brain-derived neurotropic factor (BDNF) and myogenic regulatory factors (MRFs).

BDNF controls neurogenesis, signaling your brain stem cells to convert into new neurons,8 while MRFs are instrumental in muscle development and regeneration.9 In other words, fasted exercise may actually help keep your brain, neuromotors and muscle fibers biologically young.

Fasted Exercise Improves Glucose and Insulin Parameters

Most recently, a study10 published in the October 2019 issue of The Journal of Clinical Endocrinology and Metabolism found that the timing of your meal when exercising impacts "acute metabolic responses to exercise." In other words, when you eat will impact your body's responses to your workout, for better or worse.

The study included an acute randomized crossover trial and a six-week randomized training trial involving overweight and/or obese men. In the acute trial, the researchers compared the effects of eating a balanced breakfast before versus after moderate-intensity cycling.

In the training trial, which lasted six weeks, they assessed the impact of eating a carbohydrate-rich breakfast either before or after training. Results showed:

- Exercising before eating a balanced (mixed micronutrient) breakfast resulted in higher intramuscular fat utilization in Type 1 and Type 2 muscle fibers. Stored intramuscular fat is thought to play a role in insulin resistance,11 so this finding suggests fasted exercise improves your insulin sensitivity.

- Exercising before eating a high-carb breakfast resulted in improved glucose sensitivity and lower insulin levels after eating.

- Exercising before breakfast also improved remodeling of skeletal muscle phospholipids and the protein content of the glucose transport protein (GLUT4), which are embedded in your cell membranes and facilitate glucose entry into the cell.

As a principal mediator of glucose uptake by your muscle, GLUT4 helps maintain glucose homeostasis (balance) in your body.12 By improving the ability of your muscle to store glucose as glycogen (which is then used to produce energy), GLUT4 helps improve your glucose tolerance and lowers insulin resistance,13 thus lowering your risk of Type 2 diabetes.

The finding that exercising while fasting has a beneficial impact on GLUT4 is not new. In a 2010 study,14 those who exercised fasted increased their GLUT4 levels by 28% compared to those who had a carb-rich meal before training, or controls who did not train.

This result was only for the acute impact of fasting exercise. When you exercise fasting, in the long term you also increase autophagy, which facilitates muscle growth. Muscle growth, of course, is enhanced if you are doing resistance training, especially like Blood Flow Resistance Training.

In fact, exercising while you are fasting for more than 14 to 18 hours likely activates as much autophagy as if you were fasting for two to three days. It does this by increasing AMPK, NAD+ and inhibiting mTOR. I personally only eat in a four-hour window and typically start my exercise after fasting for 18 hours.

Additionally, your muscles are the largest sink for glucose in your body. So, if you have more muscle mass you will be able to easily remove it from your blood and store it in your muscles and, as a result, you will have decreased insulin resistance.

In conclusion, the authors of this October 2019 study15 noted that "exercise performed before versus after nutrient intake (i.e., in the fasted state) may exert beneficial effects on lipid utilization and reduce postprandial insulinemia."

Medical News Today quotes16 co-author Dr. Javier Gonzalez saying their findings "suggest that changing the timing of when you eat in relation to when you exercise can bring about profound and positive changes to your overall health."

Other Health Benefits of Fasted Exercise

The combined effect of time-restricted feeding and short but intense exercise may also:
  • Boost your growth hormone production17,18
  • Improve your body composition (as described above, it specifically helps shed fat and not just overall body weight19)
  • Boost your cognitive function20
  • Boost your testosterone level21,22
  • Help prevent depression23,24
Importantly, fasting also helps boost the generation of new stem cells — cells that can be used to heal and regenerate your tissues. Regeneration can be further boosted by doing strength training the morning of the day when you're planning to break your fast.

Tissue regeneration occurs during the refeeding phase. That's when your body starts rebuilding and replacing all those damaged cells that were cleared out during the fasting (autophagy) phase.

The reason why fasted strength training can further boost tissue regeneration is because during fasting, your growth hormone level skyrockets25,26 — rising as much as 300% for a five-day fast27 — and growth hormone activates genes involved in the healing of damaged tissue.28

So, fasting can in some ways be likened to getting a growth hormone injection and a stem cell transplant, and by incorporating strength training at the right time, just before refeeding, you optimize all these regenerative benefits.

What to Eat After Exercise

All right, so you've implemented a time-restricted eating schedule in which you're eating all your meals within a six- to eight-hour timeframe each day (which means you're fasting for the remaining 16 to 18 hours), and you're exercising in the morning, before your first meal.

The question now becomes, just what should you eat when breaking your fast, having just exercised? This question is most important when you're strength training or doing high-intensity exercise, as your muscles need specific nutrients at this time.

The most effective way to optimize muscle building and repair is to follow your resistance training with a high-protein meal, leucine-rich whey being one of the most efficient. Leucine is also important for the prevention of sarcopenia (age-related muscle loss), as it helps regulate the turnover of protein in your muscle. As explained in a more recent study,29 published in 2017:
"Protein ingestion produces a strong anabolic stimulus that elevates muscle protein synthesis. The ability of a serving of protein to stimulate muscle protein synthesis (MPS) is dependent on absorption and blood kinetics of amino acids, amount of protein ingested, and the amino acid composition of the protein source.
Only the essential amino acids (EAA), especially leucine, initiate an immediate increase in MPS. Being a rapidly digested protein with a high leucine content, whey has been shown to stimulate MPS more than equal amounts of casein and soy in the first hours after exercise ...
At the molecular level the mechanistic target of rapamycin complex 1 (mTORC1) and its substrates ... are believed to largely be responsible for the protein synthetic response to resistance exercise and protein intake, with resistance exercise potentiating the effect of protein ingestion."
Leucine-Rich Whey Is an Ideal Post-Exercise Meal

Leucine is a branched-chain amino acid that serves multiple functions, one of which is signaling the mTOR mechanism, which causes protein to be created and builds your muscle.

However, for optimal results you need far higher amounts of leucine than the recommended daily allowance. The reason for this is because most of the leucine is used up as an energy substrate or building block rather than an anabolic agent.

The typical requirement for leucine to maintain body protein is 1 to 3 grams daily. However, to optimize its anabolic pathway, research suggests30 you need somewhere between 8 and 16 grams of leucine per day, in divided doses.31,32

Getting that amount of leucine from your regular diet could be pretty difficult. For example, 4.6 eggs will provide you with 2.5 grams of leucine,33 which means you'd have to eat nearly 15 eggs to reach the 8-gram minimum.

High-quality whey, on the other hand, contains about 10% leucine (10 grams of leucine per 100 grams of protein).34 So, 80 grams of whey protein will give you 8 grams of leucine.

Consuming the whey about one hour after your workout will allow you to first optimize the benefits you get from exercising in a fasted state, and then provide your muscles with the much-needed protein they need to repair and rebuild.

Should You Never Eat Before Exercise?

For all the benefits of fasted exercise, not everyone can exercise without something in their belly. This is especially true for explosive exercises that require significant amounts of muscle glycogen for fuel.

Additionally, some people, especially those that are not yet metabolically flexible, are more sensitive to changes in their blood sugar levels, which can decline during the first 15 to 25 minutes of their workout.

It is this decline in blood sugar that causes dizziness, faintness, nausea or lightheadedness. This is especially true if you exercise first thing in the morning. If you fall into this category, whey protein can also be a beneficial pre-workout breakfast.

A 2010 study35 found that consuming whey protein (20 grams of protein per serving) 30 minutes before resistance training boosts your body's metabolism for as much as 24 hours after your workout.

If you decide to take whey before strength training or high-intensity exercise, take it 30 to 60 minutes beforehand and limit it to small amounts like 10 grams or less, so you are providing amino acids during the exercise but not increasing insulin much until you finish your workout.

You may also want to follow that with a larger dose about an hour after your workout. This will help to activate mTOR and provide substrate for muscle building. It would also be good to add some healthy carbs after your workout as that will further contribute to mTOR activation and muscle growth.

For guidance on how to identify a high-quality whey protein, see "What Benefits Can You Get From Whey Protein?"

Naturally, a number of individual factors can play a role in whether it's appropriate to exercise while fasting, such as your age, when you last ate, whether or not you're pregnant, medication use, your medical history, level of fitness, and the type of workout you engage in.

As a general rule, listen to your body. If working out while fasting makes you feel weak or ill, you'll need to make the appropriate adjustments. Just be mindful of what you're putting into your body, as it can have a distinct impact on the benefits you end up reaping from your workout.

But if you haven't seriously integrated fasting into your exercise program I would strongly encourage you to consider it as it will radically increase the metabolic benefits from all your hard work at no extra cost or effort to you.