Metabolism + fueling during a workout

Author: Cate Ward, PhD, RD Imagine this: A runner out for a training run; a longer run than usual this time. They’ve been feeling great thus far, and even remembered to eat a pre-run banana to help power their movement. They’re gliding through steady miles until, suddenly, their legs start to feel heavy, each leg turnover slower than the last. They’ve run out of fuel. “Hitting the wall” or “bonking” are common terms used to describe running out of readily available energy during a workout, typically one of a long endurance format. Zooming in view to the skeletal muscle, and then even further in to the cells in that tissue, there are several metabolic processes that are occurring here simultaneously at the molecular level. Each of these pathways transforming energy substrate, such as digested food and stores kept in the body, to the useable form to power muscle. So why does this happen when we run? While running, energy stores break down to fuel each muscle movement. These stores include blood sugar (thanks, pre-run banana), glycogen stores (sinks of chained glucose), and fat. Each of these fuel source substrates breaks down to the body’s useable energy currency for muscle movement. Glucose breaks down more rapidly than fat, thus is the preferred source of energy during a workout. “Hitting the wall” is the point at which the athlete’s body has run out of available glycogen stores, thus rapidly available energy. At this point, it becomes harder to push the muscles as fast as previously in the run, albeit not impossible, since fat oxidation contributes energy (though at a slower rate). The runner slogs on to finish the run, moving noticeably slower than before, and feeling miserable by the finish. Let’s break down the metabolism of these three types of fuel during exercise — glucose, glycogen, and fat — down even further. Broadly, metabolism can be defined as all of the chemical processes that occur within the body to maintain life. Pertaining to food, metabolism not only occurs at the macronutrient level — the breaking down of carbohydrates, fat, and protein — but at the micronutrient level as well — where chemical processes transform the compounds in foods eaten into active co-factors in countless reactions. Focusing on macronutrient metabolism during exercise for now, broadly, complex carbohydrates are broken down into simple sugars, lipids are digested into species ready to be used by the body, and proteins are first stripped of their 3D structure into linear amino acid chains and then further digested into smaller peptides and single amino acids. How are these metabolic processes tapped into and altered during exercise? Let’s dive in.

  1. In the early stages of a workout, the body draws from available glucose stores. An athlete can’t use glucose — a basic sugar found in carbohydrates and fruit — in its entirety as an energy source. It needs to be broken down, and during this breakdown, releases adenosine triphosphate (ATP), the energy currency in the body. After eating, glucose from food is absorbed into the blood stream where it is transported to tissues of need (for example, muscles during exercise), or stored as glycogen stores and fat if there is excess. During exercise, when local ATP in the muscle has been depleted, glucose is transported to the muscle and broken down to harvest more ATP and thus power. Through intra-workout fueling, athletes can maintain blood glucose levels without tapping into backup fuel as heavily, thus prolonging the length of time they can continue to exert energy without “hitting the wall”.
  2. During exercise, glycogen stores provide additional glucose. Glycogen is a multi-branched chain made up of glucose. When the body needs more energy in between meals, while sleeping, or during exercise, glycogen is broken down to provide glucose and subsequently ATP. Glycogen is stored in both the liver and muscle, and intra-workout the bonds that chain glucose together to form glycogen are broken to release some ATP, and then more ATP is released again when glucose itself is broken down. On average, the body stores about 1800 kilocalories of energy in the form of glycogen. “Hitting the wall” and “bonking” are expressions used to indicate the depletion of glycogen stores, and thus, the end of readily available glucose for energy. The sink of backup glucose — and thus fact acting energy — has been emptied. Once this occurs, feelings of lethargy typically ensue, and it can be difficult to finish a workout after reaching this state.


  1. Later into the workout, fat stores oxidize to provide fuel. Fat is another storage sink in the body, and provides energy through beta-oxidation (or fatty acid oxidation). Pulling from fat stores allows for delayed depletion of glycogen stores, leading to prolonged ability to sustain exercise. Fatty acid oxidation takes place in mitochondria of skeletal muscle after lipids in the form of triglycerides have been transported to the site. Triglycerides — a species of lipid — are transported from dietary fat (for example, nuts, oil, avocado, and butter) and an individual’s adipose stores. Some pre-existing local intramuscular triglycerides exist locally as well. Breaking down of these lipids in the mitochondria of cells provides ATP in an aerobic setting, with maximal lipid oxidation occurring between 45–65% VO2max. Thus, during aerobic exercise, athletes burn fat which contributes to overall energy available to power muscle movement. However, fatty acid oxidation alone cannot keep up with powering rapid muscle movements, and thus is better paired with a faster acting glucose store (such as glucose from the blood or glycogen) to keep up with the speed of an athlete completing more intense exercise. Fatty acid oxidation typically provides a higher ratio of fuel for lower intensity, aerobic exercises, such as walking or hiking, whereas higher intensity aerobic workouts such as running or cycling, draw from a higher ratio of glucose energy sources. Fuel source percentages also shift during the duration of a workout. The body tends to pull from glucose and glycogen stores first before tapping into fat stores — thus endurance events tend to burn higher percentages of fat stores, especially towards the tail end of a workout.


What do we do with this information? Use it to time our fueling. To optimize food intake to fuel a workout, it is important to take into account the exercise type, duration, and previous food intake (not just immediately before exercising, but the whole day and even day before). What could our runner do differently to prevent their energy crash? Fueling during a long format workout keeps glycogen stores from being depleted for longer; glucose from food is absorbed in the intestines and shuttled through the blood stream to muscles in need. Many athletes struggle to properly fuel during endurance sports, thus don’t tap into their full performance potential. Fueling during a workout is tough; time, amount, and type are each important, and can be difficult variables to balance in effort to find the right formula. Each athlete is unique in their tolerance and needs of fuel during a workout, making this problem even more difficult. To add to this struggle, fueling before and after the run is just as, if not more, important. The food athletes eat before and after a workout contribute to ensuring glycogen stores are at full capacity before embarking on an endurance event, aid in muscle repair, growth, and recovery, and keep blood sugar stable to fight both high and low swings which can lead athletes to feel lethargic, hangry, irritable — the list goes on. Again, time, amount, and type of food pre- and post- workout are important to maximize both gains and recovery. Duration, type, and effort of the workout influence ideal timing, amount, and type of food to be used for fueling. For example, a pre-run banana helps athletes of all varieties harness an additional boost of energy in the early stages of a workout. However, if this workout is of longer format, the glucose harvested from that banana runs out; this is where additional fuel mid-workout is important. Post-workout, carbohydrates (which provide glucose to replenish depleted glycogen stores) and protein (to aid in muscle repair and recovery) are both key parts of an ideal recovery meal or snack. In respect to each pre- during, and post- workout fueling, questions regarding food include: How much? What types? At what times? The answer isn’t so general. Each individual is unique in their exact needs, thus these answers will be unique to each athlete. benefic nutrition helps athletes assess and reach their proper fueling needs to optimize performance, recovery, and everything in between. Using an individual’s specific workout type, duration, and effort (thanks the smartwatch or tracker so many athletes use to train), benefic nutrition provides athletes with specific fueling plans, without needing to break out the kitchen scale or calculator. benefic nutrition taps into refueling according to how an individual’s body is burning their available energy sources — glucose, glycogen, and fat — and provides complimentary goals to maximize individual fueling and performance.

Circling back to the our runner; what could they have done differently? The runner 1) may have needed to eat more over the course of the day (or day before) to ensure full glycogen stores prior to the run and optimal muscle recovery, 2) could have fueled during the run with readily available carbohydrates for fast energy, and/or 3) may not be replenishing their workouts with appropriate servings of carbohydrate and protein to optimize recovery — the list goes on. benefic nutrition keeps athletes on track with intake of ideal foods at the ideal times to optimize fueling and performance. And not just athletic performance. Fueling during a workout helps athletes feel less fatigued after a longer format exercise, and less ravenous in the pantry, leading to better recovery.