Fueling and Training Strategies to Improve Metabolic Efficiency and Flexibility

Fueling for Metabolic Efficiency 

Most athletes assume that they need to eat a ketogenic or low-carb diet in order to be more metabolically efficient. This is not necessarily true. You don’t need to become “more efficient” by restricting carbs overall. There are other strategies that you can try that don't involve an overly restrictive diet. This can include the use of periodized nutrition strategies in combination with training to improve metabolic efficiency. This might involve a period of time in which an athlete limits carbohydrates during a low-intensity base phase of training. Another example might be combining an exercise approach with a temporary lower carb intake to help "train" the body to be better at utilizing fat for fuel. You can time carbs to support quality sessions while training the metabolism with a lower carb approach on other days.

Simple approach:

• Fuel hard workouts and long runs and workouts aggressively with carbs

• Keep easy runs lower-carb (light breakfast or none)

• Stage carbs pre-long run to simulate race conditions

This builds both fat adaptation and carbohydrate utilization capacity (you need both for a fast marathon or triathlon).

Advanced approaches:

• Incorporate a lower carb approach during the "off-season" or "base-training" phase of training. Then transition to the simple approach above to maintain the adaptation during the preparation phase of training when you are actively training for your next race

• Cyclic low carb approach could possibly be used during the preparation phase of training. This is where an athlete is generally following a low carb approach but is eating strategically before, during and after harder or longer training sessions or an athlete is eating lower carb earlier in the week and then eating high carb around the long run or hard training sessions later in the week. This involves more of a daily or weekly cyclic approach with carbs. This involves careful planning and thought in order for the athlete to meet their energy needs and prevent low energy availability which can negatively impact performance.

  

Fueling Practices That Support Metabolic Flexibility

Metabolic flexibility isn’t about restricting carbs; it’s about timing them.

-High-carb before high-intensity days

→ supports quality, VO₂max, tempo, and MP runs.

-Moderate/low-carb before easy days

→ trains fat use without compromising training.

-Practicing race-day fueling on long MP runs

→ trains the gut to absorb and use carbs at high flux rates (key for marathoners).

-Consuming adequate protein after sessions

→ enhances mitochondrial adaptations

How Runners Train Metabolic Efficiency

You train it through a combination of specific workouts, long-run structure, and fueling habits.

The first thing to remember is that we automatically train metabolic efficiency during our base phase and through many hours of low intensity aerobic volume (aka zone 2 training). In other words, your biggest metabolic improvements come from consistent easy and steady mileage.

Benefits:

• Increases mitochondrial density → better fat burning

• Improves capillary density → better oxygen delivery

• Lowers the cost of running at MP

What to do: Run 70–80% of your runs at an easy pace/zone 2

Steady-State Runs (The Sweet Spot)

These are runs between easy and tempo: Roughly marathon pace + 20–40 seconds per mile.

This is often considered the goldilocks zone for improving fat oxidation. However, many runners are told not to spend time in the this zone and it is often referred to as the grey zone. This is frustrating because so many positive benefits can be gained when running in this zone. The key here is that you are not spending all of your time in this zone. Doing one moderate run in this zone once a week can be helpful for many marathon runners. Another example of this would be do start in an easy zone-2, then gradually progress to this zone (for roughly 20-30 minutes), then finish the run with the last 10 minutes at goal marathon pace.

Examples:

• 6–10 miles @ steady (MP + 20–40 seconds)

• 60–90 minutes moderate aerobic

• Progressive run starting in zone 2 and gradually getting faster

These directly increase the intensity at which you burn fat efficiently.

BOTH Easy & steady running (60–75% VO₂ max) increases:

• The amount of mitochondria in our cells which helps our cells be able to burn energy more quickly and more efficiently (greater production of ATP)

• Increases capillary density which can enhance the delivery of oxygen and nutrients to muscle fibers while accelerating the removal of metabolic waste products like lactate

• Fat oxidation- along with increases in mitochondria our body can also utilize fat more efficiently

• Enzyme systems for aerobic metabolism- there may be in increase in the amount of enzymes used for aerobic metabolism and

Training at your easy zone-2 pace or steady state pace is not the only thing that helps to improve metabolic efficiency. You also want to include:

Fasted or Low-Glycogen Runs (Use with Caution)

There is always a risk to low glycogen runs or fasted runs. Low glycogen availability can be very stressful on the body so this is not a great strategy to implement on a regular basis. Running occasionally with lower glycogen levels forces the body to use more fat, improving metabolic flexibility.

Effective method

s:

• Morning easy runs before breakfast (lasting 60 minutes or less)

• Long runs without carbs for the first 60–90 min (then fuel normally)

• Back-to-back training days where the second session is low-carb

Never combine this with high-intensity days or long MP blocks—it should be used on easy aerobic days only. Doing fasted high-intensity workouts can set an athlete up for risk of injury and illness ad increase the risk of low energy availability. Remember the ultimate goal is to improve performance and running while fasted for quality workouts is not a badge of honor.

Long Runs with Progression or Marathon-Pace Blocks

These uniquely improve your ability to burn fat at higher intensities. This is not to be used in combination with fasting. You would fuel normally for these runs. The benefit comes from the fact that you are pushing your body to run for longer durations and thus are forced to use BOTH carbohydrate and fat for fuel. The body adapts by becoming more efficient at using both fuel sources.

Examples:

• 20 miles steady (not slow, just controlled)

• 16–18 miles with 8–12 miles at MP

• 22 miles progressive (last 4–6 strong, not fast)

These shift your fat-to-carbohydrate crossover point to a faster pace.

Strength Training

Muscles that are stronger and more fatigue-resistant and when you are more fatigue-resistant you can keep good form for longer periods of time. When our running form starts to deteriorate, we start to loose our efficiency. This means that the body is burning more energy. When our muscles are more fatigue-resistant they can:

• use less glycogen at any given pace

• produce more power at the same energetic cost

• sustain good form late in the race

For most runners 2 short to moderate sessions per week is enough.

Focus on the muscles you need for running:

• Glutes

• Hamstrings

• Calves

• Core

• Eccentric loading (downhill runs, eccentric calf raises, low tempo squats)

How Endurance Athletes Train Metabolic Flexibility

Many of the same strategies for improving metabolic efficiency can improve metabolic flexibility. This includes zone-2 and steady state running. However, here are a few additional training options to help improve metabolic flexibility:

Tempo and Lactate-Threshold Training

While it is true that lactate threshold runs rely more on carbohydrate for fuel, they do have some very important benefits, including:

• Increase carb-handling capacity- this means that the body can utilize carbs for fuel at a slightly faster rate.

• Improve lactate clearance. Lactate is produced during higher intensity exercise when oxygen supply can't meet energy demands. When this happens lactate starts to build in the muscle. The faster the body can clear lactate (used by slow twitch muscle fibers or sent to the liver to be re-synthesized into glucose), the longer we can maintain higher intensity exercise.

• Help the body switch back to fat efficiently after the effort- when we go from higher intensity exercise to lower intensity exercise- our body can clear lactate more efficiently and switch from burning more carbs to burning more fatty acids.

• Raise the ceiling for marathon pace. This means we can run a faster marathon pace at the same effort.

  
  

Example sessions:

• 20–40 min continuous tempo

• 2×3 miles @ half marathon pace

• Cruise intervals (6×1 mile at lactate threshold pace with 1-2 min jog recovery between effort)

This trains the :switching" mechanism between fat and carb burning and helps improve lactate clearance.

Train high and sleep low strategy

As I mentioned already, occasionally training with lower glycogen increases fat use and metabolic signaling, but doing it too often worsens performance.

In addition to trying fasted easy runs and fuel late long runs you can try Back-to-back hard training days or do a hard session with adequate carbs followed by a period of low carb intake and an easier run at lower carb availability or decreased glycogen stores. Some athletes will do a hard training run or interval workout in the afternoon/evening. This run will be well fueled with plenty of carbohydrate prior to the run to help improve performance. There is no carbohydrate intake during the run and the athlete will prioritize a high protein, low carb dinner meal. Essentially the athlete will be low carb after the workout and will wake up and do a fasted easy run of 60-90 minutes (no more than 90 minutes!). The morning run will be on very low glycogen availability since the evening workout would deplete glycogen stores and the low carb dinner and fasting overnight would deplete glycogen stores even more. These types of workouts should be done sparingly, but can be a powerful tool. Keep in mind that easy morning run will likely not feel great.

Another option is to do two runs in one day with the first run carb-supported, second run easier with reduced carbs. This means after the first run, the athlete will eat lower carb all day and not fuel with carbs prior to the second run. This could involve a well-fueled morning tempo run followed by an easy 60 minute evening run on low carb availability.

Avoid doing low-glycogen sessions on:

• Tempo days

• Interval days

• Long marathon training block days

Used correctly, these improve the range of intensities where fat contributes meaningfully.

Final thoughts:

A well planned fueling and training plan can incorporate all of these different elements in a way that is unique to the athlete's individual needs. When implemented correctly, the athlete should be able to notice an improvement in their performance and an increased ability to run longer and faster without hitting the wall or running out of energy. A fueling plan that incorporate elements such as fasted runs or low glycogen available runs as well as high carb fueling days will provide both the benefit of enhancing metabolic efficiency while also training the gut and allowing the athlete to be able to tolerate more fuel during their race, when it matters the most. The key here is to enhance both the ability to utilize fat for fuel at higher intensities AND be able to digest and utilize more carbs. This means that the athlete can spare glycogen, enhance fuel or carbohydrate utilization, and clear lactate faster. All of these adaptations can increase the ability to run at a slightly higher intensity for longer duration (such as for a marathon, half ironman, ultra distance even or ironman).