Principles of Aerobic Foundation Training - An overview of preparing to go long!

Introduction

The human body utilizes a combination of fuels to produce energy during exercise.  This fuel mixture consists mainly of carbohydrate, fat, and a very small quantity of amino acids (protein).  Carbohydrate provides the necessary, and constant, kindling for energy so carbohydrate “sparing” during exercise is a key attribute for an endurance athlete.  In this case, the now familiar term “metabolic efficiency” refers to the degree to which fat is utilized as a fuel source at any given intensity during exercise.  I prefer to call it the Aerobic Foundation.  Therefore, the primary objective of Aerobic Foundation Training is to improve this aspect of metabolism, which is also associated with several cellular adaptations related to better overall health and fitness.  You’ll often see this referred to as “Zone 2” training, as further outlined in detail in this discussion with Peter Attia and Iñigo San Millán here.

Diving a little deeper, mobilizing more fat to produce energy can be beneficial for several reasons: fat burns more cleanly, thereby reducing metabolic “emissions;” fat has a greater energy density than carbohydrate; and burning fat for energy spares limited stores of carbohydrate, resulting in better endurance or stamina (see FIG 1). Another benefit from improved fat metabolism to endurance athletes is less dependence on exogenous sources of carbohydrate during training and competition, therefore reducing the likelihood of gastric distress occurring in long-duration events.  On the flipside, an athlete gets more speed or power at a specific rate of carbohydrate ingestion!  Most relevant, and from a broader perspective that includes overall health & wellness, improving fat metabolism: increases the ability to reduce excess body fat mass and maintain lean body mass, reduce fasting blood glucose, and blood cholesterols, and improve the anti-inflammatory response. 

Training overview

Plenty of well-trained and novice athletes alike (based on 16 years of experience working in the lab) have, or start out with, a high capacity for carbohydrate metabolism, in addition to concerns over increased body fat mass, stress and aches & pains, due to training “hard” all the time and having a carb-dependent nutrition regimen.  For these reasons, I like to employ true aerobic foundation training to reboot the system each year (or throughout an entire season), let athletes get some recovery and refresh their nutrition & training habits.  Through exercise testing, the intensity where the greatest capacity for fat burning (Fat Max) is measured and assigned (see FIG 1), this is the maximal fat-burning zone (MFZ).  The MFZ is the primary training zone – but not the only zone for optimal benefit!  To optimize this type of training, consider a visit to the lab or field test and let’s identify your MFZ, VO2max, and other applicable benchmarks. 

Having presented the case for better fat metabolism for overall health and endurance performance/capacity, it is important to recognize that as the metabolic pathways for fat burning improve, the resulting diminished ability to metabolize carbohydrate will decrease anaerobic capacity (see FIG 1).  In other words, excluding high intensity training can squash an athlete’s sprint/high-output capacity.  Here lies a cost-benefit scenario to burning more fat for muscle contraction.  Continued low intensity can also diminish an athlete’s VO2max and economy, to mention only a few of the outcomes.  The balance between training these two systems is dependent on the athlete’s age, gender, and objectives.  For example, an Ironman-distance triathlete/road or gravel cyclist would benefit tremendously by training the aerobic pathways because the discipline involves 7 or more hours of predominantly steady-state exercise.  So too would a 45+ year-old athlete who wants to get back to fitness, lose a few pounds of fat mass and shake off a nagging knee injury. By contrast, a short- or middle-distance runner/mountain bike/cyclocross rider would necessitate a balance of higher-intensity training to stimulate the glycolytic (carbohydrate burning) pathways for more speed and explosiveness.  

How this plays out in the weekly training sessions

Many of us are not training to win sprints; more likely individuals want to be healthier and sometimes a bit lighter (which typically translates to better performances anyway).  However, even though we strive to home in on training for health and stamina during training, we must also keep the glycolytic system stimulated as a key adaptation, which is why we continue to add some targeted higher intensity work.   For these reasons you’ll see a variety of “tempo” intervals and cadence variations (in cycling) with the occasional short-duration/short-rest VO2max-esque intervals mixed into each week.

FIG 1.  A professional athlete’s metabolic profile changes with adaptation to a training regimen with most of the training in the MFZ and a lower frequency of high-intensity (above the AT) training.  Changes from TEST 1(dashed lines) to TEST 2 (solid lines) represent an increase in fat metabolism with decrease in carbohydrate burn, note the 20watt increase at the anaerobic threshold (AT) and lower carb requirement at higher power.

Associated nutrition guidelines

A 6-to-8-week training period where most of the exercise is done in the maximal fat-burning zone (MFZ) plus some additional “spice,” can uptick fat metabolism.  The training is only half of the overall benefit; therefore, this phase must include a nutrition regimen that includes a reduction in refined carbohydrate and an increase in fruits, vegetables, and a bit of whole grains to help improve fat metabolism and reduce body fat mass.  This does not mean someone has to go “keto” or starve themselves of carbohydrates!

Plenty of good things happen when exercise is paired to nutrition, but if too much carbohydrate is cut out of the diet (especially without proper supervision) nutrient intake can drop as well as exercise capacity, so I like to express this as replacing one source of carbohydrate for the other rather than use the term “cut carbs.”  The duration of this type of training and the level of compliance to the nutrition guidelines depends on the goals of the athlete and the aerobic level of fitness of the athlete coming into the program.

Fueling

Hydration – water & electrolytes - is still the overriding focus during & post training. Even if it's a relatively "easy" training session, you still need to top-up with carbohydrates post session because, as mentioned before, there’s a constant underlying carbohydrate burn as kindling (see FIG 1), so recovery drinks are still a good idea.  There is a reduced need to take gels, Chews & Bloks during a session (unless it’s a later afternoon session and someone hasn’t eaten all day).  Taking in those high-octane carbohydrates during the training can shift the body’s burn to carbohydrates from fat, defeating the purpose of the session, so these types of fueling aids should be minimal to none (long rides outside are a different story!). In some of the morning training sessions people eat bars & other carb-based snacks (lower-octane than gels, etc.) but that’s okay because it might also be their breakfast for lack of appetite so early in the morning!

Conclusion

In summary, achieving the capacity and durability to participate and/or compete in events such as an Ironman or half-Ironman, requires acute attention to frequent exercise in the lower intensity domain.  However, targeted sessions that stretch us past that domain (but not too much) are required, too!  An understanding of these different energy systems is key to training them accordingly so that an athlete has well-balanced fitness, is adequately fueled, healthy, and mentally ready for event day! 

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