Metabolic Flexibility for Athletes – Part 1

Human metabolism is an amazing orchestration of complex signaling and function. The ways in which the metabolism utilizes digested food and nutrients is manyfold. Of particular interest to the Lifetime Athlete is how the body uses the different macronutrients (fat, protein, carbohydrate) to support lifelong health and peak performance. 

For discussion purposes, we can oversimplify the roles of the 3 macros concerning how the body uses them. Fat is a dual purpose macro in that we use it as a building block (brain, nervous tissue, hormones, membranes) and a fuel source (producing energy as ATP). Protein is primarily a building block from which we form bones, muscle, skin, hair, nails, and organs. And carbohydrate is for the most part a fuel source. 

Beyond those basics, macronutrient metabolism begins to get very interesting. I’ll briefly discuss each macro, in the order I previously used.

The ability to use fat for fuel is genetically wired into every human beast. Most of the cells within the body can “burn” fat, as well as related molecules produced by the liver known as ketones. Exceptions to this include red blood cells and certain parts of the brain. This fat-burning ability is akin, particularly at rest and at low-to-moderate exercise intensities dependent upon individual factors, to a clean-burning, high efficiency furnace with a small blue flame and little waste emissions. The body will typically upregulate the ability to use fat for fuel when total food availability is low, and/or when carbohydrate intake is restricted/unavailable. This feature also unlocks our ability to access and utilize stored bodyfat for fuel. Even very lean individuals have a fairly significant onboard fat energy supply. The brilliant human body can also use fat as a secondary source from which to make glucose, by using the glycerol backbone of triglycerides to do so.

Dietary protein is critically necessary for tissue maintenance and repair. And this is true not just in the muscles but in most of the structural elements of the body. Protein isn’t just for bodybuilders. New research indicates that protein requirements are higher than what was once thought, and that athletic individuals may have slightly greater needs. This is perhaps even more important in mature(ing) athletes as the efficiency with which they undergo muscle (and other tissue) protein synthesis is reduced. And, just like with fat, the body can use glucogenic amino acids from protein to make glucose as needed. We can even strip muscle protein from our own bodies to make glucose if absolutely necessary, but this is not desirable and the body treats this function as a last resort.

Carbs are going to get several paragraphs in this portion of the discussion. First of all, carbs in and of themselves are not evil. They are just a food source. Carbs can certainly have a place in a healthy, high performance diet. We just need to approach their consumption, like everything else, intelligently. If we use the ancestral lens to view carbs in the diet, we should first appreciate that humans (Homo habilis-erectus-sapiens) have inhabited the earth for about 2 million years. Current debate rages whether humans were preferential carnivores or opportunistic omnivores but you can probably appreciate that the brilliant (again) human beast is programmed to survive, and possibly even thrive, on a variety of fuel sources. I personally have a bias toward a low(er) carb diet with an emphasis on high-quality animal-based nutrition, but I don’t think that most people need to eat zero carbs to be healthy. I also think that carbs can, and many times should, be used strategically by the athlete.

So it’s true that you can never eat a single carb, just consuming fat and protein, and your body can make all the glucose you’ll ever need for baseline survival. Where we get into trouble in the modern world is that we have become conditioned to eat, and believe strongly in the necessity, of large volumes of highly processed carbs (refined sugars, flours, starches, etc.). This promotes great degrees of glycemic variability, hyperinsulinemia, and strong links to various health and disease-related disorders. Most of these products rapidly turn into glucose and it’s this chronic insult to the metabolism over time that leads to metabolic dysfunction. This is because glucose is used by much of the body (particularly the muscles) as jet fuel. It allows for rapid energy production but this occurs at a cost. Compared to the clean-burning fat-combusting furnace described earlier, sugar-burning looks different from a metabolic perspective. It’s not unlike throwing a bunch of cardboard and pallets onto a bonfire. This results in rapid, high energy production but unfortunately makes a lot of smoke, ash, and mess. In small amounts our bodies are fully equipped to easily deal with this process. But in large, chronic amounts…it tends to overload and pollute the system. 

But some will say “a carb is a carb is a carb” and you might ask “aren’t all carbs the same?” Well, yes and no. What can differentiate one carb from another is how quickly it spikes blood sugar (glycemic index) and how long this measure stays elevated (glycemic load). And of course, there are many other concerns such as what type of micronutrients might be on board with said carb ingestion, are they necessary, and whether there are also antinutrients and toxins potentially present. Those topics deserve consideration but all that would be too doggone long for this post. 

Now let’s move into exercise, training, and sports participation. How the body utilizes fuel to power these movements can be viewed in the context of intensity and duration. If I oversimplify again, I’ll divide this topic into three categories.

Very brief, highly intense movements such as sprinting, explosive lifts, and jumps lasting up to about 10 seconds represent the alactic, or phosphocreatine energy system. Our bodies have this natural, instantaneous and powerful energy-producing ability. This system allows us to go all-out or nearly so, for a few seconds, and then rest/repeat a few times. This process doesn’t require major increases in carb or fat metabolism.

Strong and moderately intense efforts lasting from about 15 seconds to maybe 3 minutes or so, rely heavily on that sugar combustion, or anaerobic, system we discussed earlier. The by-product is the lactate production and breathlessness that will be all too familiar to all you athletes out there. The anaerobic glycolysis, or combustion of stored muscle sugar known as glycogen, is how we sustain high output in this time period.

Prolonged, light-to-moderate (depending on the individual’s fitness level) output lasting beyond several minutes and heading practically toward infinity, is the domain of the aerobic system. This is where our bodies primarily use fat in the presence of oxygen to produce ATP (the energy). 

It’s important to keep in mind that these energy systems are not distinct. They tend to bleed into one another depending upon the intensity and duration of the movement. In a well-tuned human, we are always burning a mix of glucose and fatty acids, even at rest. 

There are two sides to the energy system coin. On the one side, if we eat a very high (and processed) carbohydrate diet, our metabolisms become carb-dependent, and we end up downregulating fat metabolism. The body becomes tricked into learning that carbs are always available, so it sort of “forgets” how to use fat. This reduces the amount of fat we can burn during low-to-moderate intensity exercise, and we become “bonk-prone.” This is because our muscles and liver can only store about 2000 calories worth of glycogen, and that’s only enough fuel for about 2 hours — give or take — of training or competition. A carb-dependent metabolism will blow through the sugar reserves too quickly and then not be able to get to the fat stores…leading to a crash in energy and all the symptoms of hypoglycemia and exhaustion. 

So for any athlete participating in extended training (and anyone who desires maximum health), being able to burn more fat is advantageous because it is glycogen-sparing, preserving the jet fuel for when you really need it. Many athletes will use an immediate pre-workout carb source, or “trickle in” +/- 50g carbs per hour during exercise to spare muscle glycogen. This does indeed work and it’s the one place in the athlete’s diet where simple (as opposed to complex) carbs are probably superior because they are easily digested and the exercising muscles have transporters on the cell membranes which immediately take up the glucose for fuel utilization. This decreases the residence time of the glucose in the bloodstream and offsets some, but not all, of the health concerns with sugar-burning. The truth is though, you’ve got to be going pretty hard, and fairly long, before you really need this intake. An hour workout at the gym or on the trail does not really necessitate pounding a bunch of gels and strapping on several bottles of syrup.

The other side of the energy system coin has to do with how our bodies recover from exercise and refill those glycogen stores. As you are probably well aware, there is quite a bit of controversy and debate among the experts on this topic. There have been some studies, as well as much anecdotal evidence, which show that muscle glycogen can be replenished rapidly and fully on a low-carb diet, especially in those individuals who have been on such a diet for a long term (6-18 months) and are thus highly adapted. However, much of this work has emphasized either very short (the alactic system) bout athletes or very long (aerobic endurance) session trainees. The results become much less conclusive when you start to look at frequent, high-output anaerobic (glycolytic) athletes. These are people who tend to go hard, for moderate to long sessions, multiple days per week…including frequent back-to-back days. Examples include team sports, swimming, CrossFit, and others. These folks generally respond with higher work capacity and performance, and shorter recovery periods between sessions, on diets which include the strategic use of carbs.

It’s essential to use the aforementioned ancestral lens concerning the glycogen stores. These precious energy caches were really designed to be an emergency fuel supply that we could tap into during times of need, such as fleeing a predator or outracing a storm back to the cave. Theoretically, if the glycogen stores were temporarily depleted, primitive people — just like other animals — would then “lay up” and rest until they restored this very important reserve gas tank. Overutilization of this resource, or heading out to hunt dangerous game before the reserves were refilled, was generally assumed to be unwise, risky behavior. However, modern athletes do not face the same realities regarding food acquisition and survival, and thus we can perform glycolytic activity on a more frequent basis, at least to some extent. Repeating the word strategic here, a reasonable amount of carbs can enter into the diet of the healthy, high-performing Lifetime Athlete. 

Using the bell-shaped curve of biology, under which practically everything exists in a “Goldilocks” (just-right) zone, where neither too much nor too little of something is ideal, we can apply carbs to the athletic diet. We just have to be careful about thinking if a little is good, then a lot must be better. Part of the problem with carbs is that the processed variety is hyperpalatable and quite addictive. Once hooked, we’ll believe strongly in them and defend them to the death, using all the delusion, denial, and justification-rationalization we can. So when you hear a person say  “I really, really, really NEED my carbs,” they are not lying. They have to feed a true biological addiction. This is proven scientific fact. I know also because I’ve been there. I was taught that carbs were just extra fuel, and in the athlete, the fire burned so hot you could just shovel in this crap and it would burn. But you can’t outtrain a bad diet. Even if it takes a few decades, it will catch up with you and dysregulate your metabolism. Also fact. 

I’ve learned in my wise old age to be cautiously reluctant in using terms like “always” and “never.” Whether you are talking to your spouse or discussing a certain food, this is good practice. Just like you shouldn’t tell someone you care about (or anyone for that matter) “You always do this…or never do that!” you probably don’t have to use those two words with most foods. It’s just that high-carb processed junk food should be consumed rarely and nutrient-dense foods should be consumed frequently. Hopefully that sums it up. 

Another thing to appreciate, especially with regard to the myriad dietary recommendations to which we are all exposed, is variability within our species. Unlike most animals, humans have evolved to exist with a very diverse gene pool. You see this in many circumstances, but let’s use the athletic world for an example. Some folks are built for speed, others for brute strength, and still others for endurance. Genetics drive these distinctions and lifestyle further refines it. The same can be said for metabolisms. While humans are more similar than different, some folks tolerate more or less carbs, of different types, than others. We need to respect individual differences, and always conduct our own experiments to find that Goldilocks amount of carbs in our athletic diet. And keep in mind that this will change almost daily with respect to workload intensity and volume.

So, in summing up Part 1 of Metabolic Flexibility for Athletes, here are a few bullets:

  • A well-constructed athletic diet can, and probably should, contain some amount of all three macros (fat, protein, carbs).
  • The human can get by with little or no carbs just fine, but this may be less true for the hard-training athlete. You should experiment to find the amount that helps you to perform and feel your best.
  • When our goal is to maximize performance without compromising health, we want to optimize the body’s ability to use both fat and carbohydrate as a fuel source.

This provides the foundational basis for why we want to create a dual-fuel metabolism. We can get more economy and efficiency from our food and training. In Part 2, I’ll provide some general suggestions regarding ratios of macros, the timing of feeding/fueling, calories, and food choices. Creating a flexible metabolism can help you to make 2020 a year of great success! Also, if you’d like to learn more about this aspect of peak performance, consider attending the Hard to Kill Workshop in Bozeman, MT on Feb 29th.

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