Written by Garett Reid | NSCA, CSCS, CISSN, M.S.E.S.S
Fact checked by Tyler DiGiovanni, BSBM
FACT CHECKED
"Metabolic fitness" is all the rage these days, but what does it mean? Usually, it refers to some sort of a "metcon" workout or somehow relates to diabetes. Both of these hold some truth, but metabolic fitness goes much deeper and is a vital component of both fitness and longevity but also performance. This article will dive into metabolic fitness and how it affects your life.
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What Is Metabolic Fitness?
The easiest and shortest way to explain metabolic fitness is the efficiency of your body's metabolic systems. This begs the question, what is metabolism?
Well, your metabolism as a whole is a plethora of various processes that are essential to providing energy to sustain life. The scientific definition is:
"the whole sum of reactions that occur throughout the body within each cell and that provide the body with energy"¹
We can break these processes down into 3 primary groups;
- Conversion of food into energy available to run cellular processes
- Transformation of food into building blocks for proteins, lipids, nucleic acids, and some carbohydrates
- The elimination of metabolic wastes
Therefore, metabolic fitness is how well your body is at doing these collectively.
Your Bodies Ability To Produce Energy
As we just mentioned, metabolism refers to the total sum of various processes. With that said, when people speak about their metabolism it generally refers to your body's ability to utilize different nutrients to produce energy.
The word "energy" refers to the high-energy compound adenosine triphosphate, also known as ATP. ATP is required to power a host of reactions, including muscle contraction.
As its name implies, ATP contains three phosphate groups with high-energy bonds. When energy is required, one of the three phosphate compounds breaks off which releases energy and results in another compound, adenosine diphosphate, or ADP.
For your body to continue functioning, it must be able to continually replenish the compound ATP. Essentially, this is what your metabolic systems do; they work to replenish ATP through the use of different compounds; carbs, fat and protein.
Your Three Metabolic Systems
Your body can produce energy (replenish ATP) through the use of three different metabolic systems:
- Phosphagen system (Also known as ATP-CP system)
- Glycolytic system
- Oxidative system
Each system functions optimally to supply energy during activities of different intensity and duration.
Phosphagen
- Intensity- High-Intensity/Very-High Intensity
- Exercise Duration- 0-30s
- Aerobic/Anaerobic- Anaerobic
Your phosphagen system is the primary source of energy for short bursts of very high intensity.
The general time frame is activities that can be held at a maximum intensity for 30 sec or less.
You use your phosphagen system to supply energy for maximal strength and explosive efforts.
Examples include:
- Lifting weights
- Sprinting
- Jumping
It can supply energy so fast because it utilizes a single-step process to replenish ATP. It does this by releasing stored creatine phosphate, which donates its phosphate group to ADP to become another compound of ATP.
This process is very quick but only generates a small amount of ATP.
Glycolytic
- Intensity: High Intensity
- Exercise Duration: 30 sec - 2:00 min
- Aerobic/Anaerobic: Anaerobic
The second metabolic system is called the glycolytic system, also known as anaerobic glycolysis.
Your glycolytic system is anaerobic in nature, just like your phosphagen system. However, it regenerates ATP from either glucose (blood sugar) or glycogen (a stored form of glucose in the muscles) coming from carbs.
One of the by-products is the accumulation of lactic acid, which can contribute to muscle fatigue.
The whole process is a bit more complex than the one-step process in the phosphagen system but still somewhat simple. This means it takes a bit longer to replenish ATP but results in more ATP.
This means that the glycolytic system is the primary metabolic system for sustaining high-intensity activity lasting from around 30 seconds to 2:00.
- 400-meter sprint
- High-intensity interval training (HIIT).
Oxidative
- Intensity: Low and Medium
- Exercise Duration: 2:00+ min
- Aerobic/Anaerobic: Aerobic
Finally, we have the third metabolic system, the oxidative system. The oxidative system is the only aerobic system that relies on oxygen, hence its name.
Another interesting component of the oxidative system is that it can replenish ATP stores through the breakdown and processing of all three macronutrients;
- Carbs
- Fat
- Protein
All three macronutrients require slightly different processes, but the end result is a lot of ATP. These processes are significantly longer than the other two metabolic processes, but they yield sufficiently more ATP.
For this reason, your oxidative system is your primary metabolic system for low-intensity activities. This means everything is done at an intensity that can be maintained for longer than 2:00, which is the majority of the time.
How To Target Different Metabolic Systems
Before we discuss these, it's important to understand that all three metabolic systems work simultaneously. Some people speak about these systems as if there's a switch that turns them on and off.
What happens is that the extent to which one is used will increase. This can also differ from person to person depending on their²:
- Training Level
- Training Style
- Fed or Fasted-State
- Body Fat Percent
For example, walking in a fasted state could mean a person is utilizing fats to get 85% of their energy. On the other hand, if a person in a fed state sprints 800m, they may derive 70%+ of their energy from carbs.
This is important because, ideally, you will optimize each metabolic system. They are always working so it makes sense you would want them to all be optimized.
For example, many strength athletes forgo endurance training as they feel there's no need. However, remember that the oxidative system produces a ton of ATP.
Theoretically, this means that a strength athlete would be able to perform more work and recover faster between sets as their metabolism is producing large amounts of ATP.
Keep in mind that this training will have a crossover between different systems.
Training Your Phosphagen System
Training your phosphagen system simply requires you to perform bouts of maximal effort. Most lifters don't have a problem with this as this is the main system used in the gym. However, there are some things you can do to truly optimize this metabolic system.
- Sprint intervals: Use work intervals of 10-30 sec with rest intervals 4X the work interval
- Max sled push for 20-30s. Go for either distance or max load and rest 1:00-2:00
- EMOM (Use 3 reps of your 5RM for 5:00)
Training Your Glycotic System
To train your glycolytic system, you need to perform events from 30 sec to two minutes with maximum intensity. This means you should be near exhaustion by 2:00. With that said, here are some training protocols you can use to optimize your glycolytic system.
- Sprint intervals with work intervals of: 30 sec-2:00 min and rest intervals of 2-3 times as long.
- Interval training using 30-60s work intervals and 1:00-2:00 rest intervals (box jumps, slam balls, cycling/air bike)
Training Your Oxidative System
Finally, training your oxidative system requires you to perform low-intensity exercise for an extended duration.
Now, technically, your oxidative system is the primary metabolic system used for events longer than 2:00. However, you should really shoot for activity at 70-80% HRmax for at least 30 minutes.
Train All Your Systems
One of our favorite tools to train all of your systems are with fartleks. "Fartlek" means speed play and consists of a long run that uses various bursts of different speeds. The base of a fartlek is running at low-intensity of around 70-75% intensity but again, this can vary.
From here, you will randomly perform runs with higher intensity. For example, you may pick up with 100% effort for 20 sec or maybe pick up to 90% effort for 4:00 min. There's literally no format, so you can alter it to your needs.
One thing we really like about these is after your sprints, you don't stop running. Rather, you just come back to a low-intensity speed. This teaches your body how to recover while still moving and can make a huge difference in your total work volume.
What Does It Mean To Be "Metabolically Fit"?
The problem with giving a solid definition of what "metabolically fit" is is that there is no clear definition.
When your metabolism functions properly, your body efficiently converts carbohydrates into glucose, fatty acids into ketones, and proteins into amino acids. Glucose and fatty acids provide the energy your body needs, while amino acids are used to build muscle tissue or create signaling molecules that help cells communicate.
A healthy metabolism also ensures that waste products are effectively filtered and eliminated through the liver and kidneys.
However, if your metabolism isn't working at its best, your body struggles to extract energy and nutrients from your food. This can lead to weight gain, inflammation, a weakened immune system, and other health problems.
Over time, metabolic dysfunction can significantly raise your risk of chronic diseases and make you more vulnerable to infections.
That's why assessing your metabolism is crucial to determining overall health. If your metabolism is out of balance, it can impact every other aspect of your well-being.
Metabolic Fitness And Metabolic Syndrome
Above, we largely spoke about the role of metabolic fitness in performance.
However, there's also another side of metabolic fitness, and that is when our metabolic systems become damaged. This is usually caused by our behavior and includes things such as;
- Lack of exercise
- Excess consumption of overly processed foods
- Excess calorie consumption
When these behaviors go on for too long, it can result in something known as metabolic syndrome.
5 Symptoms Of Metabolic Syndrome
Metabolic syndrome isn't a specific condition but rather a group of 5 symptoms known as metabolic abnormalities.
Specifically, a person is said to have metabolic syndrome when at least 3 of the 5 metabolic abnormalities are present³:
- The waist is more than 35 inches for women and more than 40 inches for men.
- Raised triglycerides in the blood of at least 150 mg/dL or more
- Reduced high-density lipoprotein cholesterol (HDL) less than 40 mg/dL in men or less than 50 mg/dL in women
- Elevated fasting glucose of l00 mg/dL or greater
- Blood pressure values of systolic 130 mmHg or higher or diastolic 85 mmHg or higher
If an individual is found to suffer from metabolic syndrome, they are at a significantly greater risk of various health concerns. These include;
- 2-fold increased risk of atherosclerotic cardiovascular diseases
- 5-fold increased risk of diabetes
- Accelerated atherosclerosis,
- Premature atherosclerotic cardiovascular diseases
Metabolic Fitness And Diabetes
Of all the topics related to metabolic fitness, the most common concern is building insulin resistance and diabetes.
In a nutshell, diabetes is a metabolic disorder in which your body stops responding to insulin, resulting in chronically high blood sugar levels. With that said, there are two major types;
- Type 1—Type 1 diabetes is an autoimmune disease that occurs when the body's immune system destroys pancreatic cells (beta cells). This is generally due to unpreventable circumstances such as genetics and environment. Type 1 diabetes primarily occurs in children but has recently been found to occur in adults as well.
- Type 2—Type 2 diabetes is a metabolic disorder that is largely preventable through maintaining a healthy weight and an active lifestyle. Whereas Type 1 diabetes is caused by the body stopping to produce insulin, Type 2 diabetes is caused by excessively high levels of insulin, which results in insulin resistance, which can then result in a lack of insulin production as well.
As of right now, almost 15% of the adult (age 18+) US population is living with diabetes. What's scary is that about 23% of these people are undiagnosed!⁴
And it gets scarier – almost 40% of the adult US population has what's labeled as prediabetes!
Now, the good thing is that those with prediabetes can effectively reverse this prognosis by following some of the guidelines we'll list below.
When speaking specifically about Type 2, it's essentially caused by gaining unhealthy amounts of weight and eating an excess of calories as well as carbs, usually from junk food or processed foods. These abnormalities cause dysfunction in your body's metabolic systems and they eventually shut down resulting in no maintenance of a person's blood sugar levels.
Insulin Resistance
Insulin resistance is perhaps the biggest effect of not caring for our metabolic fitness. Above, we talked about it being a predictor of diabetes, but it's actually a stand-alone condition that is part of metabolic syndrome as a whole.
Not only is it strongly associated with diabetes, but cardiovascular disease as well.
When a person has a healthy metabolism, their pancreas releases insulin when it senses elevated glucose levels, usually after they eat.
When insulin is released, insulin-sensitive tissue (muscle, adipose tissue, liver) absorbs glucose and stores it as glycogen for later energy use. At the same time, this lowers blood glucose to a normal range.
However, when someone becomes insulin-resistant, their once insulin-sensitive tissues lose their sensitivity and begin to resist insulin's presence. As a result, they stop absorbing glucose, leading to high blood sugar levels.
As a response, the pancreas may release more insulin to compensate for the lack of sensitivity. At this point, normal glucose levels can still be maintained. However, if no changes are made to diet or exercise and elevated levels continue, eventually, the body stops responding to insulin.
How To Improve Your Metabolic Fitness?
Improving and maintaining your metabolic fitness doesn't need to be overly complicated.
In fact, there really aren't a ton of special rules you need to follow apart from your basic "healthy guidelines." This includes;
- Maintain a healthy weight
- Maintain a healthy body fat percentage
- Focus on eating whole foods
- Reduce foods with added sugar or added salt
- Minimize or eliminate drinking and smoking
- Follow a resistance training program at least two times a week.
- Include some endurance training as well as higher intensity
- Increase physical activity (more is better!!)
One of the most important factors, however, is consistency. Just about any type of regular physical activity will maintain adequate levels of metabolic fitness. Of course, this assumes a person is at a healthy weight and is following a nutritious diet.
With that said, there are some tactics one can use to help improve their metabolic fitness, especially if they are dealing with one of the metabolic syndrome disorders above.
Fasting, Low-Carb Diets, And Improved Metabolic Fitness
If someone is suffering from prediabetes or showing symptoms of insulin resistance, fasting, and low-carb diets have been found to improve one's metabolic fitness.⁵ There doesn't seem to be a specific protocol, but here are some guidelines.
- A low carb-diet is defined as having less than 130g per day or getting 26% of total calories from carbs.⁶
- Using a 8:16 intermittent fasting protocol is the easiest plan of fasting for new people to follow. This means you have an 8-hour window where you can eat and then fast for 16-hours.
- Try to do some fasted exercise. This doesn't need to be (and shouldn't be) intense exercise. Rather, go out for a 30-60 minute walk in the morning.
- Eliminate junk food, processed foods, and added sugar.
Again, there are no specific protocols that have been found to be superior. However, those simple guidelines are a great place for beginners to start and should be done in conjunction with a regular training program.
Get Your Metabolic Fitness Under Control!
Metabolic fitness is an essential part of everyone's health profile. It not only gauges your overall fitness levels but your overall health profile as well. The good thing is that improving your metabolic fitness runs adjacent to other health protocols:
- Eliminate junk food and processed foods
- Concentrate on eating a balanced diet of whole foods
- Maintain a healthy weight
- Get involved in a strength training routine at least twice a week, ideally more
- Include at least 150 minutes of moderate-intensity exercise (running, cycling, rowing, etc)
- Increase your overall physical activity
- Ideally, use various levels of cardio training
Get this under control, and you'll get your health back on the right track!
Prepare to lose some pounds with our exclusive 12-week fat loss training program. Choose between a 4 or 5 day training split and torch that stubborn fat, all while retaining, or even gaining muscle...
References
- Sánchez López de Nava, Arturo, and Avais Raja. “Physiology, Metabolism.” PubMed, StatPearls Publishing, 2022, www.ncbi.nlm.nih.gov/books/NBK546690/.
- Hargreaves, Mark, and Lawrence L. Spriet. “Skeletal Muscle Energy Metabolism during Exercise.” Nature Metabolism, vol. 2, no. 9, 3 Aug. 2020, pp. 817–828, www.nature.com/articles/s42255-020-0251-4, https://doi.org/10.1038/s42255-020-0251-4.
- Swarup, Supreeya, et al. “Metabolic Syndrome.” PubMed, StatPearls Publishing, 2021, pubmed.ncbi.nlm.nih.gov/29083742/.
- Centers for Disease Control and Prevention. “National Diabetes Statistics Report.” CDC, 15 May 2024, www.cdc.gov/diabetes/php/data-research/index.html.
- Landry, Matthew J., et al. “Benefits of Low Carbohydrate Diets: A Settled Question or Still Controversial?” Current Obesity Reports, vol. 10, no. 3, 23 July 2021, pp. 409–422, www.ncbi.nlm.nih.gov/pmc/articles/PMC9621749/, https://doi.org/10.1007/s13679-021-00451-z.
- Oh, Robert, et al. “Low Carbohydrate Diet.” Nih.gov, StatPearls Publishing, 29 July 2019, www.ncbi.nlm.nih.gov/books/NBK537084/.
Garett Reid
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