Imagine a machine that can run on different types of energies…
No, it’s not a Toyota Preius.
We’re talking about a far more complex system, capable of using various fuel sources to produce power.
This amazing machine is your body, which can utilize a variety of foods as fuel to propel itself. Body’s effectiveness to extract nutrients from a variety of foods and transform them into energy is often referred to as metabolic flexibility.
In this article, we’ll discuss this important ability, how it works and type of problems that are associated within the process.
Energy fuels Life
The human body is amazing and immensely complex. It consists of tens of trillion cells, each one creating its own energy to do whatever it was designed to do, and ultimately to be a productive part for the whole body system.
Throughout evolution, our system adapted by developing numerous pathways to process different types of foods into usable energy. In the end, we consume and utilize the three main macronutrients which are protein, sugars and fats.
Now, protein is mostly for building and repair (repurposing amino acids) and seldom used as energy—to do so amino acids are further restructured (like lego pieces) into usable fuels. The main energy used by the body are Sugars and Fats.
Metabolic Flexibility
Metabolic flexibility is simply the body’s ability to effectively transition between fats to sugars and vice versa.
But, what does this mean exactly?
Being metabolically flexible is adapting to whatever nutrient content is served within any meal. An example of this would be:
- when you consume a sugary (high carbohydrate) snack, the body switches and uses sugars as fuel.
- when you eat a meal with high fat content, the body responds by transitioning to using fat for energy production.
- when you don’t eat anything, the body does not crash and simply digs into its energy reservoirs (glycogen and fat, but predominantly fat) to maintain standard function (basal metabolic rate).
Insulin and Glucose Metabolism
The body responds to any modifications within its system, and that includes food. A meal enters the digestive track, is broken down into macro- and micronutrients to be absorbed. Numerous biochemical reactions and molecules participate in mechanical breakdown, digestion and absorption of consumed meal. Eaten carbohydrates in pasta, corn, potatoes, fruit and baked goods are all broken down into monosaccharides, one of them being glucose.
Glucose is interesting to us, as it is used to produce energy (Adenosine Triphosphate – ATP) with or without oxygen. But, before glucose can be used as fuel, it needs to be shuttled from bloodstream into cells. This is where the hormone insulin plays a key role and is one of the prominent facets of metabolic flexibility.
Insulin has been extensively studied and continues to be an active research topic. In healthy individuals, depending on nutritional content, insulin is released into the blood stream to move absorbed glucose into cells. Inside the cells, glucose is used to:
- produce energy (ATP),
- temporary stored as glycogen or,
- converted to triglycerides (TGs) and stored long-term as fat.
Regardless of what you may have read or heard—yes, all foods release insulin however, the hormonal concentrations rushing into the bloodstream varies on the substrate. Foods high in simple carb content rapidly increase blood sugar levels, which signals a stronger insulin response. On the other hand, foods with low sugar content (either high in protein or fat or both) still release insulin, but in smaller, more subtle amounts.
Majority of absorbed glucose (from intestinal tube—lumen) is moved into skeletal muscle and liver for energy production and storage. Release of insulin into bloodstream is often referred to as “Insulin Sensitivity”.
Dangers of Being Inflexible
We have two major forms of fuel—glucose and TGs—used to produce and store energy. These involve four main biochemical processes, each has a potential for becoming impaired thus decreasing overall metabolic flexibility.
Constant consumption of carbohydrates continuously activate pancreas to produce and release insulin. This eating habit results in more insulin production and release into overall system. The cells begin to accustom to greater insulin concentrations which is also referred to as decrease in insulin sensitivity. If unchanged, pancreas (insulin producing gland) may become exhausted and shut down insulin production all together, leading to variety of diseases including Type 2 diabetes.
Impairment of fat metabolism often occurs inside mitochondria (cell organelle that produces energy through aerobic respiration). Mitochondrion dysfunction leads to weight gain, as TGs are continuously stored in all types of tissues including: adipose, muscle, organs and blood vessels. These common and uncommon storages of fat further impact other metabolic processes within surrounding tissues including insulin signalling, increasing the risk for metabolic syndrome, cardiovascular disease and obesity.
Some individuals develop metabolic inflexibility within both fat and glucose metabolisms.
- First, by gaining weight and not being able to properly oxidize fat, putting greater dependancy on glucose as energy driver. The body attempts to overcome this affected process, by processing more sugar with higher insulin levels, eventually creating “insulin resistance”.
- Secondly, issues within glucose and fat metabolisms can be independent of one another. There are many cases where obese individuals have un-affected glucose metabolisms and slim type 2 diabetics (working fat metabolism).
Solutions for fixing a Broken System
The two main causes of metabolic inflexibility are insulin resistance and mitochondrial dysfunction. To improve insulin sensitivity, we need to engage fat metabolism (fat loss). The following three methods have been researched and shown to produce metabolic improvements.
1. Exercise
Significant research links physical exercise to increases in both insulin sensitivity and mitochondrial function. Physical activity has also been shown to improve fat oxidation. Muscles are an active tissue requiring significant energy to produce force and sustain themselves. However, the training suggestions vary, from four-five cardiovascular sessions (cardio) amounting to about 45 minutes, to three resistance training sessions for about half hour, three times per week.
Unless your cardiovascular system is very good at burning fat, weight training sessions are a better approach for improving metabolic activity. Yes, with cardio you do burn calories during an exercise period however, the ATP expenditure slows down significantly soon after the cardio run/jog/elliptical trainer. The body recovers most of spent glycogen and fat through adjustments of feeling hungry and consuming more food.
On the other hand, strength training not only improves your associated biomotor abilities, but also increases lean body mass. More muscle increases basal metabolic rate (BMR) which burns more calories at rest. Once again, the body adjusts with increase hunger and food consumption, but what also happens is—muscle hypertrophy. Repair and regeneration of muscle fibers increase tissue density and volume, increasing glucose metabolism and insulin sensitivity. Furthermore, muscle cells have the most mitochondria density compared to other cell types, and that improves aerobic respiration and fat oxidation.
2. Diet
Consuming nutritionally dense foods should be a priority, while selecting lower carbohydrate intake at the same time. Some people avoid carbs all together, which is a mistake—Not all carbs are created equal. A pastry or bowl of pasta should not be compared to whole fruits and vegetables. Raw fruits and vegetables contain other nutrient properties including—fiber, protein, vitamins, phytochemicals and some even essential fatty acids (Avocado).
And speaking of fiber, its a complex carb that helps with slowing down digestion, avoiding a rapid insulin response and keeping you feeling fuller for longer. Feeling full decreases overall caloric consumption which is a win-win. Fiber further improves gut bacteria health, digestion process and bowel movements.
3. Rest and Fasting
Metabolic inflexibility is a problem where body cells have trouble using glucose or fat, or combination of both to produce energy. When it comes to recovery, sleep plays a major role. Some studies have linked lack of sleep with weight gain. When feeling tired we often look for food to satisfy the sugary craving that sleep-deprived brain is looking for. Making late night snacking a habit creates further problems with digestion and insulin influx, pushing all of those consumed calories into storage—body fat. So, give yourself real opportunity to recover by making sleep a priority.
Speaking of rest, food and exercise in itself are forms of stressors that we apply to the body on daily basis. Sleep allows your neuromuscular system to repair and recover, but what about our digestive side? Well, by eating a large, late dinner just before bed time, all that food will be mechanically broken down, digested and absorbed. So, the digestive system gets no rest.
The solution is intermittent fasting. Following a specific feeding time period and consuming no calories for the rest of the day allows the digestive track to get some rest. Furthermore, by not having any calories, the body still maintains its BMR but only through fat oxidation (burning predominantly body fat), while improving metabolic flexibility at the same time. Research studies and extended personal experimentations have shown just that.
Science between the Lines
Metabolic flexibility is a large umbrella for many modern day diseases including metabolic syndrome, cardiovascular disease, obesity, and diabetes. With all these disorders on the rise in every age group of each population—finding answers to why the body stops working is big business. Numerous streams of completed and on-going research produce volumes of literature and suggestions on what to do, how to do it, while hypothesizing everything under the sun.
We at Science and Strength follow a general rule when reading any research paper:
- to identify the purpose of the paper, and
- analyze its results and applicability.
You see, most of the research, no matter how interesting, is simply not easily relatable to the majority population. Studies using obese or diabetic subjects may show improvements in specific form of diet or exercise, but these results won’t provide value towards an individual who does not suffer from excessive weight symptoms. Also, much of research is conducted on small animals (mice). We understand the convenience of gene manipulation to observe effects of insulin or specific process within mitochondria, but again these results are nice metabolic models identifying potential broken links, and not easily applied to human population.
So, it’s important to read the literature, educate yourself and ask questions on how new findings can help you understand possible issues and achieve personal goals.
Final Thoughts
The body is an impressive machine capable of using various fuel sources to generate energy. The two main forms of body energy substrates are glucose (sugar) and TGs (fat). Metabolic flexibility is the body’s ability to efficiently switch between glucose and fat metabolism to generate energy, whether from consumed nutrients or its own reserves.
Metabolic inflexibly occurs on cellular level where glucose or fat metabolism is impaired. The problems within fat oxidation often occur inside mitochondria where glucose process have been linked to insulin resistance.
To begin restoring metabolic flexibility, the ultimate goal is to lose body fat while increasing muscle mass. Strength training is the proven method of burning greater amount of calories while building lean tissue. Eating low-calorie and fiber-rich foods further enhance benefits of metabolic flexibility. Getting adequate rest and fasting have also been shown to improve insulin sensitivity while increasing lipid metabolism and decreasing body fat.
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