Metabolic Efficiency: Why You Should Care

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Improving Metabolic Efficiency Series (PART 1)

I began talking about the importance of metabolic efficiency more than 5 years ago (back in 2017). At the time I held a number of workshops on the topic and planned to offer an online course that went into detail on how to become more metabolically efficient. Over the next few years, I went on to demonstrate how powerfully being metabolically efficient can affect performance as well as optimal body composition, not to mention health. This included my 2018 sub 24-hour Javelina Jundrend 100-miler (pictured above) and 2019 completion of Tahoe 200-miler. I’ve also helped many athletes see their own exciting results.

And so finally, after a hiatus of many more years than anticipated, my long held plan to ensure widespread access to this information in which my clients and I have been using for years now is now finally coming together. Today I’d like to go into detail on what exactly metabolic efficiency is – and why you will likely be interested in what becoming more metabolically efficient can offer you.

In part one of this series on Metabolic Efficiency, I’ll cover:

  • What metabolic efficiency is; 
  • The benefits to becoming more metabolically efficient (and why simply restricting calories to lose weight isn’t a good strategy);
  • How metabolic efficiency is related to metabolism (and why ‘speeding up your metabolism’ is a fallacy);
  • The different ways our body makes energy (and which one we want to develop to improve metabolic efficiency).

What Metabolic Efficiency Is

Metabolic efficiency is: 

  • a measure of the body’s tendency to direct calories consumed into fat cells for storage versus burning them for fuel (energy) immediately; 
  • a measure (percentage) of carbohydrates (glucose / glycogen) versus fat (fatty acids) the body utilizes during activity at various intensities;
  • a measure of metabolic flexibility – that is, the ability of mitochondria (the energy centers in cells) to adapt and easily switch between using sugar and fat for fuel (to make energy).

Why Become More Metabolically Efficient?

There are at least three reasons why one might wish to pursue metabolic efficiency:

1. To achieve optimal body composition. 

If an individual is carrying excess body fat they wish to be rid of, the least restrictive option for weight loss is to teach their body to use its own fatty acids for fuel (that is, to become more metabolically efficient). This will allow them to consume less calories with the deficit supplied happily from stored body fat. In other words, they won’t feel hungry with a slight deficit and weight loss will feel effortless. 

Weight loss is not simply about burning more calories than you consume (which often backfires). Achieving metabolic efficiency is a healthier and more sustainable method of weight loss than simply creating a calorie deficit (by eating less and/or working out harder) because simply eating less without first becoming more metabolically efficient means the body is not conditioned to burn fatty acids for fuel, and this will result in hunger. This hunger becomes more intense the longer the calorie deficit is present, driving the hungry individual to eventually consume the calories their bodies requires. 

Of course, some people manage to ignore their hunger and quite literally starve themselves – forcing weight loss to take place. While the discomfort of hunger is might be offset with the euphoria of weight loss achieved, unfortunately, this is to the detriment of the health of one or many of the following body systems: hormonal, immune, cardiovascular, nervous, respiratory, digestive, skeletal, urinary and/or reproductive. And while sometimes this type of weight loss is sustained; more often it’s short-lived (a couple months to a couple years) and the rebound can result in a higher body weight than before. 

Sometimes an individual sustains a calorie deficit but sees little or no weight loss, despite ignoring their appetite. In this case there has likely been past dieting attempts that negatively impacted the body’s systems resulting in a significant hormonal imbalance and other health concerns. The body is in such a state of stress that it’s in survival mode (which means it’ll do everything and anything to preserve and store fat instead of burn it). 

2. For endurance athletes – to become less dependent on needing carbohydrates for fuel.

For those already at their ideal body composition, you may be thinking there’s no point to becoming more metabolically efficient. However, as an athlete, you’ll find that for endurance sports fueling will become much easier (less fueling is needed), which typically alleviates GI issues or nausea that is often experienced with fueling during long events. 

Many endurance events can be fueled almost entirely by fatty acids. Taking in 30-60 and up to 90 grams of carbohydrates per hour (the recommended amount in sports nutrition for exercise longer than 90 minutes) can lead to GI distress, nausea and gastrointestinal distress for many athletes. But having a body that knows how to burn a higher percentage of fatty acids endogenously (meaning, your body knows how to burn your own stored fatty acids for fuel) greatly reduces the need for carbohydrate intake during long events. 

3. To improve health.

Lastly, working to improve your metabolic efficiency holds many benefits to your health. Foundationally, it helps to balance blood sugar levels – and this prevents the cascade of physiologic effects that dysglycemia can lead to including: 

  • Diabetes
  • Neuropsychiatric symptoms including anxiety, depression and insomnia
  • Heart disease and cardiovascular events
  • Dementia

How is Metabolic Efficiency different from Metabolism?

Metabolism is linked to metabolic efficiency: metabolism is simply the process by which your body converts food and drink (nutrients) into energy; while metabolic efficiency is concerned with HOW this process is completed. 

Becoming more metabolically efficient doesn’t mean requiring less calories (we cannot change how many calories our bodies require except through how active we are), it means shifting HOW we create energy from calories. 

A note of interest: there’s no such thing as ‘speeding up your metabolism’ or ‘having a faster metabolism’. In reality, people who appear to have a ‘faster metabolism’ are for the most part simply naturally more metabolically efficient (it does appear people are predisposed to have a default of being more or less metabolically efficient).

Metabolism versus BMR versus Daily Calorie Requirement

While metabolism is simply the process of our body turning nutrients into energy, basal metabolic rate (BMR) is how many calories your body requires to carry out your daily basic functions of all the systems of your body (hormonal, immune, cardiovascular, nervous, respiratory, digestive, skeletal, urinary and reproductive). 

BMR is based on your body size, composition, sex and age and for the most part cannot be changed. 

Along with BMR, thermogenesis (calories required to digest and absorb food) and physical activity (movement and exercise) determine the total number of calories your body requires each day.  

Therefore, while you can’t change metabolism (it’s a physiological process that’s always happening) and you can’t change your BMR (the calories you require for basic functions), you can change HOW metabolism happens. In other words, you can change your metabolic efficiency. 

So while you can’t speed up metabolism, you can teach your body to become more efficient at utilizing calories for fuel instead of storing them, and to burn a higher percentage of fatty acids for fuel both at rest and during exercise.

How our Bodies Produce Energy

ATP (adenosine triphosphate) is the energy currency of our body’s cells – and our bodies use five fuel sources for energy: ATP-PC, carbohydrates, fats, proteins and alcohol.

To become as metabolically efficient as possible, the goal is to train our body to rely primarily on fat for fuel, utilizing ATP-PC for high intensity where possible and metabolizing carbohydrates efficiently as well as where needed for performance. 

1 – ATP-PC

The ATP-PC (phosphagen) energy system refers to the adenosine triphosphate and phosphocreatine that is created and stored in the cells of our muscles – a naturally occurring phenomenon. 

This energy system operates without oxygen and lasts for about 6 to 10 seconds (up to maybe 12-15 seconds in a well-trained athlete, particularly one who’s supplementing with creatine). 

The only time this energy system is solely depended on would be 20 to 100-meter sprints, or a near maximal lift in the gym (1 to 4 reps), a single jump, 2-3 boxing punches or 1-4 explosive moves (plyometrics). 

The ATP-PC system requires at least 3 minutes if not more (up to 5-10 minutes) of total rest to replenish itself before you can do another set relying on ATP-PC alone.

2 – Carbohydrates

Within both the anaerobic glycolytic (lactic acid) and the aerobic glycolytic energy systems, carbohydrates (glucose, stored as glycogen) provide 4 calories per gram and are stored at a rate of:

  • ~400 grams as muscle glycogen (less in non-athletes; more in well trained and carb-loaded athletes)
  • ~100 grams as liver glycogen 
  • ~15 mg of glucose, always present in the blood and brain

This makes carbohydrates a limiting fuel that must be continually replenished (by eating many carbohydrate rich foods) if it’s to be relied upon as the primary or large source of fuel for activity. 

It should be noted that the anaerobic glycolytic system uses up carbohydrates (glucose) rapidly – it produces only 2 ATP molecules for each glucose molecule metabolized. 

In contrast, the aerobic lipolytic system uses carbohydrates (glucose) more efficiently – it produces 36 ATP molecules for each glucose molecule metabolized. 

Thus, working out at an intensity that is more anaerobic will burn through carbohydrates more quickly.

The downsides to constant carbohydrate replenishment can include: 

  • Blood sugar imbalances (dysglycemia and hormonal imbalances)
  • Digestive distress 
  • Systemic inflammation
  • Weight gain (due to improper metabolism of carbohydrates)
  • The possibly annoying logistics of eating more often

3 – Fat

The lipolytic aerobic energy system burns fat (fatty acids and fatty acid metabolites called ketones [ketone bodies: acetoacetate, acetone and β‐hydroxybutyrate]).

Fat provides 9 calories per gram and can be stored at a rate of 22 to 33 pounds in the average 154-pound person (more or less depending on your weight) as adipose tissue, and about 300-400 mg intramuscularly. 

Additionally, fat is a highly efficient fuel as it produces 80-200 ATP molecules for each fatty acid metabolized.

As you can see, this essentially makes fat an unlimited source of fuel that can be tapped into for even many days in a row – good news for endurance athletes who may wish to train their bodies to utilize this fuel source during longer events from the marathon to an Ironman to 100-milers and right up to multi-day events (although a small amount of carbohydrates and protein will be required to help burn the fat and for other bodily functions).

It also means that if you have extra body fat you wish to be rid of, training the body to burn this fuel is to your advantage.

4 – Protein

Proteins (amino acids) provide 4 calories per gram, however they are not stored or generally used for energy purposes. Instead they form muscle and organ tissue which can be broken down to release energy in extreme circumstances, but obviously this isn’t ideal. 

That said, certain amino acids are burned for energy at low percentage (4-7%), primarily the BCCA’s (branch chain amino acids: leucine, isoleucine and valine), but shouldn’t be relied upon for fuel as they need to be converted to a useable energy form. 

The amino acids isoleucine, valine, threonine and methionine can be fairly readily converted to glucose via gluconeogenesis; while the amino acids leucine and lysine are exclusively ketogenic, converted to ketone bodies via ketogenesis. While this is an example of how brilliant the body is at utilizing many different fuel sources, again, it’s not a fuel source that helps build metabolic efficiency. That said, getting enough protein daily (1.2 to 1.8 grams per kg bodyweight) is essential for athletes for many other reasons.

5 – Alcohol

Alcohol (ethanol) provides 7 calories per gram but it must be noted that alcohol calories are not a source of fuel utilized by muscles for energy during exercise. 

Because ethanol is toxic to our body’s tissues, when it’s present the liver makes breaking it down and getting rid of it the number one priority. Therefore, the body is less efficient at producing fuel for exercise – the conversion of both carbohydrates and fat to energy (ATP) is inhibited as the body prioritizes clearing out the alcohol.

Furthermore, alcohol consumption immediately following exercise reduces the rate of muscle protein synthesis (MPS), thus inhibiting muscular recovery. Additionally, ethanol calories provide zero nutrient value – and often these excess calories end up being stored as fat on the body.

Carbohydrates versus Fats for Fuel

The table below gives you approximate percentages of fuel burned by athletes who are less metabolically efficient (typically consume a carb-rich diet of pasta, breads, bagels, etc. as well as many processed foods) versus approximate percentages of fuel burned by athletes who are more metabolically efficient (typically consume a higher percentage of fat in their diet [generally 45-65%] and pay attention to food quality such as more whole foods and less processed foods).

You’ll see these percentages change based on the effort the athlete is exerting, shown here by their percentage of heart rate maximum (HRmax).

HRmax‘typical’ carb-fueled athletesmetabolically efficient athletes
low intensity
can sustain all day
(purely aerobic)
~55% (up to 72%) energy from fat
~28% to 45% energy from carbs
Up to 99% energy from fat
can sustain for
1-2 hours

(primarily aerobic)
~30% (up to 56%) energy from fat
~44% to 70% energy from carbs
~88% (up to ~98%) energy from fat
~2% to 12% energy from carbs
can sustain for
~60 & up to 90 min

(primarily anaerobic)
~10% (up to 30%) energy from fat
~70% to90% energy from carbs
~56% (up to ~76%) energy from fat
~24% to 46% energy from carbs
can sustain for
10-15 minutes

(purely anaerobic)
~2% energy from fat
~98% energy from carbs
Up to 23% energy from fat
~77%+ energy from carbs

With this understanding of metabolic efficiency, metabolism and fuel sources, it’s time to explore HOW to become more metabolically efficient. In part 2 we’ll look at two methods of training and three nutritional methods that can be utilized in order to gain metabolic efficiency. 

To deliciously healthy food and stronger faster running,

Sarah Cuff, R.H.N.
Holistic Sports Nutritionist
Run Coach
Therapuetic Coach

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