Metabolic Efficiency: How to Get There

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Improving Metabolic Efficiency Series (PART 2)

In my last post, part 1 of this series Metabolic Efficiency: Why You Should Care, I talked about what metabolic efficiency is, its benefits, how it’s related to our metabolism and how our body makes energy. Personally I’ve used metabolic efficiency to help me achieve my goals and perform well many times over – including my September 2019 completion of Tahoe 200-miler, pictured above. Additionally I’ve had the honour and pleasure of supporting many clients in finding their own successes over the past decade.

It’s helpful to know what you’re aiming for and why when making changes to either your training and/or your nutrition. If you don’t have a good understanding of why you’re doing something, the chances of failing are most certainly greater. Once you understand the mechanism of metabolic efficiency, naturally you’ll wonder how to achieve it.

Here in this post I’ll cover two methods of training and three nutritional methods that can be utilized in order to gain metabolic efficiency:

  1. Training method #1 – train 80% aerobic / 20% anaerobic; 
  2. Training method #2 – incorporate fasted / carb-depleted aerobic workouts;
  3. Nutritional method #1 – eat in a way that aims to keep glycemic load low;
  4. Nutritional method #2 – transition to JERF and possibly to LCHF;
  5. Nutritional method #3 – incorporate intermittent fasting.

1: Train 80% aerobic / 20% anaerobic 

Essentially, aerobic exercise means exercise that burns fuel in the presence of oxygen. 

In other words, your muscles have enough oxygen being delivered to them that they utilize the lipolytic energy system (burn fat for fuel). The upper end of aerobic exercise does also incorporate the glycolytic energy system (burning carbohydrates for fuel). 

In contrast, anaerobic exercise means exercise that burns fuel without oxygen, utilizing the anaerobic glycolytic energy system (and burning only carbohydrates for fuel, at a high rate). 

Building a strong aerobic base improves your ability to burn fatty acids for fuel. Thanks to this, it also makes you a more efficient endurance athlete.

Layering anaerobic training on top of a strong aerobic base improves your speed, strength and power as well as your body’s ability to burn a higher percentage of fat for fuel, even at these higher intensities, thanks to the strong aerobic base. Therefore, it is advantageous to perform about 80% of your training in an aerobic state, leaving a remaining 20% to be done in an anaerobic state. 

Note – for those who are just beginning to build their aerobic base, it is useful to spend 1 to 3 or more months just building your aerobic base, thus performing closer to 100% of exercise in an aerobic state. Once your base is built, go ahead and incorporate up to 20% anaerobic exercise. You’ll know when you have a good foundation because you’ll be able to go faster than you were previously able to (when you began building your base) while still maintaining an aerobic heart rate. 


As documented by exercise scientists including Achten & Juekendrup 2004(*), running in your aerobic zone allows the following adaptions to occur: 

  • Increases the number of key fat-oxidizing enzymes (so your body becomes more efficient in breaking down fat into fatty acids);
  • Develops a greater number of capillaries and blood vessels to carry fatty acids (as well as oxygen and nutrients) to muscle cells;
  • Increases number and size of mitochondria (part of muscle cell where fatty acid oxidation takes place and energy [ATP] is produced); 
  • Increases your muscles capacity to store glycogen by up to 50%; 
  • Allows you to run farther without getting dehydrated; 
  • Builds your resistance to injury (aerobic fibres are resistant to injury); 
  • Boosts your immune system (whereas intense anaerobic exercise depresses it);

These adaptations enable you to break down and burn fat at a faster and greater rate at any intensity, so you’ll burn more fat for fuel both in aerobic and anaerobic workouts.


Plan to spend approximately 80% of training time in your aerobic zone. For example, if you run 5 hours per week then a minimum of 4 hours would be at easy pace (aerobic zone) and no more than 1 hour at tempo or interval pace (anaerobic zone). 

Or if you have multiple sports add up all your time spent working out in either intensity. For example, if you swim 2 hours a week, bike 6 hours a week and run 3 hours a week, plus spend an hour in the gym strength training – determine how much of that total of that 12 hours total is spent in aerobic zone versus your anaerobic zone. Adjust to ensure no more than 20% is anaerobic. 

To figure out what your aerobic zone or threshold is, consider the following options:

  • Jack Daniels’ VDOT Running Calculator – input your latest race result (
  • Dr. Phil Maffetones’ 180 formula (
  • Determine what 65-79% of your heart rate maximum (HRmax) is by referring to the table below (to get a generalized HRmax number, subtract your age from 220)
%HRmaxIntensity Zone 
50-64% AEROBIC: considered very easy effort; could sustain for hours or even days; uses lipolytic energy system [fat-burning]; improves aerobic fitness.
65-79% AEROBIC: considered easy effort in exercise; high end is aerobic threshold (aim for this range in aerobic exercise); uses mostly lipolytic and some aerobic glycolytic; improves aerobic fitness.
80-87% anaerobic/aerobic: considered moderate intensity; in running often called ‘junk miles’ (this range doesn’t train either the anaerobic or aerobic fitness well).
88-92%ANAEROBIC: threshold/tempo effort; generally train with repeats of 5-15 minutes with 1-3 minutes rest; uses mostly anaerobic glycolytic system; increases anaerobic threshold and improves endurance. 
98-100% ANAEROBIC: interval effort; train with repeats of 3-5 minutes with equal recovery; uses anaerobic glycolytic system; improves speed and VO2max.

Less than 50% of HRmax is considered being at rest, with no meaningful strain being put on the body.

2. Incorporate fasted / carb-depleted aerobic workouts

By integrating fasted runs or carb-depleted runs (also called low-CHO-availability training) into your weekly schedule, you can heighten your body’s ability to burn fatty acids for fuel even further. 

This “train low, compete high” strategy prescribes strategically placed fasted workouts within a runners training (with intention of fully carb-loading before an actual race). 


Burke 2010(*) states that exercising in a fasted or carb-depleted state (resulting in low glycogen stores) enhances the adaptations made through consistent aerobic training. This strategy essentially takes fat oxidation to the next level, driving the muscles to further increase mitochondrial fatty acid oxidation. 

In one case study of three elite runners, Stellingwerff 2012(*) found 18% of their runs were done in a fasted or carb-depleted state – and each one achieved their personal best on race day. 


Fasted training can be done a number of ways:

  • Run first thing in the morning on nothing but water (eating a low-carb dinner the night before can enhance the effect); 
  • Do a hard interval workout in the morning, withhold carbohydrates during the day and then head back out for an afternoon/evening ‘carb-depleted’ easy run (refueling well with carbs and protein after the second run). 

Ideally, aim to have your fasted aerobic workout last about 1-hour (and up to 2-hours), as training carb-depleted for less than an hour appears to slow progress in achieving the desired adaptations.

3. Eat in a way that aims to keep glycemic load low 

The GI (glycemic index) is a numeric score of 1-100 given to a carbohydrate-rich food based on how much 50 grams of this food makes blood sugar rise (according to an average of 10 people lab tested). 

On the other hand, the GL (glycemic load) is a more accurate representation of how the body responds to that food because it actually takes a typical serving size into account. 

A low GL is defined as 10 or less, medium as 11-19 and high GL is 20 or higher. 

To roughly determine the GL of a meal, it’s helpful to look at the overall carbohydrate content of a snack or meal – if the percentage of carbohydrate content is less than 35%, chances are the overall GL is low enough to keep your blood sugar levels balanced.


Consuming high GL foods / meals results in chronically high insulin levels and eventual insulin resistance. 

Choosing low GL foods / meals help keep insulin levels low, which promotes fat-burning because insulin (a hormone) shuts off fat-burning. When insulin levels are consistently high, fat usage is effectively blocked. 


Structure the majority of your diet with foods rich in fibre (dark leafy greens, veggies, fruits, beans, lentils, nuts, seeds, and whole grains) as well as rich in antioxidants and phytonutrients (leafy greens, veggies, matcha, cacao, and fruits – especially berries). 

Always be sure to include healthy fats in each meal and snack (nuts, seeds, extra-virgin olive and avocado oils, grass fed butter, coconut oil, fatty grass-fed meats) – fat not only doesn’t affect insulin levels, it can help bring overall GL of a meal/snack down.

The following chart(*) provides examples of foods that fall into low, medium and high GL categories: 

Low GL foods (10 or less)Medium GL foods (11-19)High GL foods (20 or more)
¼ cup peanuts (GL = 1) 1 cup cooked oatmeal (GL = 12)2 slices white bread (GL = 20)
1 cup bean sprouts (GL = 1)1 large banana (GL = 12)1 small box raisins (GL = 20)
1 large carrot (GL = 2)2 slices pizza (GL = 13)1 cup corn flakes (GL = 21)
½ large grapefruit (GL = 2)½ cup ice cream (GL = 14)1 cup chickpeas (GL = 21)
1 cup whole milk (GL = 5)1 cup kidney beans (GL = 15)10 large jelly beans (GL = 22)
1 medium apple (GL = 6)1 cup cooked lentils (GL = 16)1 med baked potato (GL = 28)
1 medium orange (GL = 6)1 cup cooked spaghetti (GL = 16)4oz (114g) potato chips (GL = 30)
1 cup cherries (GL = 7)1 cup yogurt (GL = 16)6oz (166g) mac & cheese (GL = 30)
2 cups popcorn (GL = 7)1 medium donut (GL = 17)1 Snickers candy bar (GL = 35)
2 cups watermelon (GL = 8)1 cup cooked brown rice (GL = 18)1 cup cooked white rice (GL = 37)

4. Transition to JERF or possibly LCHF

A diet made up of whole foods can also be referred to as JERF (just eat real food). By default, it tends to include more healthy fats and less processed carbohydrates than the standard high-carb diet. 

One step further is LCHF (low-carb, high-fat) – still using real foods but with a focus on eating a higher percentage of healthy fats (60-80%). A higher level isn’t necessarily better – it’s about finding what works best for you. 

Thus, if you find yourself eating on level two (see chart on next page) and getting the results you’re looking for, you’re there! That’s the level meant for you. However, if you’re on level two, three or four and still struggling, you will need to move up to level five or six to see the results you want.


Fat-adaptation allows the body to mobilize and use the thousands of calories worth of fuel tucked away in fat cells.

Chang et al 2017(*) describes how long-term LCHF dietary intake can reduce excess body fat and stimulate metabolic adaptations to help athletes reach a maximal fat oxidation rate while lowering carbohydrate oxidation rate, resulting in a glycogen sparing effect. Burke 2015(*) states that rates of fat oxidation during exercise may be doubled by dietary fat-adaption strategies, and Volek 2016(*) shows LCHF keto results in extraordinarily high rates of fat oxidation (with similar muscle glycogen utilization). Burke et al 2017(*) found periodization strategies to be the best use of LCHF diets. 


Refer to chart below. Climb only as high as needed to achieve your optimal results.

Example – if you hit level three and are happy, stop there. However if you are on level four and still not happy, move to level five. Every body is different when it comes to what it needs to achieve optimal results. And note, some people might need to spend time at a higher level before being able to come down a level again.

 Level one

Note: this level doesn’t necessarily encourage metabolic efficiency in many people looking for change. Most will need to move up from this level to see the results you’re looking for. Makes mostly healthy choices, or at least tries to eat overall healthfully; May choose white breads/buns and/or white pasta/rice some or all of the time; Probably is consuming processed cooking oil (via restaurants, chips, fried foods); Indulges sometimes in sweets and treats; Drinks calories (such as pop, alcohol, juice, smoothies) some or most days.
 Level two

JERF (basic)
(no restrictions, with indulgences) Chooses 100% whole grain breads / buns / pastas most of the time (except very strategically for performance/gut reasons); Eats some/all of quinoa, millet, rice, buckwheat, couscous, beans, lentils, yams, potatoes, sweet potatoes; Focus is on real, unprocessed foods; Consumes moderate amount of healthy fats (nuts, seeds, oils, etc.); Indulges sometimes in sweets, ‘healthy’ (or not!) treats & ‘healthy’ packaged bars. 
 Level three

JERF (nutrient-dense)
(some restrictions, limited indulgences, focus on nutrient-density) Avoids processed foods most of the time – breads are sprouted and/or free of added gluten or yeast; any prepackaged bars chosen are nut/fruit based; Chooses high-quality real foods (organic, non-GMO, local, all natural, extra-virgin, nothing added, grass fed / grass finished, wild, free-range / pastured…); Includes lots of leafy greens and veggies with most meals and snacks; No particular restrictions, but does make a point to avoid sugars, processed cooking oils & refined foods in favour of whole foods; About 35-45% of calories come from healthy fats.
 Level four

JERF (nutrient-dense plus)
(with wheat and/or gluten and/ or legume and/or grain restrictions) Same as level one or two, but also avoids all wheat products (possibly consuming spelt, kamut, and/or rye based baked goods instead); May also choose to avoid all gluten entirely (choosing oats, rice, buckwheat and/or amaranth baked goods instead); Or may choose to avoid all grains altogether most/all of the time; May also avoid legumes (beans and lentils) most or all of the time; Typically, only consumes starchy carbs at every other meal or so; About 45-55% of calories come from healthy fats.
 Level five
LCHF (non-ketogenic)
Same as level four, but mostly avoids starchy carbs at every meal (except pre/post exercise, using carbs strategically for fuel & recovery); Makes an effort to include generous amount of healthy fat in meals & snacks; Treats look like dark chocolate (85% cacao or darker), ‘fat bombs’ and fresh berries; About 50-70% of calories come from healthy fats (less than 30% from carbs).
 Level six
LCHF (ketogenic)
Limits carbohydrate intake to 30-50 grams or less; Is careful not to consume too much protein (eats ~1 gram protein per pound of lean body mass); About 70% (to 80%) of calories come from healthy fats (less than 10% carbs).

5. Incorporate Intermittent Fasting

Intermittent fasting (IF) is a manner of eating wherein you consume your required caloric intake within a specific window of time (generally 8-10 hours) and then fast the rest of the time (drinking only water and other non-caloric fluids). 

It’s really important to note that if you are going to incorporate intermittent fasting, it must be done in a manner that still allows you to properly practice nutrient timing and provide your body the fuel it needs before and after any intense (hard anaerobic or longer than 1.5-2 hours aerobic) workouts. While there are benefits to be gained by practicing IF, these benefits could be entirely wiped out by the downsides of failing to properly support your body in fueling properly before, during and after workouts.


As shown by Welton et al in a 2020(*) review of 27 trials, intermittent fasting suggests IF can play a sustainable role in weight loss. Additionally, the studies looking at blood sugar levels found improved glycemic control. And as demonstrated by Guo et al in a 2021(*) randomized controlled trial, IF is an effective strategy to improve cardiometabolic health. 

Intermittent fasting can benefit those who still choose to eat a higher percentage of carbs (thus can be used alone or in a combination with more of a LCHF diet). This is because when you fast your body will use up any available glucose (stored as glycogen), and then switch to burning fat for fuel. Technically we begin to go into this state each day just by sleeping all night (fasting all night). Extending that window of fasting encourages the body to continue burning fatty acids for fuel until you eat again. 

Thus, notable benefits include weight loss, improved metabolic efficiency, mental clarity and longevity. Much of this has to do with its propensity to stabilize blood sugar levels, turn on autophagy (the body’s ‘clean up’ process), reduce inflammation and prevent metabolic syndrome.


Generally, the recommendation is to ease into intermittent fasting. Use the chart below to determine your starting point and possible progression. It’s important to note the goal here is not so much calorie reduction as it is to adhere to your eating window.

Eat to satiation and do not attempt to restrict calories within your eating window. Your body will naturally adjust hunger levels to allow for optimal body composition to come into being.

14:10 fastEating window = 10 hoursEx. Begin eating at 9am and stop eating at 7pm
16:8 fastEating window = 8 hoursEx. Begin eating at 11am and stop eating at 7pm
18:6 fastEating window = 6 hoursEx. Begin eating at 12pm and stop eating at 6pm
20:4 fastEating window = 4 hoursEx. Begin eating at 2pm and stop eating at 6pm

The above five methods give you an idea of training and nutritional strategies that can be utilized in order to gain metabolic efficiency. Please keep in mind that I rarely (if ever!!) have implemented all five for a client all at once. Oftentimes, I’ll choose just one to focus on – the one I feel will have the biggest impact on my clients success. Sometimes it’ll be two or three strategies, but even then the focus is more on one than the other one or two.

Thus, I encourage you to avoid believing you must incorporate all five methods, particularly all at once! Simply choose the one or two that resonate most and dive deeper into those and fully take them on. I could write a book on each of the above 5 methods (indeed, many have already been written by others), so please keep in mind the above directions are simply overviews. In my experience working one-on-one with clients over the past decade, I’ve learned we cannot follow cookie cutter prescriptions and expect ideal results – each method needs to be applied in a way that is highly individualized.

If you’re interested in diving deeper into any or all of these methods, please make sure you’re signed up to receive the eat2run newsletter. Because I’m pregnant (yay) and will no longer be offering one-on-one nutrition coaching as of end of May 2022, I am developing an online video course that will guide you through the specifics of how to apply each of the above methods in a highly individualized manner that will work for you. I’ll be sending out notification once the course is released via the newsletter.

To deliciously healthy food and stronger faster running,

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

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