ARE YOU FAT ADAPTED?
Why is it that some endurance athletes don’t burn much if any fat even though they train for long distance events such as the marathon or IRONMAN. Furthermore, how come they can have low lactate (sub 2mmol) when cycling or running in “zone 2” BUT don't burn much if any fat (circa 20%) where others burn 80%+ fat in this zone?
This question is one that goes right to the heart of modern endurance physiology and performance.
Let’s break it down step-by-step:
THE SCIENCE BEHIND FAT BURNING:
1. Mitochondrial Density and Enzyme Adaptation
Low lactate reflects low glycolytic stress, but not necessarily high fat oxidation. Some athletes (especially those with a history of high-carb fueling or high-intensity training) have plenty of mitochondria, but low expression of fat oxidation enzymes (like CPT-1, HADH, β-HAD) and poor mitochondrial fatty acid transport efficiency. In short, they can “move” easily (low lactate), but their mitochondria are “trained” to prefer glucose.
2. Metabolic Inflexibility
This is the inability to switch efficiently between fat and carbohydrate as the dominant fuel source. It’s often caused by chronic high carbohydrate intake (insulin always elevated), limited time spent in fasted or glycogen-depleted states and a lack of long, TRUE low-intensity volume. Such athletes are aerobically fit, but metabolically “stiff.”
3. Fiber Type Expression
Two athletes can have the same VO₂max and lactate response, yet very different fiber composition. Type I (slow-twitch) fibers are rich in mitochondria and fat-oxidizing enzymes. Type IIa/IIx (fast-twitch) are more glycolytic with less fat oxidation. Athletes with more fast-twitch dominance (even if “fit”) will burn less fat at any given effort.
4. Nutritional Background
Frequent high carbohydrate feeding during and between sessions suppresses AMPK activation and fat metabolism signaling (PGC-1α pathway) and keeps the body “comfortable” with glycolysis, discouraging fat oxidation enzyme upregulation. If you’re taking carbs during every session, even easy ones, the body never gets the signal to switch to fat.
5. Training Distribution
If an athlete spends too much time in Zone 3 (moderate intensity) or high-carb fueled Zone 2, they’re never forcing metabolic flexibility adaptations. In contrast, elite endurance athletes often have a high Zone 1–2 volume (70–80% of total training) with periodic low-glycogen sessions and consistent fuel periodization
6. Genetic, Lifestyle and Hormonal Factors
Some athletes have lower AMPK and PPAR-α activation. These are the genes that trigger fat oxidation pathways. They can also have a higher RER (respiratory exchange ratio) baseline due to stress, cortisol, or caffeine use making them more carb reliance.
HOW TO BURN MORE FAT
1. Fuel Periodization
This is known as the “Train Low, Compete High” plan. Here you do selected Zone 2 sessions in a fasted state or with minimal carbs (~0–20g/hour). Introduce these gradually (1–2 sessions/week max) to avoid suppressing quality training or immune function. Keep some high-carb fueled key workouts (threshold/tempo sets) in your week (20% max). This process stimulates enzymes like CPT-1 and PPAR-α that drive fat oxidation.
2. Increase Low-Intensity Volume
True Zone 2 training builds mitochondrial density and the ability to use fat. Aim for 80% of total training time here if your events are long distance focused.
3. Add “Long Slow Depletion Sessions”
An example would be doing an evening workout in low zone 2 then doing the same next morning before breakfast. These sessions drive massive increases in fat oxidation capacity.
4. Monitor and Retest
Use lactate testing (to verify zone 2 and aerobic thresholds in conjunction with the VO2 metabolic cart to track RER data or fat oxidation (FAT-MAX). Here we are looking for the shift of RER moving down from say 0.90 → 0.80 at same power/pace and a decrease in Lactate (and possibly HR) at the same pace too
5. Nutritional Support
You must support fat metabolism with adequate protein which is 1.6–2.0 grams per kg of lean mass per day. Adding Omega-3s from food and supplements will enhance mitochondrial function. Most importantly, try to avoid constant snacking on high-carb foods between sessions — maintain the 33% balance of each macronutrient at all meals and snacks.
6. Strength & Endurance Synergy
Resistance training improves muscle insulin sensitivity and mitochondrial density which promotes better substrate flexibility over time.
7. Patience
This is perhaps the most important and disliked part of the plan! Fat adaptation is slow! For example, it takes typically at least 6–10 weeks to see a measurable shift in RER or fuel use. However, once it happens, it’s durable and game-changing for the marathon, 70.3 or IRONMAN and stage racing on the bike.
WHERE DO I START?
Step 1 is to come in to my lab and do the full range of assessments to truly understand how much fat you burn (“FAT-MAX”), what your true zone 2 is (lactate test) and how many calories you need per day to fuel your engine (RMR test).
This ULTIMATE testing package is the gold standard process for setting you up for success in whatever endurance sport you do.