VO2Max test and DEXA scan: my fitness benchmarks after 6 months of Zone 2

I took a VO2Max test and DEXA scan recently and compared the results to the same tests six months ago. The tests give me an overall view of how my fitness and body composition have changed, and help check that my Zone 2 training progress is on track.

The headline: the low-intensity training is working. Here's the data.

The VO2Max test

The VO2Max test should give an accurate view of my aerobic and anaerobic thresholds, which in turn helps define my optimum heart rate zones for training. Understanding whether I can push my effort and heart rate without compromising my fuelling needs — or risking blowing up — is critical for long-distance events.

The test was carried out on a Watt Bike. I wore a mask measuring oxygen in and CO2 out. This equipment then compares those values to heart rate to establish aerobic and anaerobic thresholds.

Anaerobic threshold — the key change

The most important number from the test is the point where CO2 production exceeds oxygen consumption — where the red line crosses the green line on the graph. This is the anaerobic threshold.

In December, my anaerobic threshold was at a heart rate of 135 bpm. In the June test, it had shifted to 151 bpm. That's a significant improvement — it means I can work harder before crossing into anaerobic metabolism. At any given effort below 151 bpm, my body is still primarily burning fat.

My Zone 2 training range from this test was set at 112–122 bpm, a small shift from the previous 115–125. The machine didn't fully capture my anaerobic threshold in the calculation, but more importantly the data shows I'm pushing out more power for the same effort.

Fat burning — the most striking result

The fat burning graphs are the most interesting finding. They show calories from fat versus calories from carbohydrates at different heart rate intensities.

In December, even at very low intensities, I was burning a high proportion of carbohydrates. This was a significant surprise — I'd done a lot of Zone 2 training the previous year and assumed my fat metabolism would be well adapted. It wasn't.

My hypothesis: I was more reliant on carbohydrates because of a carb-heavy diet and evening snacking. I had relatively high blood sugar even fasted.

By June, the picture had changed dramatically. I'm now burning fat through a much wider heart rate range. At lower intensities, I'm burning a much higher proportion and volume of fat. This is exactly what you need for endurance events — you can go for far longer without having to fuel on carbohydrates, because you're running on your body's essentially unlimited fat stores.

VO2Max

My VO2Max came in at 44.7 in June, essentially unchanged from 45.2 in December. This initially seemed odd — if everything else improved, why not VO2Max?

The likely explanation: VO2Max is heavily influenced by high-intensity training, and my programme during this period was deliberately weighted toward Zone 2 base work. My Zone 2 adaptations (fat metabolism, anaerobic threshold) improved significantly. VO2Max tends to improve with Zone 5 high-intensity work, which I wasn't prioritising during this phase.

For the next phase, I'll maintain high-intensity sessions weekly to also drive VO2Max improvement.

The DEXA scan

A DEXA scan is considered the gold standard for measuring body fat, bone density, visceral fat, and lean muscle mass. The June scan showed:

• Body fat: 23.4%, down slightly from 24%. Still needs work — I need to continue losing fat, particularly visceral fat.
• Visceral fat: Not significantly changed. This requires dietary discipline as much as training volume.
• Lean muscle mass: Increased by just under 1kg — attributed to adding strength training to my routine.
• Bone density: Towards the low end of the range, but increased from December. Strength training is helping.

What I'm changing based on these results

• Continue 80% of exercise as Zone 2 sessions, maintaining HR at around 125–130 bpm (slightly higher than the previous 115–125 I'd focused on).
• Include at least one indoor Zone 2 session per week to continue pushing power output at this effort level.
• Maintain high-intensity sessions weekly to improve VO2Max alongside the aerobic base.
• Weight training at least once per week — for bone density and lean mass, both of which have responded positively.
• Stop snacking between meals, particularly in the evening. Increase protein consumption. Reduce refined carbs.

The baseline tests: where it started

Before starting this Zone 2 project, I took a full set of baseline tests — VO2Max, HbA1c blood glucose, DEXA, and resting metabolic rate. Some of those results surprised me, particularly the blood glucose (slightly elevated despite fasting) and the initial fat metabolism picture.

The baseline VO2Max established my initial Zone 2 at 115–126 bpm and an anaerobic threshold of 135 bpm. The June test confirmed the anaerobic threshold has risen 16 beats, to 151 bpm. That's six months of Zone 2 training showing up clearly in the data.