Aerobic Threshold

What is the aerobic threshold?

The aerobic threshold (also called lactate threshold 1 or LT1) is the exercise intensity where blood lactate first rises above resting levels, typically around 1.5-2.0 mmol/L. Below this point, your muscles clear lactate as fast as they produce it, and energy comes almost entirely from aerobic metabolism, predominantly fat oxidation. In untrained individuals, LT1 usually falls around 50-60% of VO2max; in well-trained athletes, it can reach 75-80% ^[1].

The aerobic threshold matters for longevity because it marks the upper boundary of the intensity range where your body builds mitochondrial density, improves fat metabolism, and strengthens cardiovascular function with minimal stress. Training at or just below this threshold is what longevity-focused physicians like Peter Attia call the foundation of a long, functional life.

Why the aerobic threshold declines with age

VO2max drops roughly 10% per decade after age 30 in sedentary adults, and the aerobic threshold tends to decline with it ^[2]. However, research shows the threshold drops more slowly than peak aerobic power. A study of 127 healthy older adults found that while VO2max decreased significantly with age, the ventilatory threshold (which closely correlates with LT1) declined at a slower rate ^[3]. This means consistent training can preserve your aerobic threshold well into your 60s and beyond.

The practical consequence of a declining aerobic threshold is straightforward: everyday activities like walking uphill, carrying groceries, or playing with grandchildren start pushing you into anaerobic territory sooner. You fatigue faster, recover slower, and accumulate more metabolic stress from the same tasks. Maintaining a high aerobic threshold through regular training keeps these activities well within your aerobic capacity.

Aerobic threshold and mortality risk

Cardiorespiratory fitness is one of the strongest predictors of all-cause mortality. The Mandsager et al. study of 122,007 patients found that low fitness carried a higher mortality risk than smoking, diabetes, or coronary artery disease (adjusted hazard ratio 5.04 for low vs. elite fitness) ^[4]. There was no upper limit to the benefit: even "extreme" fitness levels were associated with the lowest mortality.

Each 1 ml/kg/min improvement in VO2max corresponds to roughly a 45-day increase in life expectancy. Since training at the aerobic threshold is the primary way to build aerobic base fitness and raise VO2max, this marker directly connects to how long and how well you live ^[5].

How to measure your aerobic threshold

The gold standard is a graded exercise test with serial blood lactate measurements every 3-5 minutes at increasing intensities. The aerobic threshold is the intensity where lactate first rises above baseline (around 2.0 mmol/L). Lab-grade spiroergometry can also identify the first ventilatory threshold (VT1) through changes in breathing patterns, which correlates closely with LT1 ^[6].

Without lab access, several field methods give reasonable estimates. The MAF 180 formula (180 minus your age, adjusted for health status) approximates aerobic threshold heart rate. The talk test works too: you can speak in full sentences at aerobic threshold pace, but with some effort. Heart rate at 60-70% of tested maximum is another rough proxy. Portable lactate meters like the Lactate Plus or Lactate Scout let you do a simplified step test at home or the gym for under $200.

Fat oxidation and the crossover point

The aerobic threshold closely correlates with the intensity of maximal fat oxidation (FATmax). A meta-analysis found a correlation of r=0.88 between VO2 at the aerobic threshold and at FATmax ^[7]. Below the aerobic threshold, your body burns predominantly fat for fuel. Above it, carbohydrate oxidation takes over progressively until you hit the "crossover point" around 65% VO2max, where carbs become the dominant fuel source.

This has practical implications for metabolic health. Training at aerobic threshold intensity improves the body's ability to oxidize fat, which reduces reliance on glycogen, lowers insulin demand, and improves metabolic flexibility. One study found that individualized training at FATmax intensity improved body composition and insulin sensitivity in patients with metabolic syndrome, without dietary changes ^[8].

Aerobic threshold vs. anaerobic threshold

The aerobic threshold (LT1) and anaerobic threshold (LT2, around 4 mmol/L lactate) are two distinct physiological markers that define three exercise intensity zones. Below LT1, you're in the aerobic zone. Between LT1 and LT2 is the transition zone where lactate accumulates gradually. Above LT2, lactate rises rapidly and exercise can only be sustained for minutes.

For longevity purposes, the aerobic threshold is the more relevant marker. It defines the training intensity that builds aerobic base fitness, mitochondrial density, and cardiovascular capacity with the lowest injury and overtraining risk. The anaerobic threshold matters more for competitive athletes structuring race-pace efforts and interval training.

How to raise your aerobic threshold

Consistent training at or slightly below LT1 is the most direct way to raise it. This means 3-4 sessions per week of 45-60 minutes at an intensity where you can still hold a conversation with some effort. Over 8-12 weeks, you'll notice you can sustain higher speeds or power outputs at the same heart rate and lactate level.

Improvements come from increased mitochondrial density, better capillarization of muscle tissue, improved stroke volume, and enhanced enzymatic activity for fat oxidation. The 80/20 polarized model, where 80% of training stays below LT1 and 20% goes above LT2, produces the best results in both competitive athletes and recreational exercisers ^[9]. Adding 1-2 high-intensity sessions per week on top of your aerobic base work accelerates VO2max gains without compromising the aerobic adaptations.