Cardio for health and sports performance

by | Sep 17, 2023 | Nutrition

Cardiovascular training is important for general health and sports performance. It strengthens our heart, helps burn energy, and improves cell function.

However, cardio can be performed at various intensities. Some people prefer shorter, high-intensity workouts, while others prefer to go slow and steady. For optimal health and sports performance, it’s essential to train at different intensities.

Intensity determines:

  1. Which muscle fibers we use – slow or fast.
  2. What cells use for energy: sugars, fats, or lactate.
  3. Fatigue and recovery time between workouts.

In simple terms, training at different intensities teaches the body to use different types of fuel, produce more energy, utilize more energy from fat stores, and increase the body’s ability to tolerate higher loads.

What intensity to train at?

In general, it is recommended to engage in 150 minutes of cardiovascular training per week, ideally 300 minutes. Of this, 80% should be at a light intensity (Z2), and 20% at a high intensity (Z4). This ratio is suitable for endurance athletes as well as those looking to improve their health. The ratio between light and intense training may vary depending on the sport and goals.

How to estimate intensity?

Training zones are most commonly determined by heart rate, but other tools can be used:

Z1 – Recovery zone, 50-65% of maximum heart rate. Energy primarily comes from fats, improving mitochondrial function.

Z2 – 60-75% max heart rate – Basic aerobic endurance. Improves fat metabolism and achieves maximum fat-burning capacity and lactate breakdown. Develops mitochondrial function and count.

Z3 – 70-85% max heart rate. Sugar metabolism begins to dominate, and more lactic acid is produced. It is more fatiguing than lower intensities.

Z4 – 85-90% max heart rate. In this zone, we reach maximum lactate production, and fat metabolism decreases.

Z5 – 90%+ max heart rate. Maximum effort.

For sports performance and health, the most relevant heart rate zones are the second and fourth zones:

  • In the second zone, we maximize the benefits of fat-burning and cell function, mainly engaging slow-twitch muscle fibers.
  • In the fourth zone, we use sugars stored as glycogen in muscles and their byproduct, lactate. We also utilize fast-twitch muscle fibers.

Why is Z2 training important?

Running is an endurance sport where aerobic metabolism is crucial. The larger our aerobic base, the more volume and intensity we can handle during training.

The adaptations we aim for with cardio training include:

  • Improved lactate clearance
  • Increased utilization of fats for energy
  • Formation of new mitochondria and capillaries, which produce and supply energy and oxygen to muscles

These adaptations are most effectively achieved through Z2 training.


It relates to how energy is produced in cells. Cells can use fats, glucose, glycogen (sugars), and lactate (a byproduct of sugar metabolism) as fuel. But any run longer than about 3 minutes is largely dependent on fat metabolism.

We could develop fat metabolism through walking (Z1) or even sprinting, but that would require either walking for hours every day or sprinting daily. It’s not efficient.

Most amateur runners (and cyclists) train in Z3, also known as the gray zone. Although it’s still an aerobic zone, sugar, and lactate metabolism begin to dominate over fat. (This doesn’t mean it’s wrong, but more intense training is more efficient for this purpose.)

πŸ‘‰ Ultimately, it puts more strain on the body than necessary for building an aerobic base and causes more fatigue.

πŸ“Š For comparison, consider the training zone data from one of the best amateur cyclists compared to a cyclist struggling to improve performance (data from friends at the Slovenian Human Performance Centre). Notice that the better cyclist spends less time in Z3 (green line).

Wouldn’t it be better to train in Z1? That’s when we burn more fat.

πŸ‘‰ From the perspective of developing fat metabolism, it’s true that in Z1, a higher percentage of energy comes from fats compared to Z2. However, in Z2, we achieve the highest absolute amount of fat burned (grams of fat per minute).

In sports, what matters isn’t whether we use 80% or 90% of fats but whether we have sufficient energy production.

  • In Z1, 80% of energy may come from fats, but that may be only 0.23g/min.
  • In Z2, 70% of energy may come from fats, which would represent 0.53g/min.

If we were only concerned with the percentage (relative amount) of energy from fat metabolism, it would be better to walk, or even better, sleep πŸ˜‰ During sleep, we use about 99%.

πŸ‘‰ In Z2, we also train lactate clearance. Lactate is continually produced in the body, especially during sugar metabolism, which increases with exercise intensity. In Z1, lactate production is too low. In other words, the body doesn’t need to improve its utilization.

🚨 You can run easily, unnecessarily!

Some runners spend too much time in Z1, known as “junk miles” – chasing distance with a minimal training effect. They accumulate fatigue unnecessarily and expend energy that could be used for strength training, mobility, replenishing energy stores, or life outside of running.

Slow running (in Z2) is most efficient for fat burning. But it doesn’t necessarily mean weight loss.

It means that the majority of energy comes from fats. In the context of weight loss, it doesn’t matter whether the body gets energy from fats, blood sugar, or glycogen.

For reference, a trained athlete burns about 0.5g of fat per minute. The better the runner, the more fat (and glycogen and glucose) they can utilize.

Weight management is determined by the balance between energy expenditure and intake at the end of the day.

If you consume 2000 calories a day and take in 2000 calories from food, it doesn’t matter how much fat you burn during your run.

For weight loss, diet is more critical than exercise. An ideal approach combines strength training, cardio, and diet.

In summary:

In Z1, you achieve similar improvements as in Z2, but to a much lesser extent. This kind of training benefits beginners, those returning to exercise after illness, or ultra-endurance races where you’re moving for more than 12 hours straight.

For others, daily movement is in the form of Z1 and a better time investment is Z2, given its time efficiency.

The less time you have for cardio training, the more important intensity becomes. But intensity doesn’t replace training at a lower intensity!

Why train at high intensity?

During high-intensity training (Z4 and Z5), fast muscle fibers are mainly used, which relates to fuel selection. Glycogen, glucose, and lactate are predominantly used as sources of energy.

For metabolic health:

During high-intensity activities, sugars in the form of muscle glycogen are primarily used as an energy source.

Obese individuals and those with excess weight usually have their glycogen stores full. When they consume carbohydrates in their diet and don’t move enough, blood sugar remains elevated for a longer time before being stored in fat cells. Continuous elevated blood sugar leads to inflammatory processes, leading to prediabetes and later diabetes. (This only happens when there’s an energy intake surplus! Energy balance still applies).

Physically active individuals continually deplete their glycogen reserves, leaving space for new sugars (from food) to be stored in muscles.

This partly explains why physically active individuals can consume more carbohydrates without significant blood sugar fluctuations and why a carbohydrate-rich diet doesn’t automatically lead to weight gain.

For sports performance:

Athletes aim to improve endurance and the ability to use lactate as energy during high-intensity training. When we surpass the second lactate threshold (LT2), lactate accumulates faster than it’s recycled. This gradually leads to acidosis in the body and forced slowing down.

The goal of Z4 training is to enhance the body’s ability to recycle lactate, maintaining a higher pace for a longer time.

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