Why are you going to start running (if you don't already) after reading this article?

The evidence behind the risks of a sedentary lifestyle and the power of Zone 2 training.

The first time I heard about Iñigo San Millán was almost by chance, I discovered him while following on X (Twitter) news about Athletic Club de Bilbao. In addition to being the team's performance director, Iñigo is a researcher at the University of Colorado, specializing in physiology and metabolism, and also a consultant for the UAE Team Emirates cycling team. And it was as a result of a tweet he posted precisely on X when I started to delve into his studies on the impact of inactivity on our health, even in people who consider themselves "healthy".

In one of his studies entitled "Mitochondrial and Metabolic Differences Between Sedentary and Active Individuals at Rest and During Exercise", he reveals a fact that made me think about many people in my close environment: even if you don't have any disease and you feel fine, sedentary lifestyle may be affecting your health at a deep and almost invisible level. In this work, San Millán and his team demonstrated that sedentary people, although apparently healthy, have significant deficiencies in the function of their mitochondria, those small "power plants" of our cells. And, in the end, these deficiencies could lead to serious long-term problems.

Health beyond the obvious

The idea that a medical check-up or a libre life means we are healthy may be a superficial concept of our health. This study, led by San Millán and his team, makes it clear that what happens at the cellular level tells a different story. At the metabolic level, inactive people accumulate waste in the system, as if their body's engine were not at full capacity. Specifically, the study shows how sedentary people, even those without any serious disease, develop a type of "mitochondrial dysfunction," where the metabolic system becomes inefficient in its use of energy.

To try to explain this better, it is necessary to understand that our cells depend on small "batteries" called mitochondria, which are responsible for converting nutrients into energy. In active people (and we mean doing sports at least 3 days a week), the mitochondria are working at full capacity, efficiently using glucose and fatty acids as fuel. However, in sedentary people, this process is compromised. The result: an excess of lactate, a kind of metabolic "smoke" that indicates that the engine is not running at maximum capacity. This excess, as derived from San Millán's study, is one of the signs that the body is under stress that in the long term can trigger more serious problems, such as cardiovascular disease and type 2 diabetes.

The impact of zone 2 on our metabolism

So what can we do to avoid this accumulation of metabolic "junk"? One of the answers seems to lie in physical activity, and more specifically in zone 2 training, a modality that San Millán highlights as one of the most effective for keeping the mitochondrial system healthy.

At RUNNEA we have already told you many times about the benefits of Zone 2 training. Zone 2 training involves working at a low to moderate intensity, in which our body can consume oxygen constantly and, therefore, maintain fat oxidation as the main source of energy. The key to this type of exercise is that it keeps the metabolism active and the mitochondria's ability to process fuels in optimal condition. In practical terms, activities such as jogging at leisurely paces, even brisk walking, swimming or moderate paced cycling fall into this zone, which certainly makes it accessible to virtually anyone.

According to San Millán's study, Zone 2 training improves metabolic flexibility, a critical ability that allows our cells to switch from one energy source to another - such as from glucose to fat - smoothly. In active people, this ability is intact, while in sedentary people it is hampered. It is, in other words, the difference between an engine that runs efficiently and smoothly and one that does so without getting the most out of it and generating "particles" that eventually damage the functioning of that engine.

Why is metabolic flexibility important?

One of the most important concepts in San Millán's research is what he calls metabolic flexibility, which is nothing more than our body's ability to adapt to different energy sources as needed. This capacity is fundamental for maintaining long-term health, as it allows cells to efficiently manage nutrients and avoid excessive storage of fats and sugars.

However, this flexibility is more impaired in sedentary individuals. Their study shows how in these individuals fat oxidation is limited and, instead, the body tends to accumulate glucose in the form of lactate. This change in the way nutrients are used is one of the first steps towards the development of metabolic diseases, and the worst thing is that it often goes unnoticed until the problems are already serious.

This is where zone 2 training plays a very important role. By keeping mitochondrial activity at optimal levels, it helps the body remain able to oxidize fats, which prevents unnecessary lipid storage and in turn facilitates blood glucose control and reduces lactate production. Together, this means less inflammation, less risk of developing insulin resistance and a better ability to maintain a healthy weight.

Lactate: friend or foe?

One of the most interesting findings of the San Millán study is the role of lactate, a by-product of metabolism that, in active people, is recycled and used as a source of energy. However, in sedentary people, this lactate accumulates in the body, generating a "metabolic waste" effect that contributes to the premature aging of cells and the development of chronic diseases.

This phenomenon occurs because the metabolic system of an active person is equipped to handle lactate. Instead of accumulating, lactate is recycled in other tissues, such as the heart or liver, and reused to produce energy. In sedentary people, this capacity is lost, and lactate ends up causing inflammation and cell damage. Here, zone 2 training reappears as a good tool to improve lactate management, by making the body adapt and recycle this compound instead of letting it accumulate.

Mitochondrial dysfunction: the price of a sedentary life

Another critical point of the study is the deterioration of mitochondria in people who do not exercise. Mitochondria are, simply put, small energy factories. And, like any machinery, they need maintenance. Physical exercise acts as "preventive maintenance" for mitochondria, keeping them in good shape. In sedentary people, however, these mitochondria are losing capacity and efficiency, which ends up affecting not only the daily energy, but also the state of health in general.

A sedentary person may appear healthy from the outside, but his or her cells are running at half throttle. As the mitochondria lose their ability to process nutrients, a vicious cycle ensues in which the body becomes increasingly dependent on glucose and produces more lactate, creating an environment conducive to diseases such as type 2 diabetes, hypertension and obesity.

Zone 2 training: more accessible than you think

The best thing about zone 2 training is that you won't have any excuses. running at a gentle pace, where you can hold a conversation, for example, is a very effective tool. But so is brisk walking, walking up and down stairs, swimming or cycling at a moderate speed. There is no need to pay for a gym or follow a complex training plan; the important thing is to stay active and do this type of exercise consistently.

And as if the benefits weren't enough, it also helps reduce stress levels, improves circulation and boosts the immune system, all of which contribute to healthy aging. As we age, maintaining mitochondrial function becomes even more important.

Moving to live longer and better

The message of the San Millan study I think is quite clear: sedentary is not a neutral state. Although someone may be "healthy" in medical terms, lack of movement imposes a high price at the cellular level. And that price is cumulative; as the mitochondria lose their ability to process nutrients, the body deteriorates, opening the door to diseases that could have been avoided with something as simple as going for a gentle jog 3 days a week. If you're already doing it, congratulations. And if not, today is a good day to start.