Zone 2: why is it so controversial in endurance training?
Zone 2 has become the most divisive topic in endurance training in recent years: some present it as the key to all progress, others as a trend overhyped by smartwatches. The real controversy, however, lies elsewhere—it concerns what we put in the tank during these long efforts. And since the launch of our Zone 2 product—the same one, in reality, that we launched at the beginning of Holyfat—we have never taken the time to clearly explain our position on it. Let's be honest: five years ago, neither the information we had at our disposal nor the available research was at the level it is today. This text is a catch-up. And a clarification that we have owed you for a long time.
An intuition, before being proof
In 2019, when the idea of BSE took shape, we had a simple intuition: an excessive intake of carbohydrates during a long, low-intensity effort was not productive. In LSD (long slow distance) type efforts, the goal is not to maximize available caloric intake—it is to make your aerobic system as efficient as possible. And by definition, at low intensity, this aerobic system runs primarily on fatty acids, not carbohydrates. It does not exclude them completely—glycogen is still being used in parallel—but the proportion shifts largely in favor of fats when intensity drops.
That was an intuition. Today, it is no longer one.
What Maffetone and Johnston confirm
Two methodologies currently dominate the reflection on endurance training: that of Phil Maffetone, and more recently that of Scott Johnston—the coach behind the winners, both male and female, of the 2025 edition of the UTMB, whom I had the good fortune to meet and talk with in Chamonix in August. The two approaches are not identical, but they rely on a common principle: building the most powerful aerobic engine possible before moving on to strength or high-intensity training. Not the other way around. It is this aerobic base—measured by what coaches call the aerobic threshold (AeT)—that determines the speed an athlete can maintain for hours without ever touching it.
It is no coincidence that the best ultra-trail runners in the world have one thing in common: a highly developed fat adaptation, which allows them to fuel efforts lasting several hours directly from their reserves. This is not raw talent. It is built, patiently, in Zone 2.
Why provide calories if you already have reserves?
This is the question we are asked, and it is the question I asked myself at the beginning of this adventure. If the body already has significant amounts of fatty acids in reserve, why provide them from the outside?
This is where we must be precise about what BSE has always sought to do. The creation of BSE, even before Holyfat, was not intended to provide fatty acids. It was intended to provide calories, without resorting to carbohydrates.
Why do we need calories, when fat reserves would theoretically be more than enough? Because the central nervous system constantly monitors energy intake and regulates effort based on what it perceives. In other words: the body needs a feeling of satiety to maintain the effort, while avoiding carbohydrates that would disrupt the metabolic response built in Zone 2. And at low intensity—around 60% of maximum intensity—reactive hypoglycemia remains possible, especially in an athlete who is already metabolically compromised: an intake of carbohydrates during the effort triggers an insulin spike, followed by a sharp drop in blood sugar, exactly the opposite of what a long, steady effort requires.
BSE Zone 2 was designed to address this: to maintain a feeling of satiety over time, with a significant caloric intake, but never enough to prevent the body from drawing on its own reserves.
Why macadamia
This is not an arbitrary choice. The macadamia nut has a specific lipid profile—very rich in monounsaturated fatty acids (omega-9), low in omega-6—which makes it a dense, progressive energy source that is particularly well-tolerated digestively during exercise. We discuss it in more detail here.
To go further
If you want to dig into the methodologies that have fueled this reflection:
- Uphill Athlete — Scott Johnston
- Maffetone Method — Phil Maffetone
Five years later, we are not apologizing for being right too early. We are simply making up for the time it took us to explain it to you.