Underfuelling vs overtraining: why your pace has stalled
That chronic plateau isn't a recovery failure. It is a mild, persistent energy shortfall that looks and feels exactly like discipline.
~3 MIN · KEXBI RESEARCH
That chronic plateau isn't a recovery failure. It is a mild, persistent energy shortfall that looks and feels exactly like discipline.
There is a pattern running through amateur endurance sport that rarely gets named. It does not show up as an injury or an illness. It shows up as a plateau. You are training six hours a week, sleeping well, hitting your calorie targets, and your pace will not budge and your recovery metrics sit flat.
You are probably not over-training. You are under-fuelling.
Relative Energy Deficiency in Sport, updated in 2023 to the term REDs, describes what happens when there is not enough energy to cover both the cost of training and the everyday work of the body [1]. The measure at the centre of it is energy availability: the energy left over, per kilogram of fat-free mass, once training has taken its share.
Below roughly 30 kcal per kg of fat-free mass, the body starts to triage. It down-regulates reproductive hormones, eases back on the thyroid, slows bone formation, and, most relevant to your training, blunts the adaptation you are training to drive [2].
The insidious part is that most calorie apps steer you straight into that shortfall.
Tell a standard nutrition app you want to lose a kilogram a week and it will subtract a fixed block of calories, often around 1,000 a day, from your target. If you are a lean 75 kg runner covering 40 miles a week, that flat deduction can drop your energy availability well into the low range on your biggest training days.
The app does not know your fat-free mass. It does not know what energy availability is. It treats "ate under target" as success, and the harder you train, the deeper it lets you dig.
This is where KEXBI works differently. Rather than defend a single borrowed number, it calibrates your floor to you, to your sex and your body composition, and holds your intake above it. If you try to configure a deficit that would strip the energy your body needs to adapt, the engine raises the floor instead, protecting the very processes that let you mobilise fat and build muscle in the first place.
Under-fuelling is not only about total calories. It skews heavily toward carbohydrate. The low-carb message has convinced many endurance athletes that running on empty glycogen makes them "fat-adapted."
Fat oxidation handles easy Zone 2 work well. But threshold intervals and race-pace efforts run on glucose. Go into a hard session under-fuelled and short of carbohydrate, and the body leans harder on stress hormones and breaks down its own protein for fuel through gluconeogenesis. You finish feeling disciplined, having partly undercut the session you just did, and raised your stress load doing it. Fuelling those efforts, in the region of 60 g of carbohydrate per hour for longer hard sessions [6], protects both the work and the tissue.
KEXBI handles this by building the in-session fuel into the plan. When a session maps to a sustained high-intensity block, the plan puts fast-acting carbohydrate where it is needed, so you fuel the work rather than survive it, and hold on to the lean mass you are training to build.
The plateau is rarely a training problem. It is an energy problem wearing a training problem's clothes. You cannot out-train, out-discipline, or out-supplement an empty tank, because the adaptation you are chasing only happens when there is energy left to make it. Fuel the work, protect your energy availability, and the progress you have been grinding for has room to actually show up.