written by Olivia Crozier
The body has two distinct sweat systems, but only one of them matters for performance.
Sweat is not a single, uniform substance produced by a single type of gland. The human body relies on two structurally and functionally distinct sweat glands, eccrine and apocrine, which exist for entirely different reasons. One is built for cooling the body during physical exertion. The other is tied to hormonal and emotional triggers and has little to do with thermoregulation. For athletes trying to understand sweat rate, electrolyte loss, and sweat composition, knowing which gland is doing the work changes how hydration management should be approached.
Eccrine glands are the most abundant sweat glands in the body, numbering in the millions and distributed across nearly the entire skin surface. They are especially concentrated on the palms, soles, and forehead, but they are present almost everywhere.
These glands open directly onto the surface of the skin and produce a thin, watery, largely odorless fluid made up primarily of water along with small amounts of sodium, potassium, and other electrolytes. Eccrine sweat is activated by the cholinergic nervous system, meaning it responds to signals tied to heat, physical activity, and rising core temperature. When the body needs to cool down, eccrine glands are the ones doing the cooling. As sweat evaporates off the skin, it pulls heat away from the body, which is the core mechanism behind thermoregulation during exercise.
Apocrine glands are structurally different and far more limited in where they are found. They are concentrated in specific regions of the body, primarily the armpits, groin, and other areas with hair follicles, and they open into those follicles rather than directly onto the skin.
Unlike eccrine glands, apocrine glands remain dormant until puberty and are activated by the adrenergic nervous system, which responds to hormonal shifts, emotional stress, and stimuli unrelated to body temperature. The fluid these glands produce is thicker and contains proteins, lipids, and fatty compounds. On its own, this secretion is largely odorless, but once it interacts with bacteria on the skin, it becomes the primary source of body odor. Apocrine glands play essentially no role in cooling the body during exercise.
The two systems differ in nearly every measurable way:
For athletes, this is not a close comparison. Eccrine glands are the system that governs sweat rate, fluid loss, and electrolyte depletion during training and competition. Apocrine glands contribute nothing meaningful to cooling capacity or hydration status, regardless of how strong an odor they produce.
This distinction matters because it clarifies what athletes are actually managing when they track sweat loss. The fluid lost during a workout, along with the sodium and other electrolytes carried out with it, comes from eccrine activity. That is the system responsible for maintaining core temperature under load, and it is the system that determines how much fluid and how many electrolytes need to be replaced to avoid performance decline, cramping, or more serious outcomes like heat illness.
Eccrine sweat glands also adapt over time. Athletes who train consistently, particularly in hot conditions, tend to develop a more efficient eccrine response. They begin sweating sooner, produce a more dilute fluid, and conserve more electrolytes per ounce of sweat than an untrained individual. This is a direct physiological adaptation to repeated heat and exercise exposure, and it is one of the reasons sweat rate and sweat composition vary so widely between individuals with different training backgrounds. Apocrine glands show no comparable adaptation. Their output stays tied to hormonal and emotional triggers regardless of fitness level.
Because eccrine glands are the ones losing water and electrolytes during exercise, they are the only relevant target for a hydration strategy built around performance. Tracking overall sweat loss without accounting for how concentrated that sweat is in electrolytes gives an incomplete picture. An athlete who sweats heavily but conserves sodium efficiently has different replacement needs than one who sweats less overall but loses electrolytes at a higher rate. Both patterns originate entirely in eccrine gland function.
This is also why generic hydration advice, such as a fixed number of ounces per hour, fall short for serious athletes. Eccrine sweat rate and composition vary by individual, by climate, by training status, and by the specific demands of a given workout. Understanding sweat as a product of a highly individualized, adaptable eccrine system, rather than a single fixed rate of fluid loss, is what allows athletes to build a hydration plan around their own physiology instead of a population average.
Eccrine and apocrine glands may both produce sweat, but they are not comparable systems. Eccrine glands are the ones responsible for thermoregulation, fluid loss, and electrolyte depletion during exercise, and they are the only system that matters when building a hydration and performance strategy. Apocrine glands, tied to hormonal and emotional triggers rather than heat, have no bearing on athletic output. For athletes who want to train and compete effectively, the sweat worth paying attention to is eccrine sweat, and understanding how individualized that system is marks the difference between a generic hydration plan and one built around actual physiological need.