Biological energy is the capacity to perform work, and within the human body, this is accomplished by a molecule called adenosine triphosphate (ATP). ATP is the immediate energy currency, spent to power cellular processes like muscle contraction and nerve impulses. Because the body’s store of ATP is so small, it cannot sustain activity for long, necessitating a continuous supply of fuel. True long-term energy provision refers to the body’s stored reserves that can fuel activity and maintain life functions over hours, days, or even weeks without food intake. The body has evolved different storage solutions, each with a specific role in bridging the gap between immediate demand and sustained supply.
Glycogen: The Medium-Term Energy Reserve
The body’s short-term solution for sustained energy comes from stored carbohydrates as glycogen, a branched polymer of glucose. Glycogen is a readily accessible fuel source that can be quickly broken down into glucose to power cells. This reserve is primarily housed in two locations: the liver (about 100 grams) and the skeletal muscles (storing significantly more, around 400 to 500 grams in a well-trained athlete).
The liver’s glycogen is used to maintain stable blood glucose levels, releasing glucose into the bloodstream for the brain and other organs. Muscle glycogen, conversely, is retained and used only by the muscle cell to fuel movement. Because glycogen is stored alongside water, its storage capacity is limited, supplying energy for approximately 12 to 24 hours of fasting or for short bursts of high-intensity exercise.
Lipids: The Primary Long-Term Fuel Source
The most substantial reservoir for sustained energy is lipids, commonly known as body fat, stored primarily as triglycerides in adipose tissue. Lipids are the body’s preferred long-term storage because they are significantly more energy-dense than carbohydrates. Fat molecules yield about nine kilocalories of energy per gram, more than double the four kilocalories provided by carbohydrate or protein.
This high energy density allows the body to store a massive amount of fuel in a compact space, enabling survival during prolonged periods without food. The total capacity for storing fat is virtually limitless, with even a lean individual carrying enough stored fat to fuel activity for many weeks. Accessing this energy, called lipolysis, breaks down triglycerides into glycerol and fatty acids. These fatty acids are then released into the bloodstream and transported to tissues, where they are converted into ATP via beta-oxidation to provide sustained power.
The Hormonal Regulation of Energy Storage and Release
Energy storage and release are tightly controlled by a hormonal signaling system. The pancreas secretes two opposing hormones, insulin and glucagon, which act as the main metabolic switch. After a meal, rising blood glucose triggers the release of insulin from the pancreas.
Insulin acts as the “storage” signal, promoting the uptake of glucose into cells for conversion into glycogen and inhibiting the breakdown of fat stores (lipolysis). Conversely, when blood glucose levels fall during fasting or prolonged activity, the pancreas releases glucagon. Glucagon is the “mobilization” signal, instructing the liver to break down its glycogen (glycogenolysis) and stimulating the release of fatty acids from fat tissue.
Epinephrine, also known as adrenaline, works alongside glucagon, particularly during acute stress or intense exercise. This hormone acts rapidly on both the liver and muscle to accelerate the breakdown of glycogen for quick glucose release. Epinephrine also powerfully stimulates lipolysis, rapidly mobilizing fatty acids from fat cells to ensure tissues have ample fuel to meet sudden, high energy demand.
Optimizing Energy Reserves Through Diet and Activity
Strategic nutrition and regular physical activity enhance the efficiency and capacity of the body’s long-term energy systems. Maintaining a balanced intake of macronutrients (carbohydrates, proteins, and fats) is important for energy stability. Pairing carbohydrates with protein and healthy fats helps to slow the absorption of glucose, preventing sharp insulin spikes and promoting gradual, sustained energy release.
Consistent eating patterns, rather than erratic intake, help keep the insulin and glucagon system in balance, reducing the metabolic stress of constant blood sugar fluctuations. Physical activity, particularly sustained aerobic exercise, drives adaptation by increasing the number and size of mitochondria within muscle cells. Increased mitochondrial density enhances the cell’s capacity to efficiently oxidize fatty acids, improving the body’s ability to tap into its vast long-term fat reserves.