What happens to unused calories in our body?

January 4, 2026 •

3 min read

Over 80% of excess energy in the body is stored as fat. Unused calories are not simply eliminated; they undergo a complex metabolic process, ultimately converted into stored energy reserves to be used later.

Quick Summary

The body stores unused calories in two primary ways: short-term as glycogen in the liver and muscles, and long-term as triglycerides within fat cells. The hormone insulin orchestrates this storage process.

Key Points

Two Storage Methods: Unused calories are stored in the body in two primary forms: short-term as glycogen and long-term as fat.
Glycogen Stores: Glycogen, stored mainly in the liver and muscles, is the body's first energy reserve and has a limited capacity.
Fat Storage: Once glycogen stores are full, excess calories are converted into triglycerides and stored in fat cells within adipose tissue, which has a vast storage capacity.
Insulin's Role: The hormone insulin is the primary signal for storing excess calories by prompting cells to take in glucose and store it or convert it to fat.
Energy Balance: Weight management is based on energy balance; if you consume more calories than you burn, the surplus is stored, leading to weight gain.
Fat Mobilization: The body mobilizes and uses stored fat for energy when it is in a caloric deficit, meaning it's using more calories than it consumes.

The Body's Energy Management System

Our bodies are exceptionally efficient energy managers, constantly working to maintain balance. When we consume food, our body breaks it down into energy-yielding nutrients: carbohydrates, proteins, and fats. This energy is used for immediate needs, such as breathing, thinking, and physical activity. When we consume more energy than we burn, the surplus calories must go somewhere. They are not simply discarded. Instead, they are converted into two different forms of stored energy. This process is crucial for survival, ensuring we have a reserve supply of fuel for times when food is scarce. The primary driver of this storage is the hormone insulin, which signals cells to take in glucose from the bloodstream.

Short-Term Storage: Glycogen

The first destination for excess energy, particularly from carbohydrates, is the body's glycogen stores. Glycogen is a complex carbohydrate molecule created by linking together simple glucose units. Glycogen is primarily stored in the liver (about 100 grams, or 400 calories) for maintaining blood sugar and in muscle cells (around 350 grams, 1,400 to 1,800 calories) for muscle use during exercise. These glycogen stores have a limited capacity.

Long-Term Storage: Adipose Tissue (Fat)

Once glycogen stores are full, any additional surplus calories are converted and stored as fat in adipose tissue. The liver converts excess energy into triglycerides, which are then stored in fat cells. Unlike glycogen, adipose tissue has a nearly unlimited capacity to store fat. Fat cells enlarge to hold these triglycerides. This fat is stored throughout the body, including under the skin and around organs. Excessive fat storage, particularly around organs, is linked to chronic diseases.

The Role of Insulin

Insulin, released after a meal in response to rising blood sugar, is key in directing energy storage. It prompts cells to absorb glucose, promoting glycogen creation and, when stores are full, the conversion of excess glucose into fat. Persistently high insulin levels can lead to continuous fat storage and potentially insulin resistance.

The Energy Balance Equation

The fate of unused calories is fundamentally governed by the energy balance between calories consumed and calories expended. A calorie surplus results in storage, mainly as fat. Physical activity helps utilize stored calories, first depleting glycogen before tapping into fat reserves.


Feature	Glycogen Storage	Adipose Tissue (Fat) Storage
Storage Duration	Short-term (readily accessible)	Long-term (less readily accessible)
Primary Macronutrient	Carbohydrates	Carbohydrates, Fats, and Protein
Storage Location	Liver and Muscles	Adipose (Fat) Cells
Capacity	Limited (approx. 2000 calories total)	Virtually Unlimited
Primary Hormonal Driver	Insulin	Insulin
Mobilization Time	Fast (used between meals or during exercise)	Slow (tapped into after glycogen stores are low)

Conclusion

The body manages unused calories through a two-step process: initial storage in limited glycogen reserves, followed by long-term storage in potentially unlimited fat cells. This system, primarily regulated by insulin, reflects our evolutionary history of needing energy reserves. For those in modern societies with consistent food access, understanding this process is vital for healthy weight management and avoiding health risks from excessive fat. Managing energy balance through diet and exercise is crucial in determining where calories are stored. For further information on metabolism and obesity, you can consult the National Institutes of Health (NIH) website.

Frequently Asked Questions

All macronutrients—carbohydrates, proteins, and fats—will be stored as fat if consumed in excess of your body's energy needs. However, the body converts dietary fat to body fat more directly and with less energy expenditure than it does for converting carbohydrates or proteins.

Generally, no. While some small amount of consumed food is not absorbed and is excreted, the vast majority of excess calories that are absorbed by the body are either burned or stored as fat.

The process begins relatively quickly. Excess glucose from a meal can be converted to glycogen within a few hours. When glycogen stores are full, the liver can begin converting excess glucose into fat through de novo lipogenesis, a process that can start soon after a large meal.

No. Fat is the primary long-term energy storage, but the body also stores carbohydrates as glycogen in the liver and muscles for short-term, readily accessible energy.

Glycogen provides a quick source of energy and is stored in limited amounts in the liver and muscles. Fat, stored as triglycerides in adipose tissue, is a long-term, more energy-dense storage form with a virtually unlimited capacity.

Yes. Regular exercise increases the body's need for energy, which helps deplete glycogen stores. This makes the body more efficient at using consumed calories for fuel rather than storing them, particularly if glycogen stores are replenished after exercise instead of overflowing.

While exercise burns calories, the fundamental principle is energy balance. If you increase exercise but also consume significantly more calories, you can still gain weight. For sustainable weight loss, it's generally most effective to combine increased physical activity with a managed caloric intake.