Given that obesity is one of the major public health concerns worldwide, many people are looking to lose fat.

Still, a lot of confusion exists around the process of fat loss.

This article reviews what happens to fat when you lose weight.

How fat loss works

Excess consumed energy — usually calories from fats or carbs — is stored in fat cells in the form of triglycerides. This is how your body preserves energy for future needs. Over time, this excess energy results in a fat surplus that can affect your body shape and health. 

To promote weight loss, you need to consume fewer calories than you burn. This is referred to as a calorie deficit (1, 2).

Though it varies from person to person, a daily 500-calorie deficit is a good place to start to see noticeable fat loss (3).

By maintaining a consistent calorie deficit, fats are released from fat cells and transported to the energy-producing machinery of the cells in your body called mitochondria. Here, the fat is broken down through a series of processes to produce energy.

If the calorie deficit continues, fat stores from your body will continue to be used as energy, resulting in a reduction in body fat.

summary

Over time, a consistent calorie deficit frees fat from fat cells, after which it’s converted into energy to fuel your body. As this process continues, body fat stores are reduced, leading to changes in body composition.

Diet and exercise are key

The two main promoters of fat loss are diet and exercise.

A sufficient calorie deficit causes fats to be released from fat cells and used as energy.

Exercise amplifies this process by increasing blood flow to muscles and fat cells, releasing fats to be used for energy in muscle cells at a more rapid rate and increasing energy expenditure (4).

To promote weight loss, the American College of Sports Medicine recommends a minimum of 150–250 minutes of moderate-intensity exercise per week, equaling around 30–50 minutes of exercise 5 days per week (5).

For maximum benefit, this exercise should be a combination of resistance training to maintain or increase muscle mass and aerobic exercise to increase calorie burn (4).

Common resistance training exercises include lifting weights, bodyweight exercises, and resistance bands, while examples of aerobic exercise are running, biking, or using an elliptical machine.

When calorie restriction and a nutrient-dense diet are paired with a proper exercise regimen, fat loss is more likely to occur, as opposed to utilizing diet or exercise alone (6).

For best results, consider seeking help from a registered dietitian for dietary guidance and certified personal trainer for exercise programming.

summary

Diet and exercise serve as the major contributors to fat loss. A nutritious diet that provides a proper calorie deficit combined with sufficient exercise is the recipe for sustainable fat loss. 

Where does it go?

As the process of fat loss progresses, fat cells drastically shrink in size, resulting in visible changes in body composition.

Byproducts of fat loss

When body fat is broken down for energy through complex processes within your cells, two major byproducts are released — carbon dioxide and water.

The carbon dioxide is exhaled during breathing, and the water is disposed of through either urine, sweat, or exhaled air. Disposal of these byproducts is greatly elevated during exercise due to increased breathing and sweating (7, 8).

Where do you lose fat first?

Commonly, people desire to lose weight from the belly, hips, thighs, and butt. 

While spot reduction, or losing weight in a particular area, has not been shown to be effective, some people tend to lose weight from certain areas faster than others do (9, 10). 

That said, genetic and lifestyle factors play a significant role in body fat distribution (11, 12).

Moreover, if you have a history of weight loss and weight regain, body fat may distribute differently due to changes in fat cells over time (13).

Why is it so hard to keep weight off?

When you eat more than your body can burn, fat cells increase in both size and number (14).

When you lose fat, these same cells can shrink in size, though their number remains roughly the same. Thus, the primary reason for changes in body shape is a reduced size — not number — of fat cells (14).

This also means that when you lose weight, fat cells remain present, and if efforts are not made to maintain weight loss, they can easily grow in size again. Some studies suggest that this may be one reason why maintaining weight loss is so difficult for many people (14, 15, 16). 

Summary

During weight loss, fat cells shrink in size as their contents are used for energy, though their numbers remain unchanged. Byproducts of fat loss include carbon dioxide and water, which are disposed of through breathing, urination, and sweating.

Timeline of fat loss

Depending on how much weight you aim to lose, the duration of your fat loss journey can vary significantly. 

Rapid weight loss has been associated with several negative side effects, such as micronutrient deficiency, headaches, fatigue, muscle loss, and menstrual irregularities (17).

As such, many advocate for a slow, gradual rate of weight loss due to the expectation that it’s more sustainable and may prevent weight regain. However, limited information is available (18, 19, 20).

That said, if you have a significant amount of fat to lose, a more rapid approach may be warranted, whereas a gradual approach may be more appropriate for those who have less fat to lose.

The expected rate of weight loss varies with how aggressive the weight loss program is.

For those with overweight or obesity, a weight loss of 5–10% of your starting body weight during the first 6 months may be possible with a comprehensive lifestyle intervention including diet, physical activity, and behavioral techniques (21).

Some other factors affect weight loss, such as gender, age, the extent of your calorie deficit, and sleep quality. Also, certain medications may affect your weight. Thus, it’s advisable to consult your healthcare provider before starting a fat loss regimen (22, 23, 24). 

Once you reach your desired body weight, your calorie intake can be adjusted to maintain your weight. Just remember, it’s important to continue exercising regularly and eating a balanced, nutritious diet to prevent weight regain and promote overall health.

summary

Fat loss timelines vary by individual. While gradual weight loss may be more appropriate for some, those with a lot of weight to lose may benefit from faster rates of weight loss. Other factors affecting weight loss should also be taken into consideration.

The bottom line

Fat loss is a complex process influenced by a number of factors, with diet and physical activity being two of the major ones.

With a sufficient calorie deficit and proper exercise regimen, fat cells shrink over time as their contents are used for energy, leading to improved body composition and health.

It’s important to consult your healthcare provider prior to starting your weight loss journey to prevent any potential negative side effects.