Physiology of Weight Loss

If you have the perfect diet and exercise plan, you should be losing primarily fat while building some muscle. However, because there are so many moving parts when it comes to dieting and working out, chances are that you are also losing muscle or water mass when losing weight.

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Fat is an essential nutrient for the body. It supplies us with energy and makes it possible for other nutrients to function properly. Lipolysis is the scientific term to describe converting fat, lipids, and triglycerides into energy.

There are two main kinds of adipose tissue, fat deposits, that can be found within the body: white and brown. Adipose tissue has a lot of receptors on the surface of fat cells to take in cues from the body about when to intake and output their fat stores.

White adipose tissue is what we think about when we think of getting rid of fat. All fat right below the skin’s surface, the fat we see more easily and want to get rid of most, is white. Our body breaks down fats we consume into fatty acids, smaller chains of fat that can pass easily into a cell. In white adipose tissue, fatty acids are stored in the form of triglycerides, meaning a chemical change has been made to add a glycerol backbone to three fatty acids. While fatty acids are mobile in and out of cells, triglycerides are unable to cross the cell membrane. When the body is in starvation mode, due to release of catecholamines like adrenaline, triglycerides in white adipose tissue are broken into fatty acids that can be processed for energy, primarily in the liver and muscles. Processing of these fatty acids leads to an increase in energy stores, synthesizing new adenosine-triphosphate or ATP. ATP serves as a cell’s main energy source and is used for energetically unfavorable cellular processes.

Brown adipose tissue mainly serves to aid in thermogenesis, the process of producing heat to keep our bodies at a perfect 98.6ºF. Brown adipose tissue also stores fatty acids in the form of triglycerides, but there are extra proteins called fatty acid binding proteins that make it harder for glycolysis, the breakdown of triglycerides into fatty acids, to occur. Brown adipose tissue uses its own fatty acid stores to burn fatty acids internally. It does this through a slightly different process than white adipose tissue, not using ATP, because the purpose of burning these fatty acids is not to create energy, but to create heat.

What happens when you lose fat

Where does all the fat go? If you lose 22 lbs of fat, 18.5 of those lbs are exhaled as carbon dioxide in the lungs as a product of cellular metabolism as fat is converted into energy. The other ~16% turns into water which can be excreted by the body.

Although many diets will claim to “kill” fat cells, losing weight isn’t associated with a loss of cells. Those cells will instead shrink.

Hormones that affect fat loss

Hormones are chemicals that are released within the body that signal to a different part of the body. There have been many conclusions drawn about hormones and how they relate to fat loss and dieting in general. Below are hormones that are most studied in relation to fat loss and how they affect the body.

Insulin is a hormone secreted by the pancreas. When carbohydrates and sugars are consumed, the pancreas secretes insulin to remove sugar from the bloodstream, and store it in a safer form within cells as an energy source. When people consume too many carbohydrates over long periods of time, they are likely to become resistant to insulin.

Glucagon works closely with insulin to regulate blood sugar. When blood sugar becomes too low, glucagon is released to tell the body to burn fat and other stores of energy instead of sugar.

Ghrelin is a hormone that rises in concentration when you are physically hungry. It signals to the brain to find a food source to satisfy hunger needs.

Thyroid Hormone, secreted from the thyroid gland, helps to oversee the body’s metabolism. Thyroid hormone changes concentration dependent on sleep, nutrition, stress, and exercise. This hormone is responsible for slower metabolism in chronic dieters - it primes the body to process food more effectively in case of food shortage.


Muscle requires consistent working out to maintain. If a diet is solely focused on calorie restriction, then you’ll almost always lose muscle tissue. Calorie restriction also typically means macronutrient restriction, and without enough protein consumption, the body can’t rebuild muscle even if it’s complemented with strength training.

When you increase the amount of muscle in your body, you also increase your basal metabolic rate: how quickly your body burns calories while working out and at rest. This is because more muscle means more energy to perform essential functions, even at rest. Muscle also impacts insulin sensitivity. The more muscle you have, the more sensitive your body is to sugars and carbohydrates, and the more likely they’re be used for quick stores of energy, instead of being used for fat stores.

What happens when you build muscle

When you exercise, muscles are repeatedly contracted and relaxed, causing small amounts of damage along the way. After you work out, the body has a repair mechanism that repairs and replaces the muscle tissue, thickening and creating more muscle fibers in the process. Satellite cells repair muscles by moving to damaged areas and laying down more fibers and more nuclei as muscle cells grow larger and larger. Some people have greater satellite cell activation than others, which is why it is easier for some to put on muscle faster.

There are three mechanisms that help muscles grow: muscle tension, muscle damage, and metabolic stress. Muscle tension is applying more stress (i.e. weight) to your muscles than they have adapted to be able to life. Tension most dramatically affects the connection of motor units within cells. Local muscle damage releases inflammatory molecules and produces an immune response to send satellite cells into action. Metabolic stress causes muscle growth without necessarily increasing the size of muscle cells.

Hormones that affect muscle growth

Insulin Growth Factor and testosterone are the most important hormones in muscle growth.

Insulin growth factors regulate the amount of muscle mass by enabling protein synthesis, helping glucose uptake, uptaking amino acids into skeletal muscles. It’s also thought to activate satellite cells to increase muscle growth after exercise.

Testosterone facilitates protein synthesis, stops proteins from breaking down, helps to activate satellite cells, and stimulates release of anabolic hormones. It’s important to note, however, that testosterone is mostly bound in the body (~98%) and therefore unavailable. Exercise has been shown to increase free testosterone levels, as well as make the testosterone receptors on muscle cells more sensitive.  


The human body is made up of up to 60% water. Carbohydrates retain up to three times as much water as any type of macronutrient. Cutting back on carbs means that the body doesn’t retain as much water. Losing intramuscular water can make someone appear to be slimmer, but actually can lead to atrophy as the tissue adapts to dehydration.

To avoid water loss, it’s important to eat at least 0.5 grams of carbohydrates per pound of body weight for a low-carb diet or 0.75 to 1.5 grams per pound for a more balanced diet to allow for losing fat without shedding intramuscular water.

Read next: Measuring Body Composition

Hannah Nourie