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The Science of Fat and Fat Loss

Updated: Jun 21, 2023

In this science post, we are following up the conversation about the hormones and neurotransmitters involved with the regulation of our hunger and satiety cues, with how the body utilizes and releases fat in the weight loss process. So, to better understand what fat loss really is and what happens in the body to make this process happen, let’s take a deeper dive into fat and fat metabolism!

What are fats?

Fats which are also called “fatty acids” or “lipids”, are an essential component of the homeostatic function of the human body. They also contribute to some of the body’s most vital processes.

Fats and oils are categorized in chemistry as a chemical compound called an ester, which is described as any class of organic compounds that react with water to produce alcohols, and organic or inorganic acids.

Fats are fatty, waxy, or oily compounds that are soluble in organic solvents (dissolve in a substance that contains carbon and nonpolar, for example paint thinner) and insoluble in polar solvents such as water.

These include:

  • Fats and oils (triglycerides)

  • Phospholipids

  • Waxes

  • Steroids

The human body is composed of two main types of fat tissue. White fat is important in energy metabolism, heat insulation, and cushioning. Brown fat, also called brown adipose tissue, is a special type of body fat that is turned on (activated) when you get cold. Brown fat produces heat to help maintain your body temperature in cold conditions. Brown fat is found mostly in newborn babies, between the shoulders, and is important for thermogenesis (making heat without shivering). Scientists used to believe that only babies had brown fat. They also thought this fat disappeared by the time most people reached adulthood. However, researchers now know that even adults have small reserves of brown fat (Marcin, A., Hodgson, L. 2022, January). It’s typically stored in small deposits around the shoulders and neck.

A third type of fat called beige (or brite) fat has been identified in humans, and is a relatively new area of research. These fat cells function along the spectrum between brown and white fat cells. Similar to brown fat, beige cells can help utilize fat rather than store it. It is thought that certain hormones and enzymes are released when you are stressed, cold, or during exercise, which can help convert white fat into beige fat. This is an exciting area of research to possibly help prevent obesity and maximize healthy body fat levels.

When you are first born, your body does not have much white fat to help insulate and retain body heat; although there are white fat cells, there is not much fat stored in them. Brown fat cells are somewhat smaller than white, are composed of several smaller fat droplets, are iron rich (hence the brown colour), and are loaded with mitochondria which can generate heat. A newborn baby produces heat primarily by breaking down fat molecules into fatty acids in brown fat cells. Instead of those fatty acids leaving the brown fat cell, as happens in white fat cells, they get further broken down in the mitochondria and their energy is released directly as heat. Brown fat contains more mitochondria than white fat, and has generated interest among researchers because it appears to be able to use regular body fat as fuel. In addition, exercise and the exposure to cold may stimulate hormones that activate brown fat.

All people have some “constitutive” brown fat, which is the kind you’re born with. There’s also another form that’s “recruitable.” This means it can change to brown fat under the right circumstances. This recruitable type is found in muscles and white fat throughout your body.

Recap of the digestion of fat

When you eat food that contains fat, mostly triglycerides, it is digested through the stomach and intestines.

In the intestines, the following happens: large fat droplets get mixed with bile salts from the gallbladder in a process called emulsification. The mixture breaks up the large droplets into several smaller droplets called micelles, increasing the fat's surface area. The pancreas secretes enzymes called lipases that attack the surface of each micelle and break the fats down into their components of glycerol and fatty acids. These components get absorbed into the cells lining the intestine. Once in the intestinal cell, the parts are reassembled into fat molecules (triglycerides) with a protein coating called chylomicrons. This protein coating makes the fat dissolve more easily in water. The chylomicrons are then released into the lymphatic system. They do not go directly into the bloodstream because they are too big to pass through the wall of the capillaries (which are a tiny network of blood vessels). The lymphatic system eventually merges with the veins, at which point the chylomicrons are able to pass into the bloodstream. Fat molecules get broken down into glycerol and fatty acids and are then reassembled because fat molecules are too big to easily cross cell membranes. So, when passing from the intestine through the intestinal cells into the lymph, or when crossing any cell barrier, the fats must first be broken down. However, when fats are being transported in the lymph or blood, it is better to have a few large fat molecules than many smaller fatty acids, because the larger fats do not "attract" as many excess water molecules by osmosis as many smaller molecules would.

The fat that we store in our body is also stored as triglycerides. When you eat, your body converts any energy it does not need to use right away into triglycerides. The triglycerides are then stored in your fat cells. Later, hormones release triglycerides for energy between meals if you need them. Triglycerides also provide insulation to cells, and aid in the absorption of fat-soluble vitamins.

A triglyceride is made up of glycerol (a 3-carbon sugar alcohol/polyol) and 3 fatty acids. Fatty acids are chains of hydrogen and carbon. They are composed of differing lengths and various degrees of saturation. The molecule ends with something called a carboxylic acid group. How they are “bonded” allows for the creation of many different types of fatty acids. Fatty acids in biological systems usually contain an even number of carbon atoms and are typically 14 carbons to 24 carbons long.

Check out this video for more information on the molecular structure of triglycerides!

Fats are also an essential component of the cell membrane in our body’s cells. In the cell membrane, phospholipids are arranged in a bilayer, providing cell protection and serving as a barrier to certain molecules. The structure is typically made of a glycerol backbone, 2 fatty acid tails (which are hydrophobic, meaning the structure tries to avoid water molecules), and a phosphate group (which are hydrophilic, meaning that group is attracted to water molecules). These molecules are called phospholipids, and are amphipathic (having parts that are both hydrophobic and hydrophilic). The hydrophilic part faces outward and the hydrophobic part faces inward. This physical arrangement of the cell membrane helps to monitor and act as a gatekeeper, as to which molecules can enter and exit the cell. For example, nonpolar molecules and small polar molecules, such as oxygen and water, can easily diffuse in and out of the cell. Large polar molecules, for example, glucose, cannot pass freely so they need the help of transport proteins.

Another type of lipid is wax. Waxes are esters made of a long-chain alcohol and a fatty acid. Waxes provide protection. Cerumen (also known as earwax) helps protect the skin of the ear canal.

Another class of lipids includes steroids, which have a structure of four fused rings. One important type of steroid is cholesterol. Cholesterol is produced in the liver and is the most common steroid. Cholesterol is the precursor to many steroid hormones such as estrogen, testosterone, and cortisol. It is also the precursor to Vitamin D as well as bile salts, which help in the emulsification of fats and their subsequent absorption by cells. Cholesterol is also an important part in the composition of the cell membrane. Cholesterol is inserted in the bilayer of the cell membrane and has a big influence on the fluidity of the membrane.

Why is dietary fat important?

Most of the fat that we need can be made by our bodies, however, there are certain fats that our bodies cannot manufacture but are essential for our health and bodily functions.

These fats, are called “essential” fats, essential fatty acids (EFAs), or polyunsaturated fats. Essential fats include Omega-3 fats (found in foods such as fish and flax seed) and Omega-6 fats (found in foods such as nuts, seeds, and corn oil).

Other components of omega-3 fatty acid that are found in fatty fish and shellfish are called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fish can contain the metal mercury which acts as a toxin in the body, so choosing a good quality Omega 3 supplement, or fish lower in mercury like salmon, anchovies, herring, sardines, Pacific oysters, trout, Atlantic mackerel, and Pacific mackerel will help provide the EPA and DHA that your body needs.

Fat is an important component in our diet because:

  • Fat helps the body absorb and process fat soluble vitamins like vitamins A, D, E, and K

  • Fat helps to keep our skin healthy

  • Essential fats like Omega-3 are important for heart health and brain health. Omega-3 fatty acids may reduce inflammation throughout the body. Inflammation can cause damage to your blood vessels, leading to heart disease and stroke. Omega-3 fatty acids may benefit heart health by: decreasing triglycerides levels, slightly lowering blood pressure, reducing blood clotting that could lead to clots therefore decreasing your risk of stroke and heart attack, decreasing heart failure risk and reducing arrhythmias (Mayo Clinic 2019, September).

The omega-3 fatty acids EPA and DHA are critical for normal brain function and development throughout all stages of life. EPA and DHA seem to have important roles in the developing baby’s brain, and are also vital for the maintenance of normal brain function throughout life. They are abundant in the cell membranes of brain cells, preserving cell membrane health and facilitating communication between brain cells. In older adults, lower levels of DHA in the blood have been associated with smaller brain size, which is a sign of accelerated brain aging. It is also thought that Omega-3 fatty acids also reduce inflammation in the brain.

  • Healthy unsaturated fats, like monounsaturated fats and polyunsaturated fats, can help lower levels of LDL cholesterol. Monounsaturated fats are found in avocados, peanut butter, nuts like almonds, hazelnuts, cashews, and pecans, and seeds, such as pumpkin, sesame, and sunflower seeds. They are also found in plant oils such as olive, peanut, safflower, sesame, and canola oils. A study from Harvard researchers (Harvard Health Publishing 2021, April) found that consuming monounsaturated fats, especially from nuts and olive oil, can lower a person's risk of heart disease — especially if the healthy fat replaces saturated fat and refined carbs (which can also raise LDL low density lipoprotein levels, the type of cholesterol that causes plaque build-up in the arteries). Unsaturated fats help to raise the HDL high density lipoprotein (good) cholesterol levels. HDL picks up excess LDL in the blood and moves it to the liver, where it is broken down and discarded. The goal is to have a high HDL-to-LDL ratio for a healthy blood lipid profile. The researchers added that any benefit from consuming monounsaturated fats may be negated if a person continues to consume too much saturated fat.

  • Fat adds flavour to food by making food taste better.

  • Fat keeps you feeling satisfied longer after a meal as it helps to regulate blood sugar levels and the hormones involved in hunger and satiety. Fat also takes the digestive system longer to process and digest, thus also making you feel fuller longer.

Although fats are a healthy part of our diet, it is still important to acknowledge that only up to 10% of our fat intake should come from saturated fats. Saturated fat includes foods like meat, dairy and coconut oil. Those monitoring their cholesterol should be mindful of their saturated fat intake.

Fats are generally recommended to account for about 30% of our diet, with the majority of fat intake coming from the healthy unsaturated fats listed above. Trans fat (partially hydrogenated fats) found in processed food can be detrimental to our health, so avoid these as much as possible.

Check out this video for more details discussing the differences between these kinds of fats.

Now that we discussed the fundamentals of fat, let’s talk about how our bodies use it for stored energy, and how it is released from the body!

Recap on the process of metabolism

When you are not eating, whether active or at rest, your body must draw on its internal energy stores of complex carbohydrates, fat, and proteins to provide your body with the energy it needs to do all the things it needs to do to keep you alive! Important activities like breathing, keep your heart pumping, digesting, or providing energy for that run you enjoy.

Most of the cells in your body use glucose along with amino acids (the building blocks of protein) and fats for energy. However, glucose is the main source of fuel for your brain. Nerve cells and chemical messengers located there need it to help them process information, and having access to glucose is very important for overall brain function. Your brain uses about 60% of the glucose that our bodies use. However, glucose does not always have to come immediately from foods and beverages. Glucose is also generated by the body to ensure that we always have the amount that is needed. One way that this is achieved, is by breaking down something called glycogen, in order to free up the glucose it contains.

Glycogen is released by an important hormone called glucagon. When the body doesn't need to use glucose for energy, it is stored in the liver and muscles by insulin. Glycogen is this stored form of connected glucose molecules. When the body needs a quick boost of energy or is not getting glucose from food, glycogen is broken down to release glucose into the bloodstream to be used as fuel by the cells. Your body can store enough glycogen in order to keep you fueled for about a day. This process is called glycogenolysis.

The body can also produce glucose through a process called gluconeogenesis. This process occurs when the body (mainly the liver, then kidneys and to a less extent the small intestine) makes glucose from non-carbohydrate sources which include lactate (what our bodies produce during exercise), glycerol (is produced when fats are digested, can be used for energy or stored in adipose tissue) and amino acids.

Gluconeogenesis occurs when glycogen stores become low and glucose consumption is too low or nonexistent, such as during periods of starvation or prolonged fasting. Although the body has enough glycogen stored in the muscles and liver to last about a day, after approximately 14 hours in a fasted state it will begin to increase its percentage of gluconeogenesis, generating energy in increased ratios as time goes on. In this stage, energy that is stored in adipose tissue all around your body in the form of triglycerides will be liberalized. In the fat cell, other types of lipases (enzymes that break down fats) work to break down fats into fatty acids and glycerol in a process called lipolysis. These lipases are activated by various hormones, such as glucagon, epinephrine, and growth hormone. The resulting glycerol and fatty acids are released into the blood and travel to the liver through the bloodstream. Once in the liver, the glycerol and fatty acids can be either further broken down directly to get energy, or used to make glucose.

One very important consideration though, is that skeletal muscle (composed of amino acids) can be used as a source when generating glucose through the pathway of gluconeogenesis. This can lead to muscle wasting and loss over time which can have a major impact on the body!

An additional alternative form of energy that can be generated if needed, is the formation of Ketone bodies from our fat reserves. Ketone bodies can serve as a fuel source if glucose levels are too low in the body. Ketones serve as fuel in times of prolonged starvation, carbohydrate deprivation, or when patients suffer from uncontrolled diabetes and cannot utilize most of the circulating glucose. In these scenarios, fat stores are liberated, generating ketone bodies to the body and brain for energy.

So, in losing weight, what happens to the fat we use for energy?

The body disposes of fat through a series of very complex metabolic pathways. For those of you that are interested in the biochemistry behind each of these processes, these videos are a great way to understand the different pathways of fat metabolism described simply above:

This is where things become very interesting!

As the body begins to draw on our fat stores for energy, although the number of fat cells remains the same, each fat cell simply gets smaller.

The byproducts of the metabolism of fat are processed by the body in the following ways:

As water, through your skin (when you sweat) and your kidneys (when you urinate), and as carbon dioxide (CO2) through your lungs (when you breathe out). Exercise also increases your respiratory rate, so more CO2 (and the by-products of fat) leaves your body when you work out. Being well hydrated is well documented in supporting the body in fat loss (also allowing for the release of fat by-products through urine). Fat breakdown also generates heat, which keeps body temperatures normal.

What happens to body fat when you exercise?

Your muscles first utilize stored glycogen for energy. After about 30 to 60 minutes of aerobic exercise, your body starts utilizing mainly fat stores depending on the intensity of the activity. If you are exercising moderately, this takes about an hour. Building your muscle mass will raise your basal metabolic rate, and the amount of energy your body uses at rest.

This leads us to the most interesting theory about how most of our fat leaves our body.

A study conducted by a team of Australian researchers Meerman and Brown (2014), calculated exactly what happens to our fat when we drop weight, and revealed that we don't convert our missing mass into heat or energy. We actually breathe it out.

Their results, reveal that 22 pounds (10 kg) of fat turns into 18.5 pounds (8.4 kg) of carbon dioxide, which is exhaled when we breathe, and 3.5 pounds (1.6 kg) of water, which we then excrete through our urine, tears, sweat and other bodily fluids.

Lead author of the paper Ruben Meerman, a physicist, first became interested in the biochemistry of weight loss when he dropped 33 pounds (15 kg). However, when he asked his doctors where this weight went, he was surprised by the fact no one could tell him.

After surveying 150 doctors, dieticians, and personal trainers, he discovered that more than half thought that fat was converted into heat or energy as we break it down.

But, as a physicist, Meerman knew that this would violate the law of conservation of mass. This law states that in a closed or isolated system, matter cannot be created or destroyed. It can change forms but is conserved. This law also helped scientists understand that substances did not disappear as result of a reaction (as they may appear to do), rather they transform into another substance of equal mass.

We put on weight in the form of fat when excess stores of energy are converted into triglycerides (compounds made up of carbon, hydrogen, and oxygen), and are then stored in lipid droplets inside fat cells. To lose weight, you need to break down those triglycerides to access their carbon.

The results of this study showed that in order to completely break down 22 pounds (10 kg) of human fat, we need to inhale 64 pounds (29 kg) of oxygen (and somewhere along the way, burn 94,000 calories). This reaction produces 62 pounds (28 kg) of CO2 and 24 pounds (11 kg) of water. Their calculations show that the lungs are the primary excretory organ for fat!

However, they couldn't work out exactly what was happening to the fat cells in this reaction. After months of research, Meerman discovered a formula from a paper published in 1949 that solved the problem. It showed that oxygen atoms are shared between the carbon and hydrogen in fat at a ratio of 2:1 (forming carbon dioxide and water). This allowed them to come up with the final figure of 84 percent of a fat molecule's atoms being exhaled as carbon dioxide, and the remaining 16 percent ending up as water.

However, this doesn't mean that simply breathing deeply will help us lose weight, energy is still required to unlock the carbon and break down the fat in the first place.

Meerman stated that "You can only breathe so many times a day; on a day of rest, you breathe around 12 times a minute so 17,280 times you'll breathe in a day and each one takes 10 milligrams of carbon with it, roughly."

Check out this video for more details!

Hopefully you found that interesting! Stay tuned for the next science post, where we will be discussing added sugars and artificial sugars (otherwise known as non-nutritive sugars or NNS) and their impact on the brain and body.


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