The Science of Fat Loss: How to Lose Belly Fat and More

Our body needs fuel to provide us with the energy we need to practice our favorite sport, to get a HIIT workout done, or to simply go about our daily activities. In fact, even in stillness, our body requires and uses energy (BMR, i.e., our basic metabolic rate) to power our brains, organs and other bodily functions.

As you may know, there are three main types of fuel (also known as “substrates” or “macronutrients”) that our body uses to produce energy:

  1. Glucose (carbohydrates)
  2. Fatty acids (fats)
  3. Amino acids (proteins)

However, it’s not just the foods we eat that provide the fuel needed to function normally, our body has room for storage too. Glucose from carbs (stored as glycogen in your liver and muscles) and fatty acids (stored as triglycerides and found all over the body in adipose tissue) are stored as reserves to be used by your body under different conditions.

The fat around your midsection (i.e., that pesky belly fat) is a result of your body storing fuel that it doesn’t need right away. Bringing us to our next big question: How do we burn through body fat and where does it go when we do?

The science of fat loss

Before we answer those questions, it’s important to understand how fuel (carbs, fat and protein) is transformed into energy and in which cases they are stored as fuel. To be used by our cells, the different types of fuel must go through several stages of transformation to become the one and only form of  “usable” energy: adenosine triphosphate — more commonly referred to as ATP. ATP is the universal energy currency for many living organisms from mammals to insects and fungus to plants.

To explain exactly how this works, and appease our more visual learners, we created a graphic to accompany Coach Ame’s very detailed knowledge bombs. Each number in the graphic corresponds to the details below.

weight loss fat

Dietary fats (1)

See that food going into our character’s mouth? That food happens to be high in dietary fat. Dietary fats aren’t always bad (we talk about that in this article), but here they are ranked best to worst:

  • Unsaturated fats like poly- and monounsaturated fats can lower cholesterol levels, which reduces the risk of heart disease. Unsaturated fats are typically liquid at room temperature and provide our bodies with essential fats that they can’t create on their own. These fats mainly come from plants (avocado, nuts and seeds, olive, canola, etc.) but some come from the sea too (fish and seafood).
  • Saturated fats are considered healthy when eaten sparingly, but since too much consumption of these fats raises the level of cholesterol in our blood, they come with a heart disease warning. Saturated fats are solid at room temperature and come mainly from terrestrial animals and some tropical fruits (dairy, eggs, meat, coconut, cocoa, palm oil, etc.).
  • Trans fats are artificially created by humans and are to be avoided as much as possible. They are used to lower the production cost of fake foods and improve the texture and shelf life of pre-made products. They have no health benefits.

Fat molecule composition (2)

Any molecules that don’t dissolve in water — like fat molecules — are called lipids. Most fats we eat are made of triglycerides and, molecularly speaking, triglycerides are made of three fatty acids (the yellow squiggle) bound to one glycerol i.e., the “backbone” of all lipids (the green oval). Those fatty acids can be used directly as an energy source by most tissues in the body which means… they’re important.

Consumption of fat (3)

As we mentioned above, the fat we eat is a fuel that our body uses to produce energy (ATP). Fat is actually a great source of fuel as it provides nine calories of energy per gram (carbs and proteins give us four calories of energy per gram). Also, consumption of fat — especially omega 3s — is essential since our body can’t create certain fat molecules that are essential for proper cell function.

Pro tip
WHO and the USDA recommend that adults consume less than 30% of their total daily calories from fat. Less than 10% of total fat calories should come from saturated fat and less than 1% from trans fat -- making unsaturated fats the main source.

Digestion of fats (4)

Your digestive system isolates fats from the other macronutrients you eat (i.e., carbs and proteins) to distribute them where you need them. But, dietary fats — those triglycerides — can’t be absorbed by your body as is, they need to go through mechanical and chemical digestion processes first.

So, digestion of fats starts in your mouth and ends in your small intestine, where it’s finally absorbed. Compared to other nutrients, fats take the longest time to digest. Here’s what the process looks like:

  • Mouth (a): While we chew and break down food into smaller particles (mechanical digestion), our saliva helps by bringing necessary enzymes to start the chemical digestion of fats and other nutrients.
  • Stomach (b): Once we swallow, food comes down into our esophagus and reaches our stomach where more enzymes continue to break down the nutrients.
  • Liver (c): Before we even start to chew, the liver starts the digestive process. The sight and smell of food signals the liver to produce bile salts that break fat down into small droplets so it becomes more “accessible” to enzymes.
  • Gallbladder (d): This bag-like organ is there to store the bile produced by our liver. This bile is essential for the digestion and absorption of fats and fat-soluble vitamins (A, D, E, K).
  • Pancreas (e): This large gland hiding just behind our stomach produces other enzymes that are secreted into our small intestine to help with the digestion of all nutrients.
  • Small Intestine (f): The food residue finally moves from our stomach to the small intestine where bile salts and pancreatic enzymes “liquefy” the fats molecules down to their simplest forms – fatty acids and mono– and diglycerides.

Absorption of fats (5)

Still with us? Let’s move onto absorption. The absorption of fats happen in our small intestine, but as mentioned above, triglycerides can’t be absorbed in our intestine unless they are broken down, absorbed into the intestine walls, then transported to our lymphatic system first and eventually into our bloodstream, where they’ll reach adipose, cardiac, and muscle tissue.

Storage of fat (6)

The fatty acids absorbed into the small intestine walls can be used as fuel or be stored into adipose tissue for later use. Without food, we store enough glucose (glycogen) to go about our normal activities for nearly one day.

That place where fat is stored — the adipose tissue — is our body fat. It’s brought there via the bloodstream (i). Adipose tissue is made of adipocytes, cells specialized in the storage of fat. These cells look like bubbles packed close to each other. When our body uses the fat contained in the bubbles, they decrease in size. But, when we store excess fat (coming from any foods), the bubbles increase in size (hypertrophy) and number (hyperplasia).

Adipose tissue is primarily located beneath our skin (j) (subcutaneous fat), but it’s also found around our internal organs (abdominal or visceral fat). Subcutaneous fat (h) is not related to health issues and is totally normal and healthy to have. This type of fat acts like an organ and is responsible for functions like hormone secretion, insulation from the cold, and cushioning the organs and muscles (g). Excess abdominal fat can be stressful on our internal organs and is linked to type 2 diabetes, insulin resistance, and other obesity-related diseases.

Pro tip
High-intensity interval training (HIIT) is one way to effectively reduce total abdominal fat.

Role of fat: Energy! (7)

Fats are used by the body — together with glycogen stores — to fuel between meals, while sleeping, or when aren’t quite meeting our caloric needs. An important role of fat is to supply energy to our cells so that they can create ATP. Fat is a highly concentrated energy source but it needs to be broken down to participate in the creation of ATP. This process happens in special energy factories called mitochondria (7). We aren’t going to get into the specifics of the fat-to-ATP-process now but if you’d like some further reading, go here.

Here are some other ways fat participates in bodily processes:

  • Cell membranes: Some fatty acids are necessary for the maintenance and upkeep of all the cell membranes (walls) of our body. They act like bouncers in a nightclub, only allowing the appropriate molecules to cross in and out of our cells.
  • Messenger: Fat helps proteins do their jobs properly.
  • Chemical reaction: Fatty acids are the starting point of many important processes happening in our body that help control growth, immune function, inflammation reactions, and reproduction.
  • Ketone bodies: Ketone bodies are produced when the carb reserves (glycogen) start declining and the body must rely on fat reserves for fuel. Our liver then starts producing ketone bodies — a little molecule that can acts as a substitute for glucose (our brain’s favorite fuel). You can force your body to produce these ketone bodies by following a very low-carb, ketogenic diet.

So how does all of this relate to fat loss or weight loss? Initially, during aerobic exercise, glycogen is used. But, in its absence or when the activity lasts a long time, fat metabolism is initiated and fat is broken down to be turned into ATP. Generally, a workout performed at a moderately high level of intensity over a long period of time will use fats for energy. But, research also shows that HIIT can activate fat metabolism as well.

Fun fact
Here you’ll notice that our cells can convert sugars to fats, but they can’t convert fatty acids into sugars.

Excretion of fat (8)

Where does the actual fat go when it’s used? Well, the last steps of fat usage happen in the mitochondria when it’s broken down and turned into CO2 and water while the energy is released as ATP.

Yes, you read it right: The vast majority of the fat we eat — or that comes from our storage rooms (adipose tissue) — is converted into CO2 and lost in the air! The leftover weight is lost in the form of water via sweat, tears, urine, etc. In fact, almost everything we eat exits the body via CO2 and water — it’s only fiber that remains undigested and makes its way out your backend.

To answer your question…

“If I breathe more or breathe deeply, can I lose more fat?”

The answer is no. We explain all of this a bit more here.

Now that you know more than you thought you ever would about the science of fat loss, we have an important reminder: Always focus on moving daily and eating a well-balanced, healthy diet. Don’t avoid carbs, protein or fat — your body needs it all. Another great thing you can do for yourself is to sign up for 8fit to get meals that are tailored to your preferences, your wellness goals, and your different macronutrient needs.