In the first post in this series, we discussed calories; in the second, protein; in the third, carbohydrates. Next up are fats.
Chemically, fats are made of carbon, hydrogen, and oxygen, like carbohydrates, but have different structures and functions. Unlike carbohydrates, which are water-soluble and stored with water, fats are hydrophobic (they don’t mix with water) and are a dense, efficient form of energy storage. At 9 Cal per gram, fat provides more than twice the energy of carbohydrates.
Types of Fatty Acids
Fats are chains of carbon atoms, classified as short, medium, long, or very long chain fatty acids.
These chains are often bundled together into molecules called triglycerides. A triglyceride is made of three fatty acids attached to a glycerol backbone. Glycerol is made from glucose, and as mentioned in the previous section, can be used to make glucose when needed via gluconeogenesis.
In addition to their length (number of carbons), fats are further categorized based on the chemical structure of their fatty acid chains, including the number of hydrogens.
Think of a fatty acid chain as a long dinner table made of carbon atoms. When a fatty acid is saturated, every seat is filled with hydrogen atoms, sitting at every possible spot. The table is full, straight, and orderly, with no room for extra guests.
But if a hydrogen is missing and a seat is left empty, carbon will double-bond with its neighboring carbon instead. This creates a bend or kink in the table, like someone pulling two chairs together to make a closer conversation. That’s an unsaturated fat.
- One double bond = monounsaturated fat (one kink in the chain)
- Two or more double bonds = polyunsaturated fat (multiple kinks)
These kinks affect how the fats behave. Saturated fats are straight and stackable, which makes them solid at room temperature, like butter or coconut oil. Unsaturated fats, like olive oil, are liquid at room temperature.
Saturated fats (no empty spots at the table) are stable, even under heat. Monounsaturated fats are moderately stable, especially in whole foods like olives and avocados. However, the more double bonds, the more fragile the fat becomes.
Those empty seats where a hydrogen atom was missing? They’re like an unguarded entry point, and oxygen — an uninvited guest — takes full advantage to crash the party.
When oxygen sneaks in, it interacts with those double bonds and starts causing trouble. This process is called oxidation. It breaks the structure of the fat, creates unstable free radicals, and damages the fat molecules, giving them a rancid taste or smell. Once this happens, the fat loses its function, becomes pro-inflammatory, and can damage surrounding cells.
The more double bonds a fat has, the more likely oxygen is to crash the party and ruin the vibe. Polyunsaturated fats, especially when extracted from seeds and industrially processed, are more fragile and prone to oxidative damage via exposure to heat, light, or air.
Trans Fats
You have likely heard by now about the many dangers of trans fats. These artificial fats were created through a process called hydrogenation, where hydrogen atoms were added to liquid vegetable oils to make them solid at room temperature. This process was initially developed to harden engine lubricants in the early 1900s, but food manufacturers quickly realized it could be used to create a cheap, shelf-stable alternative to butter and lard. The result? Margarine and shortening, made from cheap oils that are easy to mass-produce. But the process of hydrogenation produces trans fats, an unnatural structure that disrupts cell membranes, promotes systemic inflammation, and significantly increases the risk of heart disease. It was a chemistry experiment gone wrong, with these “Frankenfats” allowed into the food system for decades before the true extent of their toxicity was understood. These industrial fats are now banned in many countries, but still lurk in some imported goods and ultra-processed foods, making label-reading more critical than ever. Always be on the lookout for hydrogenated or partially-hydrogenated oils on the ingredients list of anything you buy.
What is fat for?
While glucose can provide “high octane fuel” during high-intensity activity, fat is the primary fuel used to make energy the rest of the time, all day and night. In terms of saving energy for later, carbohydrates can only be stored in small amounts (a few hundred grams or ~2,000 Cal), whereas fat can be stored in a much more dense and efficient manner. Your stored body fat is why you can go hours (or even days) between meals and still function. Even the leanest athletes have 50,000-100,000 calories of stored energy as fat.
However, beyond its role in both storing and making energy, fat has many critical structural and regulatory functions as well:
- Building cell membranes
- Forming steroid hormones (like testosterone and estrogen)
- Supporting brain and nervous system function
- Transporting fat-soluble vitamins (A, D, E, and K)
- Providing insulation and organ protection
Essential Fatty Acids
Unlike carbohydrates, certain fats are essential for human health. The term “essential” in nutrition means your body cannot make that molecule on its own — you must get it from food.
There are two essential fatty acids (EFAs):
- Linoleic acid (omega-6)
- Alpha-linolenic acid (omega-3)
EFAs are required for critical biological functions like:
- Maintaining the structural integrity of cell membranes
- Supporting the development and function of the nervous system
- Regulating inflammation and immune response
- Enabling the production of eicosanoids, which control blood clotting, vascular tone, and more
Although rare in developed countries, essential fatty acid (EFA) deficiency can occur when dietary fat intake is extremely low or when fat absorption is impaired (as in certain medical conditions).
Deficiency can lead to a range of symptoms, including dry, scaly skin, brittle hair, impaired wound healing, and increased susceptibility to infections. In more severe cases, it can also impact cognitive function, growth and development, and reproductive health, even leading to multi-organ failure and death. Thankfully, EFA deficiency is preventable, with as little as 1-2% of total calories coming from essential fats.
While you technically only need EFAs to prevent deficiency diseases, an extremely low-fat diet (typically <10–15% of total calories from fat) can lead to functional deficiencies of other important but non-essential fatty acids and fat-soluble nutrients. Here’s what that looks like:
#1 – Low Hormone Production
Cholesterol and saturated fats are the building blocks for steroid hormones like testosterone, estrogen, progesterone, and cortisol, as well as Vitamin D. Without enough dietary fat, hormone synthesis may decline, leading to:
- Irregular or absent menstrual cycles
- Low libido
- Poor stress tolerance
- Fatigue or mood changes
#2 – Impaired Absorption of Fat-Soluble Vitamins
Vitamins A, D, E, and K require dietary fat to be absorbed. A low-fat diet can result in subclinical or overt deficiencies, which can cause:
- Poor vision (vitamin A)
- Weak bones, poor immune function (vitamin D)
- Easy bruising or bleeding (vitamin K)
- Oxidative stress and muscle weakness (vitamin E)
#3 – Suboptimal Cell Membrane Health
Even “non-essential” fats like oleic acid (from olive oil and animal fat) and saturated fats (like stearic acid in coco butter and beef tallow) contribute to cell membrane integrity, nerve conduction, and metabolic signaling. Lack of these can affect:
- Cognitive performance
- Muscle function
- Recovery and adaptation from training
#4 – Reduced Satiety and Overeating
Fat slows digestion and promotes satiety. Diets too low in fat often lead to:
- Blood sugar swings
- Constant hunger
- Over-reliance on refined carbs or protein
#5 – Increased Inflammation (Ironically)
Low-fat diets are often high in refined carbs and omega-6-rich seed oils, disrupting the omega-6:omega-3 ratio and promoting inflammation, even if total fat intake is low.
Bringing It All Together
As we’ve explored throughout this series, the macronutrients — calories, protein, carbohydrates, and fats — form the foundation of our dietary needs, each playing distinct yet interconnected roles in our health and well-being.
Fats, like the other macronutrients we’ve discussed, are essential for optimal health. They provide energy and critical structural components for our bodies. While carbohydrates may be our high-octane fuel for intense activity, fats are our reliable, long-burning energy source that sustains us between meals and rest. At 9 calories per gram, they provide more than twice the energy density of carbohydrates or protein, making them an efficient form of energy storage.
But as we’ve seen, fats do much more than provide energy. They’re integral to cell membrane structure, hormone production, brain function, vitamin transport, and inflammation regulation. The different types of fats — from saturated to monounsaturated to polyunsaturated — each have unique properties and functions, with some being more stable than others under different conditions.
Like carbohydrates and protein, quality and context matter tremendously with fats. Traditional, minimally processed fat sources provide essential fatty acids and other nutrients in their natural matrix. Meanwhile, industrially processed trans fats and fragile polyunsaturated oils subjected to heat, light, and air can promote inflammation and oxidative stress.
A balanced approach to nutrition recognizes that all three macronutrients work in concert. Protein provides the building blocks for our tissues, carbohydrates offer readily available energy and fiber, and fats enable countless structural and regulatory functions while providing sustainable energy.
The key takeaway from our series isn’t about vilifying or glorifying any particular macronutrient, but rather understanding their unique contributions to our physiology and choosing nutrient-dense, minimally processed sources of each. By focusing on whole foods that provide these nutrients in their natural context, rather than isolated components manipulated by industrial processes, we support our body’s intricate systems with the tools they need to function optimally.
Your body is an extraordinarily complex and adaptive system that requires diverse nutrients to thrive. Armed with this understanding of macronutrients, you can make more informed choices about your nutrition, finding an approach that nourishes your unique body and supports your health goals for years to come.
About the Author
Jocelyn Rylee (CF-L4) and her husband David founded CrossFit BRIO in 2008, starting in a modest 1500 sq ft space and focusing on personal training. Her dedication to excellence has also earned her a position on CrossFit LLC’s Level 1 Seminar Staff, a role that allows her to share her passion and expertise with aspiring coaches. Jocelyn holds specialties in Endurance, Gymnastics, Competition, and Weightlifting and is also a certified Strength and Conditioning Specialist through the NSCA. As a Level 2 Olympic Weightlifting Coach and a Level 3 referee, she has been deeply involved in the sport, even serving as a board member of the Saskatchewan Weightlifting Association for five years. Her achievements include being Saskatchewan’s top-ranked female Olympic Weightlifter from 2012 to 2015, during which she held provincial records in the Snatch, Clean & Jerk, and Total in her weight class. With an MS in Human Nutrition, Jocelyn loves sharing her knowledge on nutrition and performance through her blog and Instagram as “The Keto Athlete,” where she delves into the science of nutrition and its impact on athletic performance.
NUTRITION 101: Part 4 - Fat