There are 4 main areas within the body where fat/triglycerides are stored as energy:
IMTGs (intramuscular triglyceride stores)
Plasma free fatty acids
The majority of our energy is stored as triglycerides in our subcutaneous adipose tissue. Some energy exists in IMTGs, and there is an insignificant amount found in plasma triglycerides and plasma free fatty acids.
There are 2 major types of fat, which are either saturated and unsaturated. Whether or not a fatty acid is saturated depends on the number of carbon-carbon double bonds a fatty acid has. If a fatty acid chain has no carbon-carbon double bonds, it is considered saturated since it binds with the maximum number of hydrogens possible.
Essential Fatty Acids
There are two essential fatty acids (EFAs) that cannot be synthesized by the body and must be obtained in the diet. These two EFAs are alpha-linolenic (omega-3) and linoleic acid (omega-6). It should be noted that there is no data to support that omega-3s aid in improving exercise performance, but there is available research showing omega-3 fatty acids possessing anti-inflammatory properties. This is likely because it competes with arachidonic acid, a pro-inflammatory fatty acid, to replace it.
Omega-3s are comprised of two main constituents: EPA and DHA. As stated previously, one of Omega-3s most powerful benefits are it’s anti-inflammatory properties. In addition, the proper dosage of EPA and DHA have been shown to lower triglycerides, improve cognitive performance and improve insulin sensitivity. High levels have been shown to reduce muscle soreness, too. Overall, a quality fish oil supplement is a great investment for your overall health.
The primary goal of fat metabolism is to transport fatty acids to the mitochondria of working muscles in order to be broken down for the energy (ATP) we need for activity. We obtain a large amount of energy from fats, since they hold more energy per gram (9kcal/g) compared to protein and carbs (4kcal/g).
There are 3 parts to this process:
Lipolysis (breaking fatty acids from their glycerol backbone)
Transport via blood to muscles
Transport into mitochondria for beta-oxidation
The fatty acids released from lipolysis are either pushed into circulation or stay in adipose tissue to be used towards new triacylglycerols (a process known as reesterification).
Rest and Activity
Whether we are exercising or not, carbs and fats are always oxidized as a mixture. Although, whether carbohydrate or fat is the dominant fuel source depends on a few variables – specifically (and most importantly) being the intensity and duration of our exercise.
At rest or after an overnight fast, most of our energy comes from oxidizing fatty acids coming from our adipose tissue. This is because we have the lowest glycogen levels upon waking up (aka after a long period of fasting). As we transition from rest to low-intensity exercise, we stimulate lipolysis, which raises the availability of fatty acids to our working muscles, thus increasing rates of fat oxidation.
The general rule of thumb here is that as exercise intensity rises, the contribution of fat oxidation for energy decreases. Fat oxidation is dominant at low exercise intensities, where carbohydrate oxidation is the main energy source for high exercise intensities. Once we get to intensities above 75% VO2 max, fat oxidation significantly drops.
A drop in fat usage during high intensity training is due in part to the decrease in circulating fatty acids resulting from the decrease in the release of fatty acids from adipose tissue. This drop in releasing fatty acids is likely because of decreased adipose tissue blood flow and efficient fatty acid removal via the bloodstream during high intensity exercise bouts.
Guidelines in Performance
The AMDR (acceptable macronutrient distribution range) for dietary fat is between 20-35% of our total energy intake. This comes out to about 0.8 – 1.25 grams/kg of body mass. According to the available research, there is no inherent reason athletes or recreationally active people should not follow the recommended 20-35% range for dietary fats.
With my clients, this is typically also where the fat ranges lie for optimal health, body composition and performance. The ranges may change for the individual as some clients are more fat sensitive (meaning they can tolerate less) while others use and need more fat in their diet (typically those who are more carb sensitive).
Of course, if someone is on a ketogenic diet, they will be consuming a very high percentage of their total energy intake from dietary fat (65-75%). While fewer clients use a true ketogenic diet approach (read: actually keto, not just low carbs), there is a subset of people who do thrive here. As always, it depends on the individual client.
As mentioned before, fasting elevates resting free fatty acid concentrations. This seems like a good thing for us – we would essentially be able to burn more fat for fuel, right? While this does happen metabolically, it doesn’t translate this way realistically. When we burn calories, whether they are derived from carbs or fat doesn’t show any significant differences or benefits to athletes. Additionally, training fasted has not been shown to improve endurance performance in the current literature.
Each macronutrient plays a significant role in our overall physiological function. Setting targets for each will depend greatly on the individual and their body composition, activity level, and much more. Optimizing everything from our training down to our diet specifically can be very underestimated, but an important consideration to make when evaluating your progress.
Simopoulos, A.P. Omega-3 fatty acids in inflammation and autoimmune diseases. Journal of the American College of Nutrition. 2002; 21:6, 495-505.