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Fats Vs. Carbs: Which is the best source of energy?

Fats vs. Carbs as Energy Sources

With the Ketogenic Diet becoming an ever-popular trending concept, I think it is important to look at how this may pertain to energy utilization during various forms of activities. Fats aka lipids and carbs aka, carbohydrates are our bodies two major fuel sources. While proteins can contribute some energy in the form of amino acids, the amount is limited (10%) and not efficient.

The contribution of fat versus carbohydrates for energy will depend on a few factors, and each has its advantages and disadvantages depending on the individual and their exercise or activity level. Olympic weight lifting is an excellent example of where carbohydrates are a necessary fuel source for energy. Any athlete who incorporates anaerobic exercises such as lifting, jumping or sprinting should think before considering a diet which cuts out any nutrient. Specifically carbohydrates, as they may be hindering not only their performance but recovery as well.

Depending on the intensity and duration of exercise performed, the body will recruit muscles, and require specific energy pathways. For this to happen, there must be a certain amount and quality of kcalorie intake. We require a certain percentage of all macro nutrients for basal metabolic functions, but the role of glycolysis becomes even more important with an absence of oxygen availability. Carbohydrates may be the most critical of all the macro nutrients for the performance of anaerobic exercises. 

When the body is performing anaerobic exercise, it is in the absence of oxygen, and immediate energy is required. The body uses carbohydrates consumed to break down glucose, which is converted to pyruvate, then to lactate. This pathway gains its energy quickly, but cannot last for long periods of time. For the body to utilize this energy pathway properly, carbohydrates are critical. Adenosine Triphosphate (ATP) is responsible for delivering energy to the body for functions. For different types of exercise, certain muscles are recruited and ATP is utilized towards those muscles being used. There are three different energy systems used during physical activity; the phosphagen system (creatine phosphate), lactic acid system (glycolic), and aerobic (or oxidative) system. Depending on which of these energy systems are being used, macro nutrients will have different roles. 

The American Council on Exercise describes anaerobic exercise as it pertains to ventilatory threshold. Higher intensity intervals that load the cardio-respiratory system recruiting the second ventilatory threshold where lactate is rapidly increasing with intensity (ACE, 2010). Typically, in this threshold we will also see the use of fast twitch muscle fibers, as they have a capacity for anaerobic glycolysis. These muscle fibers are utilized primarily for any rapid, powerful movements such as jumping or sprinting (ACE, 2010 p. 82). A person who claims to have a lack of endurance, but ability to move quickly for short periods, is better at recruiting the use of these fast twitch muscle fibers. Where as a person who moves much more slowly, but can last for long periods of time are better at recruiting their slow twitch muscle fibers.

Carbohydrates play a significant role in the body for regular metabolic functions, and along with fat they are our bodies main energy source. However, carbohydrates yield more energy per unit of oxygen than fats. Since anaerobic exercise is in the absence of oxygen, the body uses carbohydrates immediately. There are many factors that go into the recommended intake such as the duration of the workout, intensity, frequency, body weight, and sex of the individual. In general, 6-10 g/kg of body weight is recommended as a daily intake (Clifford, J. & Maloney, K. 2015). 

Advantages of Fats:

-This macro nutrient is widely available in food supplies. We can find fats in many of the foods we eat.

-They are energy dense because they are high in caloric content. Remember fats yield 9 kcalories per gram.

-The amount which can be stored in the body is high

-They produce a high amount of ATP when metabolized


-There are many steps to metabolize this macro nutrient, making it time consuming as an energy source

-Our bodies can only produce ATP from this macro nutrient aerobically, so its use is limited to those activities which involve oxidative phosphorylation (low to moderate intensity exercise)

To determine the fuel utilization of macro nutrients, the respiratory exchange ratio must be determined during aerobic exercise. This provides the ratio of carbon dioxide produced to the amount of oxygen consumed, allowing us to see which macro nutrient is most efficient in producing energy.

Carbohydrates require use of an amount of oxygen equal to amount of carbon dioxide produced for higher RER. Oxidation of fat requires larger consumption of oxygen to CO2 produced. A case study conducted showed during low HR activities, the energy provided was through fat oxidation, and a smaller percent from carbohydrates. In the study as the participant began to increase his HR, the amount of energy provided through fat oxidation decreased, and carbohydrate oxidation increased along with the total energy expenditure.

In conclusion, the utilization of fats versus carbohydrates as an energy source is dependent on the activity conducted. During those activities which are anaerobic, where the creatine phosphate or lactic acid system is in use carbohydrates are the most fuel-efficient energy source. During aerobic exercise or physical activity, fat oxidation is abundant in its stores as glycogen stores are limited. Athletes should look to incorporate a certain percentage of each macro nutrient in their diet, paying attention to timing. Carbohydrates closer to exercise are best as they can provide immediate glucose, versus fats which take longer to metabolize.

Mackenzie Davidson

  •  MS in Nutrition Education
  • CrossFit Level 1
  • NASM Certified Personal Trainer and Behavior Change Specialist



Bryant, C. Ph.D., FACSM. Green, D. (2012). Essentials of Exercise Science for Fitness Professionals. American Council on Exercise.

Clifford, J. Maloney, K. (2015). Nutrition for the Athlete. Colorado State University Extension. 9.362. Retrieved from

Dunford, M., & Doyle J.A. (2015). Nutrition for sport and exercise. Stamford, CT: Cengage Learning.

Whitney, E & Rolfes, S. R. (2016). Understanding nutrition, 14e. Stamford, CT: Cengage Learning