How to Lose Fat and Maintain Muscle with Crossfit Training and Good Nutrition

One of the most desired dreams of average gym goers and Crossfitters is building muscle mass and at the same time losing fat. The body cannot turn fat into muscle, however, it can enhance fat burning while maintaining lean muscle mass by choosing the right diet and exercises with strength components.

The first law of thermodynamics says: When energy passes, as work, as heat, or with matter, into or out from a system, the system’s internal energy changes in accord with the law of conservation of energy. It means that the body cannot produce more energy than it gets, which rules out building muscle and losing fat at the same time.

crossfit nutrition quote card
Get on the right track

However, our body is an adaptable organism and this adaptability is managed by hormones. In a hormonally favourable environment, the body can switch into a mood where it preserves muscle mass and uses stored fat for energy.


The main hormone responsible for directing energy into muscles is insulin produced by the pancreatic cells. When the muscle cells are refilled, insulin directs the excess energy into fat stores. Insulin blood level can be manipulated by diet, activity and sometime it is the result of medical conditions, such as diabetes. Therefore insulin sensitivity is the key to losing fat while building muscle mass.

Insulin is released into the blood, when there is too much sugar. Low carbohydrate intake keeps the sugar low and insulin is not triggered, therefore it won’t store the excess energy in fat tissue. Insulin sensitivity can also by improved by exercise. According to a study published in the Journal of Sport Medicine, a single bout of exercise can increase insulin sensitivity for up to 16 hours post workout.


Before workout the athlete loads up on carbs in order to provide energy into the working muscles. Many professional CrossFitters manipulate the carb intake during the day. Mat Fraser also changed his diet in this fashion before his victory at the 2016 CrossFit Games. He cut out the junk food and included more natural food, fruits and vegetables. Fraser observed positive changes in his body composition. He dropped ten pounds, which improved his performance in gymnastics.

Several studies showed positive results on fat loss while maintaining muscle mass on low carb diet. Body maintains its energy requirements by using the fat stores. Sufficient protein intake is recommended and supplementation with BCAA can prevent muscle breakdown and improve insulin sensitivity.

Obviously, you can afford low-carb approach if you do a sedentary job and hit the gym 2-3 times a week. But a professional athlete like Fraser must include more carbs in his diet to fuel the multiple workouts. But the idea of cleaning up the diet and focusing on quality foods applies universally.


Testosterone is important for the growth processes in the body, including building muscle mass. Any type of exercise increases testosterone levels but lifting weights has bigger effect. It was proved by research on exercise physiology at the University of Southern California (USC). Dr. Todd Schroeder, associate professor at USC, listed heavier loads and shorter rest periods as the main testosterone boosters. Apart from building muscle mass, testosterone, improves body´s efficiency to burn fat.

Intensive workout uses up the energy stored in the muscles and also breaks down the muscle fibers. Naturally, they respond to this stimulus by repairing themselves in order to get stronger and more resistant. Sufficient dose of protein, which is the main building block, and healthy fats that take action in healing mechanisms, is needed. Therefore protein intake of 1 gram per pound of bodyweight is recommended, while also focusing on healthy fats, such as fish oil.

intermittent fasting protein sources nutrition salmon salt tomato testosterone
Salmon is a nutritious source of protein.


Intensive workout causes that the metabolism will be working at faster speed few hours after the workout. Drop sets, supersets, complexes and interval training are the best to call out the hormonal response and to increase the Basal Metabolic Rate (BMR). It also results in the post-exercise after burn effect, when your metabolism stays elevated up to 36 hours post workout.

Barbell exercises used in CrossFit (squats, deadlifts, presses, weightlifting) have huge muscle recruitment potential. Dynamic movements activate the fast-twitch fibres, stimulation of which is important for strength and speed development. By practicing them with heavy load (squats, deadlifts) or at high speed (cleans, snatches) your body is using large part of the muscle fibres. Adaptation mechanisms cause that you won´t lose the mass because it is required for the performance.

crossfit girl emom workouts outside beach
Stay strong!

These principles are also used in CrossFit workouts. They leave practically no time for recovery in between the sets, which forces the body to adapt to the conditions. When controlled, these changes can have positive effects on losing body fat and building work capacity of the organism.

Combination of heavy loads, multi-joint exercises and intensity switches the body into a fat burning mood while preserving active muscle mass. Combined with diet rich in protein and healthy fats with limiting carbohydrates, it has synergic effect on the body´s ability to maintain muscle mass while burning fat. Moreover, CrossFit sessions increase the sense of community and often motivate the people to hit the bar at much higher intensity.

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  1. Schroeder ET. and col., Are acute post-resistance exercise increases in testosterone, growth hormone, and IGF-1 necessary to stimulate skeletal muscle anabolism and hypertrophy?, Med Sci Sports Exerc. 2013 Nov;45(11):2044-51. doi: 10.1249/MSS.0000000000000147. Accessible at:
  2. Anssi H Manninen, Very-low-carbohydrate diets and preservation of muscle mass, Nutr Metab (Lond). 2006; 3: 9. Accessible at:
  3. Borghouts LB, Keizer HA., Exercise and insulin sensitivity: a review, Int J Sports Med. 2000 Jan;21(1):1-12. Accessible at:

Mat Fraser: Making a Champion, YouTube. Accessible at:

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