Clash of the Titans – HIIT vs LISS – Part 2
Naturally, high-intensity physical activity, such as high-intensity interval training (HIIT), creates endocrine and defence reactions due to the acute stress it triggers, forcing our body to adapt!
Besides the increase of body temperature and decrease of body fluids and metabolic supplies, common to any kind of exercise, it is known that the way certain hormones and growth factors respond to exercise relies on the exercise intensity.  When you reach 85-95% of the VO2max, the levels of several hormones increase — hormones that will have effects on body composition. 
At endocrine level:
1 – Increase of anabolic hormone levels
The studies have shown that in the 24 hours following the HIIT workout, the body can produce 450% more growth hormone, reaching levels well above those caused by any other type of training. 
While low-intensity steady-state exercises don’t cause significant changes in the cortisol level, VEFG (Vascular Endothelial Growth Factor) levels, or other hormone or growth factor levels, during high-intensity exercises these levels increase exponentially. Similar results were also demonstrated in testosterone levels. 
2 – Increase in insulin sensitivity
Insulin, also an anabolic hormone that plays a role in the metabolisation of glucose for energy production, normally increases during and after physical exercise.
Studies have shown that insulin sensitivity, meaning the degree of response to the stimulus, can improve by 23-58% with HIIT workouts performed during 2 to 16 weeks. How this works, is linked to a major stimulus to the glucose carriers through the induced muscular contraction. [4-6]
3 – Increase in nitric oxide
As referred in the first part of the article, HIIT has proven to be more effective in improving the endothelial function (that is, the regulatory and protective functions of the cardiovascular vessels against aggressions), and has shown its contribution to the synthesis and availability of nitric oxide in individuals with Metabolic Syndrome (a set of factors that increase the risk of heart disease and other health problems such as CVA and diabetes). Nitric oxide is an important intracellular and extracellular marker that promotes muscle relaxation. Most likely, these markers’ synthesis and availability increase because HIIT induces a different type of stress on the artery wall. Since its vasodilatory effect grants protection to the vascular function, the natural loss of vessels’ physical properties is slowed down and their elasticity lasts longer. [7, 8]
4 – Improvement of HDL cholesterol levels
Despite the effects on the overall cholesterol and triglyceride levels not being statistically significant, after 8 weeks of HIIT improvements in the HDL cholesterol levels were found. Thus, this type of workout is recommended as one of the methods to improve the blood lipid profile. 
5 – Increase in fat oxidation
Another metabolic benefit of HIIT is the increase in post-workout energy expenditure, also known as EPOC (Excess Post-exercise Oxygen Consumption). After a high-intensity workout, our body tries to restore the cells’ physiological and metabolic factors to the levels of the pre-workout period. This means that our metabolism will stay high. 
Since 2008, it has been demonstrated that after 6 weeks of HIIT workout, the fat oxidation is significantly higher , and since then several studies have already proven that the oxygen consumption, and consequent calorie expenditure, can remain high for up to 48 hours after training, increasing basal metabolism and fat burning. Besides, in individuals with Metabolic Syndrome who followed a 16-week HIIT programme, it was found that the values of the fatty acid synthase enzyme, responsible for promoting fat production, decreased in comparison with Low-Intensity Steady State training (LISS). [5, 12, 13]
On the musculoskeletal system:
1 – Increase in mitochondrial oxidation:
The mitochondria use oxygen to produce high ATP amounts (the cell energy molecule) through fatty acids utilisation and oxidation during cardio exercise. The increase in mitochondrial density is an adaptation to the exercise. With the increase in mitochondrial density and its oxidative enzymes, more energy is made available for the muscles to produce more strength, for a longer time. For many years, it was thought that this only happens with long-lasting resistance workouts, but more recent studies show higher results with a consistent HIIT workout, in just 2 weeks. [14, 15]
2 – Muscle switch
Another HIIT benefit is that it helps develop fast-twitch muscle fibres due to the high speed in which exercises are performed. The slow-twitch fibres respond better to resistance training, while quick-contraction fibres respond better to boosting periods, in other words, intense short-lasting exercises. [16, 17]
3 – Increase in lean body mass
We know that one of the main risks and disadvantages of LISS training is the loss of gains — which take you so long to get. In case of inhibition of the mTOR path, one of the essential muscle growth regulators, the body will end up burning the same amounts of muscle and fat. [18, 19] But as previously described, HIIT increases the anabolic hormones and the mitochondrial synthesis, leading to an opposite effect — hello, gains!
Yes, this kind of training is quite intense but we can say that it is equally or more efficient, and definitely effective for you to get results! With just 4 to 30 minutes of HIIT, 2 to 3 times per week, you can get results as good as or better than with 60 minutes of LISS, 5 times per week. It’s not easy… but the secret of life lies in seeking constant challenge.
Inês Fernandes – She’s a medical doctor and passionate about fitness. Inês dedicates a lot of her time to her own workouts, based on the effectiveness of the exercises and on scientific study results, looking to mash up them with complex, unusual and challenging exercises. She wants to take a Personal Trainer course during her medical internship, combining the two things she likes to do most in life: heal people and help them achieve a good physical shape!
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