Metabolic training involves a combination of strength and cardio exercises, performed in rapid succession with minimal breaks. High-intensity interval training (HIIT) affects hormones in several ways, contributing to better physical performance and overall health. 

Combining short periods of intense exercise with short periods of rest or lower intensity helps burn calories, improve metabolic efficiency and increase muscle strength. The main benefits of how HIIT affects hormones are outlined below:

1. Increasing adrenaline and noradrenaline levels: HIIT stimulates the release of adrenaline and noradrenaline, which increases heart rate, blood flow and the mobilisation of energy from fat stores. This hormonal response helps to increase performance during exercise and improve fat burning.

2. Improving insulin sensitivity: HIIT can increase insulin sensitivity, which means cells can more easily absorb glucose from the blood. This reduces the risk of developing insulin resistance and type 2 diabetes.

3. Increasing growth hormone (HGH) levels: after intense exercise, growth hormone levels increase, stimulating muscle growth and regeneration and increasing fat burning. HGH also helps in tissue repair and improves overall metabolism.

4. Endorphin release: HIIT releases endorphins, also known as "happy hormones". This reduces pain, improves mood and reduces stress. Endorphins can also increase motivation to keep exercising.

5. Cortisol regulation: HIIT can regulate levels of cortisol, the stress hormone. While cortisol increases during exercise, regular exercise can help maintain a healthy cortisol trajectory throughout the day in the long term.

6. Increasing testosterone levels: in men, HIIT can increase testosterone levels, which helps to build muscle mass, improve strength and reduce body fat. In women, it can help balance hormonal fluctuations and improve overall energy.

7. Stimulating myokine secretion: myokines are hormones released by muscles during exercise. They have anti-inflammatory effects, improve the metabolism of fats and sugars, and affect cardiovascular health.

High-intensity interval training (HIIT) is so effective not only because of its direct effect on calorie burning and metabolic rate increase, but also because of the positive hormonal changes it triggers. Regular HIIT can lead to better health, increased muscle mass and improved body composition.

Scientific research

1. A 2023 study published in Frontiers in Endocrinology found that the timing of exercise is an important factor in optimising metabolic benefits. Exercising early in the morning or late in the afternoon can synchronise the biological clock and improve metabolic outcomes.
2. A 2022 study published in "eLife" found that high-intensity interval training (HIIT) remodels the proteome and acetylome of human skeletal muscle, contributing to better metabolic health.
3. A 2023 study published in Frontiers in Endocrinology found that HIIT improves insulin sensitivity and glycaemic control in older, sedentary people with type 2 diabetes.