Everyone is looking for a magic pill, potion or program for easy weight loss and a killer body. The truth is there simply is no secret to success but HIIT and a sensible diet are the closest you can come. In this article we explain the reason why Weight Loss with HIIT is excellent.

A HIIT workout is the ideal way to bring your body to the point of regular anaerobic metabolism so that glycolysis takes over to create the energy needed for continued cellular function.

Based on the research, published in the British Journal of Sports Medicine, HIIT or Hight Interval training is an attractive alternative to address overweight and obesity given. It’s potential to offer benefits similar to moderate-intensity continuous training while requiring less time.  Although all the participants lost weight, those doing HIIT saw a 28.5% greater weight loss. High-intensity interval training (HIIT) has been shown to improve cardio-metabolic health during supervised lab-based studies but adherence, enjoyment, and health benefits of HIIT performed independently are yet to be understood. However the researchers cautioned that HIIT may not be suitable for everyone.

The Body Becomes a Fat Burning Machine

Glucose is the primary source of fuel that powers the body. For the most part, it is a product of the digestion of carbohydrates, starch and sugars in the foods we eat, and is in abundant supply as long as there is oxygen present to help with the conversion to energy.

After the body has used up glucose and glycogen, non-carbohydrate elements can be broken down to provide glucose. These elements include amino acids from proteins and glycerol from fat. It is possible for the body to use up almost all the fat stored in adipose tissues for the creation of energy in the absence of adequate glucose.

Weight Loss with HIIT

Weight Loss with HIIT

ATP, the energy from the metabolism of glucose, is continually produced during moderate exercise. As the heart rate and breathing rate increase, more oxygen is taken in and spread through the body to promote aerobic metabolism. When the demands on the body continue to the point where the lactate threshold is passed, the body automatically switches to anaerobic metabolism until the buildup of lactic acid prevents further activity.

The high intensity of HIIT workouts forces the body into the anaerobic mode, completely depleting the body’s stores of glycogen in the cells of the muscles. The lower intensity or resting recovery periods provide the break the body needs for aerobic metabolism to break down fat and protein to create more carbohydrates that can then be converted into glucose.

It is through alternating high and low intensity workouts that the body is ‘programmed’ into burning fat during resting periods –12 and even up to 24 – 36 hours after the actual workout is complete – the After Burner Effect!

HIIT Makes the Entire Body Work Hard

That may seem obvious but it means that along with the sweating and aching muscles, all the cellular processes within the body become involved and get a good workout. With the intense activity incorporated into a HIIT session, the body reaches a number of critical thresholds that stimulate defensive reactions:

  • Reduced oxygen supply and increased blood carbon dioxide
  • Increased body temperature
  • Reduced available fuel stores – low blood sugar
  • Reduced body fluids – dehydration
  • Tissue damage – injury

In reacting to these danger signals, the body must adapt and that is where the benefits of HIIT are observed, especially in respect to weight loss:

  • Reduction in body fat without losing muscle mass
  • Strengthening of the cardiovascular system
  • Increased ‘work capacity’ (tolerating more intensity for longer periods of time)
  • Improved oxidation of carbohydrates and fat in skeletal muscle
  • Challenging fast-twitch muscles (that build power, strength and physique)
  • Improved mental attitude

Lose Weight While You Rest

Now that sounds like magic! After a HIIT workout, your body continues to burn calories in order to return to its normal physiological and chemical resting state – homeostasis. This is the beauty of anaerobic metabolism! Glycogen does not need energy to be broken down for use by the cells but it needs energy to replenish the supply. In order for the blood sugar to be made available for the metabolic system of the muscles during periods of rest, energy is used and this is part of the ‘after burner’ effect.

Because of the tremendous demands created by HIIT activities, a suitable balance between supplying body needs (rest and carbohydrates) and expending energy (muscular and stress-related) is necessary for overall health and wellness. Stated another way, there has to be balance between activities involving the parasympathetic nervous system (‘rest and digest’) and the sympathetic nervous system (‘fight or flight’).

You work hard for 20 to 30 minutes during a HIIT session which is just up to the point of depletion of glycogen stores. In the following hours, your body converts fat to replenish that glycogen and you lose weight while you are ‘resting’!

What About Calories?

If you reduce food intake, the body goes on the defensive and slows the metabolism so that it does not run out of fuel. When the metabolism slows down, it is harder to lose weight. That is why crash dieting and severely limiting caloric intake result in increased weight gain when the ‘diet’ is over!

If you increase your caloric needs by exercising, you actually need to have adequate calorie intake. The problem with most people, though, is that their calorie intake is already higher than their energy demands and the calories come from the wrong types of foods. Adopting a better overall diet will bring the available calories and nutrients into better alignment with the needs of the body.

For those of you who need to see some numbers, there are general rates for caloric burn based on the intensity of the activity and the body weight of the person involved. The heavier the person, the more calories will be burned performing the same activity for the same length of time.

When and What to Eat?

Given the intensity of a HIIT workout, it is best to eat a meal at least 1 to 1 ½ hours before exercising. Not only do you want to give the food a chance to pass through the stomach to avoid nausea, it needs to be digesting so that the nutrients are available as soon as the workout is over. A healthy nutrition plan plays a big role in HIIT results.

Also, don’t eat immediately after your session because you don’t want simple carbohydrates available for quick conversion to energy while the ‘after burn’ effect is working at its maximum. In half an hour or a little longer after working out, a protein-based snack or shake is the best boost for your body.

Numerous small meals are better than 3 regular meals when it comes to providing an even fuel supply for intense muscle workouts. That does not mean a wide range of foods at each time, but more of a healthy snack interspersed in between regular meals. It comes down to a matter of personal choice, circumstances and convenience.


  • Gibala MJ. High-intensity interval training: a time-efficient strategy for health promotion? Curr Sports Med Rep 2007;6:211–3.
  • Vella CA, Taylor K, Drummer D. High-intensity interval and moderate-intensity continuous training elicit similar enjoyment and adherence levels in overweight and obese adults. Eur J Sport Sci 2017