HIIT

 

Introduction

Exercise is a proven way to improve overall health and decrease the risks of developing different chronic diseases. (Figure 1.) ^{[2], [12], [24]} The traditional recommendation for time spent on doing physical activity (PA) is 75 minutes of vigorous or 150 minutes of moderate activity per week. ^{[1], [2], [22]} A different, very effective and non time-consuming type of exercise is HIIT which although does not meet the accepted PA time recommendations makes up by increasing the intensity. ^[1]

 

Figure 1. Benefits of exercise on body. Comparison between Continuous endurance training and High Intensity Interval Training (HIIT) showing their ways of improving health. They both activate the peroxisome proliferator-activated receptor-g coactivator-1α (PGC-1α) which results in many benefits as shown. The difference is the signaling pathways. Continuous endurance training uses the Calcium–calmodulin kinase (CaMK); a mitochondrial biogenesis messenger; by increasing the intramuscular calcium concentration. HIIT signals PGC-1α by reducing Adenosine triphosphate (ATP); a nucleotide that stores free energy for cells to use; concentrations as a result of the exercise so that the production of higher concentrations of Adenosine monophosphate (AMP) is triggered which activates AMP-activated protein kinase (AMPK). ^{[24], [25]}

 

 

What is HIIT?

High Intensity Interval Training or HIIT, is an exercise technique where you repeatedly have a very high intensity work out for a short period of time, followed by a short period of active recovery or rest. There are 2 types of HIIT – Aerobic and Resistance. As suggested by their names Aerobic HIIT focuses on improving the aerobic fitness using jogging or cycling, while Resistance HIIT is focused on building muscle mass by weightlifting. Although both types of HIIT are beneficial to health, Aerobic HIIT shows higher efficiency on chronic diseases like Type 2 Diabetes Mellitus (T2DM) in comparison with Resistance HIIT. ^{[1], [2], [22]}

 

 

Why choose HIIT?

The Aerobic HIIT includes physical activity such as running and cycling. It is highly linked to the metabolic health and has proven benefits on cardiovascular function, VO2max and prevention and management of Type 2 Diabetes. ^{[1], [2], [12]} (Figure 2.) Studies show that HIIT is a more efficient type of exercise for improving insulin sensitivity and VO2max compared to continuous moderate training and it also increases muscle glycogen consumption. (Figure 3.) ^{[1], [2], [5], [6], [7], [8], [9], [11], [23]} These improvements are very important to T2DM and pre-diabetic patients, as well as to the healthy population to improve the general health and lower the risks of developing a disease. The Aerobic HIIT is more efficient for diabetic patients as T2DM is an endocrine disease, also classified as metabolic and should be treated like e cardiovascular disease and it is proven that aerobic training highly improves cardiovascular function. ^{[1], [7], [16]}

Figure 2. Plasma glucose concentration and insulin response in both men and women pre- and post- exercise. Data is taken from oral glucose tolerance test and is shown in means ± Standard error (n = 16 for men, n = 18 for women). ^[9]

     

Figure 3. Mean VO2max (L/min) in HIIT and MICT (Moderate intensity continuous training) groups over 8 weeks period. ^[23]

 

 

How it works

 

Improving VO2max

VO2max is the maximal oxygen uptake i.e. how much oxygen we use during an intense maximal physical activity. The higher the VO2max is, the longer and harder exercises we can endure. Increasing the VO2max leads to increased fat oxidation meaning that we are burning calories even when we are resting. HIIT is recommended to people suffering from Obesity, T2DM and other chronic diseases linked to the cardiovascular function as it helps us burn fat and boosts our blood circulation.  ^{[1], [4], [6], [10], [17]}

Metabolic health 

The normal regulation of blood glucose is shown on Figure 4.

 
Figure 4. Glucose metabolism ^[21]

 

Studies have proven that HIIT helps control Glucose tolerance by dropping the mean plasma glucose after 2 weeks of exercise. ^[12] What happens is that by exercising we increase the translocation of a transporter protein that carries glucose to muscle and adipose tissues called ‘GLUT4’ to the cell membrane. (Figure 5) When we have higher concentration of this protein on the surface of cell membranes, we are able to increase glucose intake ^[14] without using high amounts of Insulin and therefore we start producing less Insulin. As a result, the body becomes more Insulin responsive. ^{[2], [15]} It has also been proven that fat percentage is strongly linked to insulin resistance and reducing fat improves glucose uptake. This is a key way to improve Insulin sensitivity in T2DM patients and sedentary or obese people. ^[19]

 

Figure 5. GLUT4 and Insulin function ^[20]

 

 

Improving cardiovascular function

T2DM is associated with cardiovascular diseases such as Atherosclerosis; a condition where arteries are clogged with fat buildup. (Figure 6.) Researches show that HIIT improves endothelial function by increasing flow-mediated dilation (FMD) which means the blood vessels widen and allow blood flow to increase and therefore lowers the risks of cardiac ischemia e.g. Heart attack. HIIT is proven to be more effective than moderate-intensity continuous training (MICT) because of its bigger impact on VO2max which is highly linked to FMD. ^{[1], [2], [3], [4], [11], [13]}

 

Figure 6. Build up of plaque in Atherosclerosis ^{[29], [30]}

 

 

More benefits of HIIT

• HIIT is also proven to improve the effectiveness of medicaments prescribed for T2DM patients such as Metformin. ^{[5], [22]}
• It is not time consuming, so it appeals to everyone. It does not necessarily require special equipment, which makes it relatively easy to perform anywhere. ^{[1], [2]}

• There are different levels of intensity and variable training options depending on the individual. ^{[1], [2]}

 

• Enjoyment response – HIIT is reportedly more enjoyable than Continuous moderate-intensity and continuous high-intensity exercise. ^{[1], [18]} (Figure 7)

 

Figure 7. Enjoyment (Mean±SE) of the three exercise trials. (HIT – High Intensity Interval Training; CVI – Continuous vigorous intensity; CMI – Continuous moderate intensity) ^[18]

 

 

Safety considerations

• Although HIIT is proven to be beneficial for variety of medical conditions, it must be taken in consideration the established guidelines. ^[22]
• It is only appropriate for low to moderate risk individuals and people who are being supervised by a specialist. ^{[1], [22]}
• The frequency, intensity and type of exercise should be carefully considered as it is a key point for management and prevention of a disease. (Table 1.) ^{[1], [22]}

 

Table 1. Recommended Physical activity for T2DM patients. Data was taken from Aagaard P. (2010) ^[22]

 

 

 

Written by Lilly Konyarska

 

 

 

 

 

 

 

 

 

 

 

 

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