High intensity cardio vs low intensity cardio

You've likely seen people doing lots of different variations of cardio while in the gym. Some are walking, some jogging, while you will see others flat out sprinting. So which version is the best? In short, cardio of any type is beneficial to health, from walking to skiing, for all ages in any state of health.



Like any other muscle, the heart grows stronger when it is challenged and worked. Conversely if it is not worked it will become weaker. A strong heart means a decreased resting heart rate, resting blood pressure, improved circulation (important for nutrient delivery and waste removal), which all means that your heart isn’t having to work as hard while at rest.


A consistent increase in heart rate also speeds up many processes which collectively are known as metabolism. Studies have shown that the higher the intensity of cardio the greater the metabolic effect. This is due to what is known as EPOC (excess post-exercise oxygen consumption). Following exercise, EPOC is part of the process that restores and replenishes body energy stores (ATP and creatine) back to a resting state, repairs cells and balances hormones creating an anabolic environment. An increased metabolism aids weight management.


Exercise increases the release of endorphins (feel good hormones), which have been shown to alleviate the symptoms of depression and fatigue.

Ghrelin is a hormone which increases appetite. It has been shown to rise when performing low intensity cardio, while the opposing hormone leptin, which decreases appetite, falls making you hungrier. Interestingly studies have shown that high intensity training decreases ghrelin concentrations, helping to curb appetite and subsequently aiding fat loss and weight management.

Human growth hormone (HGH) - contributes to fat loss by aiding muscle tissue growth, which in turn increases metabolism. As well as sporadically during the day and when we are sleeping, high intensity training has also been shown to help increase human growth hormone (HGH). It has also been shown that high intensity training, as opposed to low intensity, increases cell sensitivity to the appetite suppressing hormone leptin up to 48 hours post training. As well as contributing to bone and muscle strength, HGH also regulates fat metabolism.

Irisin (also known as the exercise hormone) was shown in one study to re-programme fat cells to burn rather than store fat, in another it was shown to regulate undifferentiated stem cells to become bone building cells instead of fat storage. On another note, people with higher amounts of irisin in their blood were more likely to have longer telomeres (caps at the end of chromosomes). This is significant because shorter telomeres are associated with health issues such as heart disease, Alzheimer’s and cancer.   

Testosterone the male sex hormone (women also have testosterone in smaller amounts) is increased during exercise, especially when working large muscle groups. It is needed for bone, hair and muscle growth as it repairs muscle cells damaged through exercise. As muscle is biologically more active than fat, the more muscle the faster the metabolism. Testosterone is increased during and after exercise, low testosterone has been shown to increase the risk of blood clots. 

Oestrogen is an important female sex hormone, needed for the development of breasts, for reproduction and the menstrual cycle. Like most hormones it has to be in the correct balance. Both oestrogen and progesterone decline with age, however progesterone declines more creating an oestrogen dominance. It’s a vicious cycle because an increase in oestrogen leads to an increase in fat which in turn produces more oestrogen. Progesterone stimulates fat burning, and is also a diuretic, while oestrogen increases water retention. Too much oestrogen is associated with breast, endometrial, prostate, colon and ovarian cancer. Conversely, progesterone slows cancer growth. Other symptoms include weight gain, excessive menstruation, thyroid problems, headaches, fatigue, hot flushes, infertility, depression, endometriosis and uterine fibroids to name a few. Oestrogen dominance could also be due to other factors:

  • Xenoestrogens - skin care, some food dyes, industrial materials and pesticides to name a few.

  • The contraceptive pill.

  • Obesity - due to an increased oestrogen production.

  • Stress - cortisol is the stress hormone and when elevated it uses up a hormone called pregnenolone, which is needed to produce progesterone. It also depletes magnesium, vitamin C, and B vitamins needed to neutralise bad oestrogen metabolites in the liver.

  • Heavy metals - these slow liver detoxification function, which in turn leads to an increase in hormones that should have been eliminated.

  • Liver problems - for the same reason as detailed above.

  • Blood sugar imbalances - lead to an increase in both insulin and cortisol, which subsequently increases oestrogen release, depletes magnesium, vitamin C, and B vitamins needed to neutralise bad oestrogen metabolites in the liver.

  • HRT - hormone replacement therapy, synthetic oestrogen.

  • Poor sleep - increases cortisol, which when elevated uses up a hormone called pregnenolone, which is needed to produce progesterone.

  • Caffeine - increases oestrogen production, depletes magnesium, vitamin C, and B vitamins, which are needed to neutralise bad oestrogen metabolites in the liver.

  • Cortisol - also known as the stress hormone, as it is released in large quantities when we are anxious and stressed. Ironically cortisol is a catabolic hormone, and when released in a short period it will increase fat burning. Chronic stressful lifestyles, however, panic our body into thinking the constant circulating catabolic hormone will use up our important energy reserves so it up regulates the fat receptors on cells (especially around the abdomen) subsequently increasing fat storage. Studies have shown that low intensity training will either lower cortisol levels or they will stay the same. High intensity training was shown to acutely increase cortisol levels, returning to normal after exercise. Intense endurance training has been shown to chronically increase cortisol levels. Vitamin C can help reduce cortisol levels.

  • Peptide YY - is secreted in the gut, and acts upon the brain increasing satiety after a meal. It appears that exercise increases peptide YY levels.


Exercise increases the muscles ability to utilise glucose, helping balance blood sugar and preventing blood sugar swings. This is extremely important with diabetics as they struggle to control blood sugar. Low testosterone may also be a contributing factor.  


Low intensity cardio (a walk after a workout) helps remove by-products of exercise thereby decreasing recovery time. It can also help increase blood flow and therefore oxygen and nutrients to muscles, which helps to speed up the repair and replenishment phase, reducing DOMS (delayed onset of muscular soreness). 

What is low intensity cardio?

Often a favourite of body builders, who perform very slow, low intensity cardio for a long period in the belief they will be solely working in the fat burning zone and prevent any muscle breakdown. Working at 50-70% of your maximum heart rate (220-your age), walking swimming and light jogging are all examples of low intensity cardio. A person of 40 working at 50% of maximum heart rate would be working at 90 beats per minute, this is known as the fat burning zone. It is true that a higher percentage of fuel burned during low intensity cardio comes from fat. Walking at 2.5 miles per hour burns 257 calories per hour and a low intensity bike ride will burn 343 calories per hour (based on an 80kg person). Low intensity exercise is aerobic, meaning it uses oxygen. If you are training for a long-distance event like the marathon obviously low intensity cardio is what you should be doing, accompanied with the correct exercise and nutrition plan. Simply breathing produces free radicals (reactive unchained molecules), and the more and harder we breathe the more we produce. Conversely, exercise also produces the antidote, namely antioxidants. The lower intensity training however produces more antioxidants compared to free radicals, while the opposite is true of high intensity training, making lower intensity training better for the immune system.

What is high intensity cardio?

This is performed at 70-90% maximum heart rate and works in the anaerobic (without oxygen) energy systems (ATP-PC and glycolysis) using short intense bursts with an ATP replenishment rest in-between. The fuel used in these particular energy systems comes predominately from stored carbohydrate (glycogen). There is a limited supply of glycogen, around 2 hours depending on weight. High intensity workouts include short explosive bursts of energy, sprints for example. You will never be exclusively burning one fuel whilst training, it is true that working at this intensity there will be an inevitable use of protein as an energy source and this will be taken from muscle tissue, as the body simply cannot break down fat fast enough. This will subsequently lead to a loss of muscle tissue, however due to the hormonal response elicited from this type of training (an increase in testosterone and HGH) it has also been shown to aid muscle growth. As long as the high intensity training is accompanied with the correct nutrition and nutrition timing, there should be no muscle loss. Insulin is the anabolic hormone needed to take sugar from the blood stream and push it into cells, to be either used for energy or stored as glycogen or fat. Insulin sensitivity (how well your body processes glucose and stores body fat) is improved through high intensity training. The more insulin sensitive your body, the smaller amount of insulin is required to balance blood sugar, preventing an over-reaction and an increased fat storage. As well as the negative affect this style of training has on the immune system (mentioned in the previous section), another negative is the parallel increase of lactic acid with intensity. An increase in acidity will cause the body to leach alkaline reserves in the form of minerals from bones to buffer the acidic environment. This however is not a reason to avoid this type of training, with the correct nutrition plan in place you can ensure you are getting plenty of alkalizing foods and antioxidants to counteract the negative aspects of high intensity training, although if you are already feeling under the weather you will be better choosing the low intensity option until you are recovered. 

Which is best for fat loss?

A woman weighing 60kg working at low intensity (60-65% of MHR) for 30 minutes burns 146 calories, 73 calories from fat (50%). The same woman performing high intensity training (80-85% MHR) for 30 minutes burns 206 calories, 82 from fat (39.8%). This shows that although a higher percentage of calories burnt at low intensities come from fat, working at a higher intensity actually burns more calories and subsequently a higher portion of fat. High intensity training, by its nature is hard due to the energy system you will be working in, this type of training can only be performed for shorter periods of time. It is important to remember that training at high intensity on a regular basis can lead to over training and injury. Make sure you stretch and listen to your body. High intensity training is best for results, but it is worth throwing in the odd low intensity, recovery session. recovery will be faster with low intensity training.

High intensity training has been shown to increase mitochondria concentrations in muscle. This is significant because mitochondria are the organelles found inside cells that burn fuel for energy. The fitter you become, the more mitochondria you will have, the more energy we can burn at one time, making our workouts more efficient. Another added benefit with the high intensity training is the EPOC factor previously mentioned in this article.

This article has
been written by
Terry Fairclough