A lot of individuals have developed an old approach with regards to exercise and fat loss. The concept that running for numerous hours (this is only one example) will get you toned is totally false. Whilst both low intensity and high intensity training do have their benefits, this post will look at which is best for fat loss, and touch a little on calorie intake.
High intensity training or low intensity for fat burning?
Most individual’s either fall into the low intensity training or high intensity training camp; each of these have their benefits. Low intensity training has cardiovascular related benefits but it is very time consuming, plus an increased volume of training is required to increase metabolic responses for fat oxidation (the body’s ability to burn fat as an energy source). In a study by Gillette, Holland, Vincent, and Loy (1991), running for 20 min at a steady pace had only a 1º increase in body temperature, with little to no change in fat free mass. This small increase in body temperature is an indication that the subjects in the above study were not increasing their heart rate enough to increase body temperature and metabolic responses that would actually support fat burning through fat oxidation. However, running at a steady pace for a longer period of time ≥45min does have increased cardiovascular health benefits and fat burning properties (Fisher et al., 2015). Whereas, high intensity training can be performed in half the time or less than that of low intensity training, plus, there is increased benefits from high intensity training i.e., increased muscle mass, increased metabolic responses that aid in burning fat (Trapp, Chisholm, Freund, & Boutcher, 2008), increased VO2 max (Fisher et al., 2015), which has been found to improve the endurance of athletes and improve cardiovascular health (Shaw, Gennat, & O’Rourke, 2006).
Trapp et al., (2008) conducted a study comparing two groups of women who performed either high intensity interval training at max effort or steady state cardio training at 60%VO2 max. The group that performed the high intensity training lost more subcutaneous fat (the fat that lies directly under your skin) compared to the steady state cardio group, up to 2.5kg fat lost. Furthermore, Dunn (2009), performed a similar test and reported a similar amount of subcutaneous fat loss. Maillard, Pereira and Boisseau, (2018) conducted a meta-analysis on previous research studies that compared the amount of fat loss on individuals during high intensity training and low intensity. They found that high intensity training had a significant reduction on whole body fat stores, with the highest reduction seen in groups that maintained a calorie deficit.
High intensity training
A recent review was conducted by Kuo and Harris (2016), where they reported moderate to high intensity training was more efficient at reducing body fat, but why is this? High intensity training will break down muscle fibres more-so than low intensity training. As a result, cell regeneration from the muscle breakdown is required due to the muscle damage from the intense training, thus, leading to greater muscle growth, as a result, enhanced fat oxidation (Kuo & Harris, 2016). The fat burning concept that is due to increased energy consumption via muscle contraction may be explained by a greater fatty acid oxidation resulting in an observed abdominal fat reduction as the outcome from high intensity training (Kuo & Harris, 2016). Further, high intensity training has been shown to increase resting metabolic rate for up to 24 hours after training, hence, an increase in calories burned (King, Broeder, Browder, & Panton, 2002).
Consequently, Zhang et al., (2017) reported the same amount of fat loss in groups of women using both high intensity training and moderate intensity training, with both groups performing exercise equalling 300kj which is equivalent to about 80 calories burned. The use of calories compared to volume of work, may be an overlooked confounder due to each participant burning the same number of calories per session, therefore, each individual was in the same calorie deficit, as a result they were guaranteed to lose fat. Furthermore, the individuals in this study also had their calorie consumption monitored. While this study does seem a little biased, the training groups did see a reduction in fat mass, but this is due to the individuals been in a calorie deficit and increasing their energy expenditure through regular exercise. Changes in body composition are the primary result of energy consumed vs energy expenditure [calories in vs calories out] (Swift, Johannsen, Lavie, Earnest, & Church, 2014). This further supports the evidence that consuming less calories and/or burning more calories through exercise will result in a reduction in body fat stores.
What types of fuel does the body use during exercise for fat loss?
Fat and carbohydrate are both important fuels for energy production within the body. However, the length and intensity of exercise, and the availability of these substrates both inside and outside of the muscle will determine how both of these energy sources are oxidised within the body (Spriet, 2014). For example, during exercise, the metabolic rate (the rate at which the body burns calories for energy) is increased well above the resting metabolic rate, therefore, the energy substrates from both fat and carbohydrate are activated at the duration of the exercise until a steady state is established. Once this occurs, the body will use either fats or carbohydrate as a fuel source, but this will depend on a number of things i.e., the duration and intensity of the exercise, the intracellular and extracellular metabolic environments (Spriet, 2014; van Loon, Greenhaff, Constantin-Teodosiu, Saris, & Wagenmakers, 2001).
So, WTF does this all mean?
Increasing carbohydrate intake before or during exercise (via supplementation) will only increase carbohydrate oxidation (the body’s ability to use carbohydrate as a fuel source) and inhibit whole body fat oxidation (Spriet, 2014), meaning, less fat burning tends to occur, however, there are still the health benefits. Additionally, the rate at which fat oxidation occurs depends on the level of the individuals VO2 max. It has been well documented that training at higher intensity’s, above 75% VO2 max will decrease fat oxidation and favour carbohydrate oxidation, and once 50% and above of VO2 max has been achieved, muscle glycogen stores will become decreased, hence, fatigue may become more apparent (Gejl et al., 2017; Martin, 1996; Romijn, Coyle, Sidossis, Zhang, & Wolfe, 1995; Spriet, 2014; van Loon et al., 2001).
Achten and Jeukendrup, (2003) reported, that training at an average intensity of 63% of VO2 max, which corresponded to an average 73% heart rate max, was sufficient for maximum fat oxidation. This can also be supported with the research conducted by Romijn et al., (1995) with findings suggesting a VO2 max of 65% is sufficient for fat oxidation, and that an increased VO2 max from 65-85%> inhibited fat oxidation, consequently, switching to carbohydrate oxidation. Similar findings were reported by van Loon et al., (2001) with maximum fat oxidation occurring around 55% Vo2 max. Although high intensity training does use carbohydrate as its main fuel source, the increase in resting metabolic rate from an increase in calories burned is more effective at burning fat when compared to the length of time per training session (Schoenfeld & Dawes, 2009).
There is a lot of evidence to support the fact that training at a lower intensity can reduce fat stores, but the disadvantage to this is the increased training times of each session that these studies were performed at, with a minimum training time of ≥45 min (Fisher et al., 2015) and increasing >2 hours per session (Martin, 1996). On the other hand, high intensity training can be performed in less time, ≥30min for maximum benefits (King et al., 2002; Trapp et al., 2008).
To maximise fat burning through exercise the individual is required to work at an intensity for the required time to achieve the best results. Running at 65% VO2max for less than 30 min will not achieve optimal fat burning, whereas, high intensity interval sprints that are not performed at the minimum intensity ≥ 85% VO2 max will also not achieve optimal fat burning. For instance, if you can talk during some interval training, you are well below the minimum intensity for optimal training responses. One last note, exercise does not increase 24-hour fatty acid oxidation when energy balance is maintained i.e., calories consumed are not in a deficit. Both aerobic endurance training and anaerobic weight training do not appear to increase 24 hour fatty acid oxidation if energy intake increases to compensate for the energy expended in daily exercise, then daily fat oxidation will not increase, and fat balance will be maintained (Melanson et al., 2009). In other words, you cannot eat over your required calories if the goal of your training is fat loss.
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