Fit and Lean in 4 Min / Week: 1kg Fat Loss, +9% VO2Max, +13% Fat Oxidation - Men Lose Trunk, Women Leg Fat

No excuse: You don't need an ex-pensive spinning bike for the workout.
This is not an article for the hardcore trainees among you... unless, obviously you are a trainer or have friends and family who fall into the same "I just wannabe fit and healthy" category as the subjects of a recent study by scientists from the Manchester Metropolitan University and the Cambridge University School of Clinical Medicine (Bagley. 2016),  24 men and 17 women with a mean age of 39 (±2) years, a normal weight (BMI 24.6 +/- 0.6) and average fitness levels.

In this group of "normal people", Bagley et al. aimed to examine the hypothesis that very short duration, very high-intensity sprinting exercise (on cycle ergometers) could not just improve their subjects fitness (as measured by VO2max), but also their ability to burn fat and to actually lose it.
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After baseline measurements on the DEXA scan (body fat and lean mass) and cycle ergometers (VO2max), the participants were told to do only one thing: A sprint-interval training (SIT) program on a standard cycle ergometer.
"The training consisted of a 2 min warm-up at a self-selected moderate intensity. This was followed by four bouts of 20 s ‘maximal effort’ sprints at a workload that was set at 175% of the workload attained in the VO2max test. Each of these intervals was separated by 2 min of very low intensity cycling (a workload of approximately 20% of that attained at VO2max). Thus, each training session lasted less than 10 min and only 80 s was completed at an inten sity that would be expected to improve physical fitness" (Bagley. 2016). 
The first training session for each participant, who were told to maintain their their usual dietary and exercise habits throughout the intervention, was fully supervised in the research laboratory. To ensure that the subjects would indeed do their three weekly workouts 80s-workouts, the participants were then provided with clear instructions on the use of the cycle ergometers and the training regimen.
But you said "fit in 4 minutes", now the subjects train for almost 10 minutes? Yes and no. They train for 20 minutes, but the actual "exercise", which is something I define as being significantly exerted is 80s per workout. With three workouts per week, that's 3x80s = 240s = 4 minutes! So, I don't want to hear complaints ;-)
The training work load was increased by 5% every 2 weeks. Gym staff were fully informed of the research and training protocols, they logged the training session and were available to offer advice to research participants if needed during training sessions. Participants maintained a training-log to record workloads during training sessions.
Figure 1: Maximal oxygen uptake and rates of fat oxidation measured during exercise in men and women before and after 12 weeks of SIT; all changes were stat. sign. p < 0.05 (Bagley. 2016).
As you can see in Figure 1, the effects these short, highly time-efficient, and absolutely manageable (everyone can workout at max intensity for 4x20s) had on the subjects' fitness were not just statistically significant, they were also practically relevant and, at least for VO2max, differed significantly for men and women.
But how did they lose weight without dieting? The secret is the proven lack of compensation for SIT sessions, which have been show to be as low as <50kcal/week - compared with endurance exercise where compensation is 10x higher, i.e. 500 kcal/week (Burgomaster. 2008). Still, the direct energy expenditure during the short SIT sessions cannot fully explain the fat loss. Therefore, Bageley et al. speculate that "[o]ther contributing factors might include an increase in post exercise energy expenditure [that's unlikely, learn why] or overall shift towards greater fatty acid oxidation during habitual activities throughout the day" (Bagley. 2016).
Overall, the increase in VO2max averages out at 9% - the reasons for the sex-differences is not clear. After all, the scientists point out that men have been shown to have higher gains in VO2max following conventional endurance exercise. The mixed results of previous studies into the effects of sprint interval training, however, are mixed and thus not necessarily contrary to the evidence from the study at hand. While Scalzo et al. (2014), for example, found that young women had similar gains in VO2max as young men, the results Allemeier et al. (1994) et al. presented in the Journal of Applied Physiology suggest that men don't see any increase in VO2max. What could be the reason? Well, this is what the scientists say:
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"A higher relative amount of lean mass in men compared to women, coupled with a higher relative body fat mass in women compared to men, may go some way in explaining the differences between men and women in maximal oxygen consumption. However, the supply of oxygen to the working skeletal muscles is thought to be a limiting factor in VO2max, so the higher VO2max response in women might point to higher adaptations of oxygen supply than those in men following SIT, but more focused studies examining cardiac output, blood volume, haematocrit and blood flow distribution are needed to clarify this finding.

Conversely, after regular endurance training, men had higher gains in VO2max compared with women. It is possible that the training volume (higher in endurance) and training intensity (higher in SIT) lead to disparate adaptations between men and women in the oxygen carrying capacity of blood (eg, total blood volume, haemoglobin or cardiac output) or local vasculature, but physiological mechanisms driving such responses are unclear" (Bagley. 2016).
No sex differences were observed for the measured health markers, namely glucose, insulin, HOMA, triglycerides, total cholesterol or LDL - only for HDL there was a significantly more pronounced increase in the female vs. male subjects. Eventually, the improvement of the total cholesterol to HDL ratio was yet similarly pronounced in both sexes (-16% in the men, -11% in the women).
Figure 2: Body composition before and after 12 weeks of SIT; * after the categories denotes p < 0.05 (Bagley. 2016).
The previously discussed changes were accompanied by a significant loss of total, leg and trunk fat, as well as significant increases in lean mass in both groups - with inter-sex-differences in total body mass, body fat %, leg fat, and lean mass. That's quite a result, if you take into account the total and actual exercise time the subjects had to invest.
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Bottom line: I am not sure how feasible this protocol would be for an obese person, but in the healthy normal-weight subjects in the study at hand, the 12x4 minutes of working out intensely made quite a change. Ok, you have to work out thrice a week, but 10 minutes on an exercise bike? That's something you could easily do every morning before showering or when you come home from work.

Ah, and before I forget to highlight that - even though the fat loss in the female subjects may have been smaller than in the male subjects, the women lost fat where many of them hate it the most: on their legs - not bad!? Comment!
  • Allemeier, CRAIG A., et al. "Effects of sprint cycle training on human skeletal muscle." Journal of Applied Physiology 77.5 (1994): 2385-2390.
  • Bagley, Liam, et al. "Sex differences in the effects of 12 weeks sprint interval training on body fat mass and the rates of fatty acid oxidation and VO2max during exercise." BMJ Open Sport & Exercise Medicine 2.1 (2016): e000056.
  • Burgomaster, Kirsten A., et al. "Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans." The Journal of physiology 586.1 (2008): 151-160.
  • Scalzo, Rebecca L., et al. "Greater muscle protein synthesis and mitochondrial biogenesis in males compared with females during sprint interval training." The FASEB Journal 28.6 (2014): 2705-2714.
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