High-intensity interval training (HIIT) is the most time-efficient way to get a workout. You already knew that, right? High-intensity interval training (HIIT) is a form of interval training, an exercise strategy that alternates periods of short intense anaerobic exercise with less-intense recovery periods. It’s one of the best approaches for improving aerobic and anaerobic fitness and boosting stamina. HIIT sessions may vary from 9–30 minutes. These short, intense workouts improve athletic performance but also enhance glucose metabolism and fat burning.
The goal of HIIT training is to boost your heart rate to at least 85% of maximum heart rate. Each intense interval is followed by a short recovery period to maintain a tolerable level of exertion. After a brief recovery period, you launch into the next active interval. A high-intensity workout can be as simple as alternating between 10 to 20 seconds at a near-maximal intensity balanced with 60 seconds of rest and keep repeating for 10 or 20 minutes.
There’s no doubt that HIIT is a challenging way to train and one that leads to significant fitness gains but what if you combine HIIT with time restricted eating? In case you’re not aware, time restricted eating is a broad term for any eating pattern that cycles between periods of fasting and non-fasting.
The Metabolic Benefits of Time-Restricted Eating
Time-restricted eating, a form of intermittent fasting, is an eating pattern in which you restrict your food intake to a specific time period. There are several different types of intermittent fasting:
- Alternate day fasting: alternate days of consuming no calories (fasting) with days of regular calorie intake
- 5:2 Diet: five days of normal eating and two days of consuming a quarter of your usual caloric intake (fasting)
- The 16/8 Diet: 16 hours of daily fasting (no calories) and 8 hours of non-fasting (eating normally)
The goal is to eat within a short window of time each day and not eat the rest of the time. For example, from noon to 8 p.m., you can eat anything you want if it fits within that timeframe. Anything outside that window is off-limits. The focus is on the timing of meals rather than the quality of those meals. That’s one downside to this approach. However, you can still choose nutrient-dense foods with time-restricted eating.
The most common form of time-restricted eating is referred to as 16:8 or 18:6 because those numbers represent the number of hours, you’re eating each day (16 hours if you’re doing 16:8 and 18 hours if you’re doing 18:6).
5:2 time-restricted eating is where two days each week (for example, Monday and Thursday) you restrict your calorie intake and eat normally during the other five days.
HIIT and Intermittent Fasting
Research shows both HIIT and time-restricted eating is beneficial for metabolic health – but what about the combo? In a new study published in the journal Cell Metabolism, researchers looked at the effects of combining HIIT training and intermittent fasting.
The study participants consisted of 131 obese women with risk factors for cardiovascular disease and type 2 diabetes. One group did time-restricted eating alone, another did HIIT alone, a third did a combination of HIIT and time-restricted eating, and a control group did neither.
For the time-restricted eating portion of the study, the participants consumed all their calories within 10 hours each day and fasted the rest of the time. HIIT training consisted of exercising at 90% of their maximum heart rate three times weekly for 35 minutes. The study lasted 7 weeks. During this time, researchers measured various metabolic parameters.
The results? The combination of HIIT and intermittent fasting was more effective than either intervention alone for reducing total body fat and visceral fat and improved longer-term blood sugar control more. This was confirmed by measuring hemoglobin A1C, a marker of longer-term blood glucose control. The fat loss results were two times greater than either HIIT or time-restricted eating alone.
Diet and Exercise Work in Synergy
As this study points out, HIIT is effective for improving fitness, metabolic health, and fat loss, but you can get even greater results if you combine high-intensity interval training with nutritional strategies and, as this study shows, time-restricted eating. There are some limitations to the study. The participants only included women and it occurred during COVID-19- lockdown, which could have changed the participants’ behaviors. The study was also only 7 weeks in length. So, they plan on doing a larger and longer study that includes men.
Still, it shows the potential benefits of combining high-intensity interval training with nutritional strategies, like time-restricted eating.
HIIT and Time-Restricted Eating: A Match Made in Heaven?
The idea behind time-restricted eating is eating at night disrupts your circadian rhythm (aka “body clock”). Your body naturally burns fat during daylight hours, so if you eat late at night, you’re disrupting this natural process.
Another theory behind time-restricted eating is that restricting your food intake to a certain number of hours per day forces you to eat more carefully. Instead of grazing all day long, you’re forced to finish all your meals within an eight-hour window. It creates more focus and awareness of what and when you eat.
Also, there’s the belief that we’re not wired to eat all day long — evolutionarily speaking, we’re designed to get our food during daylight hours and then hide from predators until morning comes around again.
HIIT is a time-efficient way to work out and one that works, but time-restricted eating could help you boost the results you get. If you try time-restricted eating, ensure you’re still consuming enough calories and making nutrient-dense food choices. There’s nothing healthy about eating junk food in a time-restricted manner. This study further shows that how you eat is as important as how you exercise and they’re synergistic.
- Haganes et al. Time-restricted eating and exercise training improve HbA1c and body composition in women with overweight/obesity: A randomized controlled trial. Cell Metabolism, 2022 DOI: 10.1016/j.cmet.2022.09.003.
- Queiroz JDN, Macedo RCO, Tinsley GM, Reischak-Oliveira A. Time-restricted eating and circadian rhythms: the biological clock is ticking. Crit Rev Food Sci Nutr. 2021;61(17):2863-2875. doi: 10.1080/10408398.2020.1789550. Epub 2020 Jul 14. PMID: 32662279.
- Boa Sorte Silva NC, Petrella AFM, Christopher N, Marriott CFS, Gill DP, Owen AM, Petrella RJ. The Benefits of High-Intensity Interval Training on Cognition and Blood Pressure in Older Adults With Hypertension and Subjective Cognitive Decline: Results From the Heart & Mind Study. Front Aging Neurosci. 2021 Apr 15;13:643809. doi: 10.3389/fnagi.2021.643809. PMID: 33935686; PMCID: PMC8082143.
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