By Kevin Cheng
From the almighty Chris Hemsworth to the godfather of modern day bodybuilding, Arnold Schwarzenegger, intermittent fasting has long been adopted and practiced by celebrities and athletes around the world, claiming its unparalleled effects on one’s physique, strength, performance, and mood.
With its ever increasing popularity and endorsement by various trainers and athletes, I have often been asked by clients about whether or not I recommend it. I decided to make a post about it to guide the fitness community through the basics of IF, discuss the pros and cons, and to keep you more informed to make a decision for yourself. Before we start, it is imperative to acknowledge that intermittent fasting is not a diet, but rather a pattern of eating. It is also not a magic bullet to getting jacked or losing weight, but a dietary approach that requires discipline, commitment, and careful planning.
Our bodies can generally be in two states: fed or fasted. After you eat a meal, your body is in a fed state where it carries out functions of digestion and absorption, normally lasting around 4 hours. During this state, insulin (i.e. the hormone released to stimulate muscle and fat cell’s uptake of glucose) is high and minimal fat is burned. Your body does not enter a fasted state until roughly 12 hours after the meal, at which point insulin levels are low and fat burning becomes much easier. The gist of IF is to lead your body to the fasted state to maximize fat burned and physique goals. Insulin sensitivity, which is your cells’ responsiveness to this hormone that starts the cascade of glucose storage, has also been cited by multiple studies to improve with IF. For reference, insulin insensitivity is what leads to type II diabetes and can also contribute to weight gain and coronary heart disease.
The BDNF (brain-derived neurotrophic factor) has been demonstrated to be boosted with IF. The BDNF’s role in your body includes its neuroprotective effect, mood elevation, suppression of energy intake, and reduction of body weight. Other self-explanatory benefits of intermittent fasting include detoxification of the digestive system, the possibility of eating less food and consuming less calories, and the potential to save money and time preparing food.
What IF Model Is Best For You?
A) 16/8 Model (Time restricted feeding)
16-hour fast combined with 8 hours of eating (it does not matter when the 8 hour window occurs).
- Claimed benefits: good for losing weight, controlling appetite, and learning to eat within the right timeframe.
- Possible challenges: unsure if it is a good way to preserve lean body mass and increase muscle gain.
- Research: In conjunction with resistance training it could improve health, decrease fat mass, and maintain muscle mass (Moro et al., 2016). Research on TRF is limited and clear conclusions cannot be made at present (Tinsley & La Bounty, 2015). TRF produced weight loss results in obese women, but could be a direct outcome of calorie restriction (Gabel et al., 2018). TRF was established to prevent and treat metabolic disorders (Chaix et al., 2014).
B) Weekly Fasting Model
Dedicating one or two non-consecutive 24 hour timeframes of fasting per week.
- Claimed benefits: good for weight maintenance, lowering blood lipids, and detox for gut health.
- Possible challenges: unsure if it is good for weight loss or muscle gain.
- Research: IF was effective for weight loss and cardio-protection in obese women (Klempel et al., 2012). IF was as effective as calorie restriction for weight loss and insulin sensitivity improvement for overweight women (Harvie et al., 2010). IF is a viable diet option for obesity individuals who wish to lose weight (Varady et al., 2009).
C) Alternate Day Model:
Eating normally for one day, and limiting the next day to up to 20h of fasting.
- Claimed benefits: good for weight loss, reduction of heart disease and metabolic disorder risks
- Possible challenges: short eating window makes it difficult to incorporate into a training routine
- Research: IF does not help with weight loss any more than daily calorie restriction in obese individuals (Trepanowski et al., 2017). IF effectively promotes healthy weight loss in obese, overweight, and normal weight adults, but sample size is too small to be conclusive (Varady et al., 2013). ADF is safe and tolerable but not necessarily more effective than calorie restriction when it comes to weight loss (Catenacci et al., 2017).
You may have probably already seen the pattern. Many of the studies were “inconclusive”, “lacks evidence”, and “requires further investigation”. I purposely cited fairly recent articles to emphasize my point, name that to this day no consensus has been reached regarding the effectiveness of IF. Furthermore, it would be an understatement to say that in a non-controlled and non-experimental environment, such as the gym, IF is practiced very differently (in terms of models, type of food consumed, and calorie intake) and gym-goers’ bodies potentially react considerably variably with IF.
IF also has practical concerns, including the challenges to maintaining a social life and difficulty keeping up with such a strict eating schedule. Finally, and unsurprisingly, not everyone is able to starve themselves for hours before eating. I personally get very cranky and irritable when I’m starving!
The Bottom Line
Although studies do not yet all agree on the benefits of IF on your health and fitness, IF has not been shown to have adverse effects on our body. For the general population whose primary goal is to lose weight, gain muscles, improve physique, and feel better, sticking with our conventional eating habits of 3-5 nutritious meals a day and training appropriate and frequently are more than adequate. However, if you are curious about IF and want to give it a try, there is no harm. For clinical or athletic populations, IF might be more relevant. Last but not least, if you are unsure whether or not intermittent fasting is for you due to previous conditions or other health concerns, it’s always a good idea to consult a medical professional.
Kevin Cheng is a rehabilitation specialist, personal fitness trainer, biomechanics researcher, and physician-in-training. He specializes in sports and car accident injury rehabilitation, physique and figure training, and powerlifting. He is currently running a volunteer service, “Access Fitness”, to provide by-donation fitness consultation services and program design for the general public in the hopes of gathering funds for fitness equipment for children in rural areas. For more information, please visit accessfitness.org or contact him at firstname.lastname@example.org.
Catenacci VA, Pan Z, Ostendorf D, et al. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (Silver Spring). 2016;24(9):1874-83.
Chaix A, Zarrinpar A, Miu P, Panda S. Time-restricted feeding is a preventative and therapeutic intervention against diverse nutritional challenges. Cell Metab. 2014;20(6):991-1005.
Gabel K, Hoddy KK, Haggerty N, et al. Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutr Healthy Aging. 2018;4(4):345-353.
Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011;35(5):714-27.
Klempel MC, Kroeger CM, Bhutani S, Trepanowski JF, Varady KA. Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutr J. 2012;11:98.
Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290.
Tinsley GM, La bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015;73(10):661-74.
Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938.
Varady KA, Bhutani S, Church EC, Klempel MC. Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. Am J Clin Nutr. 2009;90(5):1138-43.