Updated: Oct 20, 2021
I have wanted to do a deep dive into the research about the difference between these different diet strategies for some time. I will define the three strategies and then go through some studies covering how keto and intermittent fasting compares to traditional calorie restriction for weight loss, some drawbacks to each, and how they could apply to real-life coaching strategies.
Before we get into it any further, I want to admit my own confirmation bias in this area. I have been interested in nutrition science for the better part of a decade and spent countless hours studying what the leaders in nutrition coaching say about these dietary strategies. The consensus (including my own) is that these strategies only increase compliance to a caloric deficit. There is nothing inherently magic about the results that happen in any case study other than finding a way for the subject to lower their caloric intake in a consistent a repeatable manner. Due to my beliefs regarding the subject-at-hand, I found myself cherry-picking papers to cite and made conscious efforts to find well-designed studies with various relevant findings. What resulted was a new perspective to increasing buy-in and outcomes for my clients because, in the end, delivering results through education and empathy is where lifelong success is.
Explanation Of Terms
To start with a short explanation of the three diet strategies.
The ketogenic diet is a diet strategy that is extremely low in carbs characterized by elevated production of circulating ketones. The claim is by placing your body in nutritional ketosis - you will find yourself feeling less hungry and will experience a higher percentage of your weight loss from adipose tissue. Keto also claims more significant improvements in other health markers versus its competitors.
Intermetinant fasting or IF is characterized by set periods of fasting and set windows of time where you are allowed to eat ad libitum. Various guidelines exist for fasting windows and how strict of fast is to adhere. Strategies range alternating days to two days per week, to 16 hr fasts every day. The amount of restriction also various from water only to small meals during fasting periods.
Claims range from better lean body mass retention to feelings of reduced hunger than traditional calorie restriction.
The third style of dieting is a continuous energy-restricted diet. Continuous energy restriction - referred to as macros, IIFYM, diet templates, or pre-made meal-type programs. This genre of dieting allows for flexibility in day-to-day eating but requires more work from the dieter. Either in the form of meal planning and measuring or by buying expensive, pre-made meals. This style of dieting has the added benefit of data accumulation and seamlessly transition between different phases of nutritional periodization.
What does the research say?
A systematic review of intermittent fasting shows promise as a treatment for obesity with findings of increased fat loss, less weight regains, and improvements in lean body mass . The majority of the studies that were cited calorie intakes were self-reported. Self-reporting of calories is notoriously incorrect, Lichtman SW, Pisarska K, Berman ER, et al found self-reporting to be nearly 50% higher than actual consumption . This could be the explanation for why we see better outcomes of ad libitum IF than with CR. There is less opportunity to eat and incorrectly calculate calories. A 2017 pilot study shows similar weight loss between calorie restriction and IF but with more favorable fat loss outcomes in the IF group .
In 2016, Tatiana Moro, et al found that in trained individuals, there was a higher amount of body fat loss using IF and similar retention of lean body mass compared to habitual eating . An interesting point in this study was this study did not create a calorie deficit in either group. It aimed to compare the timing of meals and based calorie targets on a self-reported isocaloric state. Promising as this study may seem, it was a small sample size (n=34) and only lasted eight weeks.
One of the more notable drawbacks to long periods of fasting is the reduced number of potential spikes in muscle protein synthesis that occurs with sufficient leucine intake. This phenomenon is capped regardless of an increase of ingested protein and has a cooling-off period of two to three hours where no further increase in MPS for several hours. When cramming your food into a small window of time in a day or when not eating all day, you reduce the potential for spurring distinct periods of MPS.  
The keto diet has become quite popular in recent years. There are many claims of varying health benefits to the keto diet, but the aim is to look at weight loss compared to a traditional balanced macronutrient diet. I will comment on health markers and all-cause mortality a bit at the end.
While there is a lot of research on keto that creates some eye-catching abstracts, it is important to note that there is more to a paper than only the abstract. For example, Cunha GM, et al found "significantly more weight loss in a ketogenic diet versus a standard low-calorie diet'. What they don't mention in their abstract is that the "low-calorie diet" the subjects were eating around 1800k/cal per day while the ketogenic subjects were INSTRUCTED to eat only ~800 k/cal per day .
In studies with similar and more controlled methodology, there is conflicting fat loss efficacy data between keto diets and low-fat diets in free-living situations with comparable caloric deficits. Encouraging and noteworthy, but these studies also show repeated findings of higher rates of lean mass loss even with higher protein consumption in the ketogenic subjects  . Particularly concerning to those who train with any performance or net muscle gain metric in mind. Coupled with the lack of readily available glucose for anaerobic creation of ATP for bouts of high-intensity work reduces outcomes that would be important for long-term health, function, and muscle growth and retention.
A quick note on ketos and glucose, while many body functions can use ketones, this is not true for all processes. Our brains, nervous system, and red blood cells need glucose to function, and the only way cells produce ATP in an anaerobic environment is glucose. Our bodies need about 100g of glucose per day, our liver produces about 120g per day using amino acids the leaves very little glucose for training and consumes amino acids that are otherwise used for tissue remodeling.
Conclusions and Real-World Applications
In clinical settings, there is no difference in weight loss between the three diets. But this is the crux of the argument. In free-living situations, there is some compelling evidence that shows better outcomes than a static caloric restriction. While there are many factors as to why this may be the case, things like hormone signaling, more satiating foods, or being bound by time or food group limitations, the result is more compliance to a caloric deficit and usually a larger one at that. And this matters... A lot.
Obesity and the other symptoms of metabolic syndrome are a very big deal and affect over 34% of the U.S. population . All markers of metabolic syndrome improve with weight loss and with it, an improvement in quality of life.
It is my opinion that a high protein, balanced fat/carbohydrate macros assignment is a more correct way to manipulate body weight versus ad libitum eating in either intermittent fasting or a ketogenic diet. Tracking calories ensures adequate protein, fat, and fiber, sufficient glucose for fueling high-intensity activity, builds better athlete data, allows for dietary flexibility and a wider variety of micronutrients, and allows for dietary periodization.
However, my goal is to improve the quality of life of the people I work with and, to help them with their goals, it would be a disservice not to use any tool available to do so. Even though I may believe one method is optimal, this does not always translate into better outcomes. Not everyone has the desire or time to prep food, weigh food, or enter food into a diary and, that is okay. A less than optimal approach with results is better than the optimal design and no compliance.
Being less rigid with the approach while continuing to educate will ultimately lead to an understanding between coach and athlete. This understanding increases buy-in and allows the eventual implementation of more direct and significant dietary protocols to keep weight off for a lifetime.
 Welton S, Minty R, O'Driscoll T, et al. Intermittent fasting and weight loss: Systematic review. Can Fam Physician. 2020;66(2):117-125.
 Lichtman SW, Pisarska K, Berman ER, et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. N Engl J Med. 1992;327(27):1893-1898. doi:10.1056/NEJM199212313272701
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):1874-1883. doi:10.1002/oby.21581
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. Published 2016 Oct 13. doi:10.1186/s12967-016-1044-0
Areta JL, Burke LM, Ross ML, et al. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. J Physiol. 2013;591(9):2319-2331. doi:10.1113/jphysiol.2012.244897
Stokes T, Hector AJ, Morton RW, McGlory C, Phillips SM. Recent Perspectives Regarding the Role of Dietary Protein for the Promotion of Muscle Hypertrophy with Resistance Exercise Training. Nutrients. 2018;10(2):180. Published 2018 Feb 7. doi:10.3390/nu10020180
Cunha GM, Guzman G, Correa De Mello LL, et al. Efficacy of a 2-Month Very Low-Calorie Ketogenic Diet (VLCKD) Compared to a Standard Low-Calorie Diet in Reducing Visceral and Liver Fat Accumulation in Patients With Obesity. Front Endocrinol (Lausanne). 2020;11:607. Published 2020 Sep 14. doi:10.3389/fendo.2020.00607
Volek J, Sharman M, Gómez A, et al. Comparison of energy-restricted very low-carbohydrate and low-fat diets on weight loss and body composition in overweight men and women. Nutr Metab (Lond). 2004;1(1):13. Published 2004 Nov 8. doi:10.1186/1743-7075-1-13
Kelly A. Meckling, Caitriona O’Sullivan, Dayna Saari, Comparison of a Low-Fat Diet to a Low-Carbohydrate Diet on Weight Loss, Body Composition, and Risk Factors for Diabetes and Cardiovascular Disease in Free-Living, Overweight Men and Women, The Journal of Clinical Endocrinology & Metabolism, Volume 89, Issue 6, 1 June 2004, Pages 2717–2723,