This is excellent, thank you! One thing I would love to see added to this research is the keto effect of just water over those 120 minutes, and THEN the keto effect of adding a tad bit of protein, such as collagen powder (I use the green tub great lakes powder; it dissolves great in coffee). I feel like that might put your test even more into an annals of science, and for us folks who don’t have the finger prick blood testers, it would be fun to know. Also, sugar free sweeteners like splenda (!!) or stevia–do they have any effect? Anyway–great read, exactly what I was looking for when I googled my question.


I’ll share some of my experiences, now doing heavy strength training for 3 years in a fasted state: For my first “fasted” workout or two after starting an IF protocol, it was very weird to not eat before training. However, after a few sessions, I learned that my body could certainly function (and even thrive) during my training sessions despite not eating a pre-workout meal.
To make “down” days easier to stick to, Johnson recommends opting for meal replacement shakes. They’re fortified with essential nutrients and you can sip them throughout the day rather than split into small meals. However, meal replacement shakes should only be used during the first two weeks of the diet — after that, you should start eating real food on “down” days. The next day, eat like normal. Rinse and repeat! (Note: If working out is part of your routine, you may find it harder to hit the gym on the lower calorie days. It may be smart to keep any workouts on these days on the tamer side, or save sweat sessions for your normal calorie days.)
Finally, note that fasting to induce ketosis can also trigger a set of symptoms often known as the “keto flu.” Fasting ketosis symptoms may include decreased energy levels, increased cravings, digestive issues, muscle aches and dizziness. These symptoms can last between a few days to a few weeks when starting the ketogenic diet, but they typically subside once your body enters ketosis and begins to adapt.
Make sure you still eat enough. Intermittent fasting does help you naturally eat less during the day, but be sure you’re still eating nutritious ketogenic foods to avoid any deficiencies or metabolic issues. Use a website or app to calculate ideal caloric intake and your ketogenic macros for each day, then track them to make sure you’re getting sufficient nutrition.
Those studies above, in working with small sample sizes, and different types of fasting than recommended here, would lead me to believe that fasting affects men and women differently, and that many of the weight loss benefits associated with intermittent fasting (that affect insulin and glucose responses) work positively for men and negatively for women.

Another program called the 5:2 Fast Diet involves eating 5 days a week and fasting for the other 2 days, when women can get no more than 500 calories and men no more than 600. That’s a quarter of the amount you likely eat on the days when you don’t fast. Whether you eat those calories in one sitting or spread them across micro-meals throughout the day is up to you.
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