Hi, Scott. I don’t know much about plant-based diets, so I can’t say for certain how well it would work with the combination of intermittent fasting and exercise. There are three reasons why I believe keto is ideally suited for the IF/exercise combo: insulin control, satiety, and abundant protein and energy.
Insulin is really the key to a healthy diet, and the best way to control it (along with IF) is to limit carbohydrates, which might be harder to do without eating meat, eggs, cheese, etc. Plant sources of protein usually also have plenty of carbohydrates (beans come to mind), which triggers a heightened insulin response. Fats do not trigger a significant insulin response, so going keto results in much lower amounts of insulin being released into the bloodstream. This allows the body to access its stores of body fat much better, as insulin causes fat to be stored in fat cells rather than be released. When the body can access its own fat stores, you feel less hungry and have more energy, both of which make it easier to combine intermittent fasting and exercise. I do a kettlebell weightlifting routine (not the short circuits, more like a barbell/dumbbell workout) in the morning after not having eaten for 14 hours. Because my insulin is low and my body is keto-adapted (uses fats rather than carbs for energy), I have no problem using body fat as fuel for 16 hours (sometimes 20 on my off days). If your body is still running primarily on carbs, higher insulin levels will interfere with the body’s ability to access fat stores for fuel. This is what makes keto ideal for fasting. Fasting isn’t actually about eating fewer calories. I eat as much as I always did. It’s about insulin. An excellent book on this is The Obesity Code by Dr. Jason Fung.
Another reason I believe keto is highly effective for fasting with exercise is that protein and fats sate hunger much more than carbohydrates. Carbohydrates are the only one of the three that doesn’t cause the body to produce satiety hormones. This is why I’m not hungry when I wake up in the morning like I used to be. Because my body relies on fat stores for fuel, those satiety hormones are always being produced. Probably the most striking difference I noticed when going keto was that my hunger was significantly decreased. I find it fairly easy to go for sixteen hours without eating whereas before I had trouble going twelve.
And for building muscle, keto is particularly well-suited. Because fats are not vilified by the diet, but rather become the primary (virtually only) source of fuel, eating eggs, cheese, steak, etc. are not limited. Those satiety hormones I mentioned will regulate your caloric intake. You literally will not have to think about it. Since starting the keto diet on July 4th this year, I have lost 35 pounds without restricting calories (there is an explanation of this in The Obesity Code as well as in articles here on Medium by Dr. Fung) while still being able not only to maintain but to build muscle mass. I’m currently 213 pounds at 6 feet tall, and I have about another ten pounds to lose to get to the point where I’m very lean. You get plenty of protein from the keto diet while also getting plenty of insulin-free energy from fats, both of which are much harder if one is eating only plants.
Two other books are worth reading: The Art and Science of Low Carbohydrate Living, and The Art and Science of Low Carbohydrate Performance, both by Jeff Volek and Stephen Phinney. Especially in Performance, they really lay out how keto is an excellent diet for athletes. The body has virtually unlimited amounts of energy if body fat is the primary source of fuel rather than carbohydrates. All of the glycogen in the average body provides about 2000 calories, while a mere five pounds of body fat provides 17,500 calories. And the keto diet makes fat stores more accessible than any other diet.