Of course, we know that an all-or-nothing approach isn't always warranted or sustainable—and that there are still plenty of benefits to be had in the territory that lies between a strict water-only fast and eating a full-blown meal.
Some foods maintain many of fasting's benefits—while keeping you sane. So, many experts say it's OK to consume certain caloric foods in small quantities to help you stick to an intermittent fasting plan.
Case in point: To help patients stay happy and compliant with their fast, integrative physician Amy Shah, M.D., allows the consumption of 30 to 40 calories from sources that won't spike blood sugar, like coffee or tea with a splash of unsweetened almond milk. "I call this 'dirty fasting,'" says Shah. "For me, I like to fast as long as possible with just water—that might be 13 or 14 hours of a water fast—then I'll have my tea with almond milk (the start of my 'dirty fast') and end at around 16 hours."
Even Horne agrees that some perks of fasting remain with minimal food intake: "One mechanism that is known to remain active when a small amount of food is consumed is ketosis—as long as you consume less than 50 grams of carbohydrates [in a day]," he says. "Some effects, such as the impact of fasting on the gut microbiome, may be different depending on whether it's a water-only fast or a very-low-calorie diet with a small amount of food consumed, but both may provide some level of benefit when compared to eating a standard amount of food."
Integrative dietitian Ali Miller, R.D., appreciates a more flexible approach as well. "A fast is broken with consumption of food or a caloric substance; however, many people who enjoy the benefits of fasting and want to incorporate it as a daily ritual may take a more flexible approach such as a 'fat fast' using coconut oil, MCT oil, grass-fed butter, or cacao butter blended into a warm liquid during their fasted window."
Of course, not everything is on the table. Here, our experts elaborate on a few items that technically have calories (and technically break a fast) but still deliver on many of intermittent fasting's perks: