Popcorn would definitely kick you out of ketosis. It is high carb. Almost pure starch.

That said a person needs to realize that there is a difference between a low carb diet and a ketogenic (keto) diet. A keto diet IS a low carb diet but a low carb diet is not necessarily a keto diet. A person could potentially have some popcorn on a low carb diet. A lot of people just refer to a low carb diet being a keto diet but it is not necessarily so. A keto diet is a VERY low carb diet. Generally under 50 grams a day carbs for people metabolically healthy and for over weight people it likely takes 20 grams or below to reach nutritional ketosis where the body becomes fat adapted and creates considerable ketones, utilizing them for energy as an alternative to glucose.

If a person follows the food pyramid and avoids lots of sugar and sweets a standard American diet would be around 300 grams carbs a day if they ate all the vegetables recommended. Could be 500 or more if a person drinks lots of sodas or sports drinks and eats candy bars and such.

So a keto diet is VERY low carb. For a person just wanting to maintain weight (not lose a lot) anything under 120 grams carbs a day is considered to be low carb. Some of the studies that talk about low carb use higher numbers like 250 but that is not really low carb at all.

Bottom line, you could probably have some popcorn on a 100 gram low carb diet if you limited other carbs for that day. But on a 20 gram keto diet it would be such a small amount of popcorn it would be just something to tempt you to over eat carbs.

So it kind of depends on your needs and goals. If you are just trying to maintain your weight and gain the health benefits of low carb eating, occasional popcorn could be part of those carb limitations. If a person wants to gain the additional health benefits of being in nutritional ketosis or wants to lose weight, best skip it.

I know most people turn up their nose at pork rinds, but that is what my wife and I went to for something crunchy to replace popcorn. Some brands are better than others so if you do not like the first ones, try another different brand. Pork rinds are mostly fat and protein with very little carbs.

John