We have a full guide to low carb for vegans and vegetarians here.
Achieving nutritional ketosis while being a vegan is possible but this style of eating will require a large amount of pre-planning. However, outsiders to both a vegan and ketogenic diet claim these diets are “too restrictive” for the “normal” person to maintain so this could be a perfect match for a creative vegan!
There are plenty of plant-based fat sources but in order to meet your protein needs you will need to incorporate high protein, vegan friendly foods such as nuts and seeds, tempeh, tofu, and vegan protein powder. Food companies such as Beyond Meat are now providing alternative meat products that are a great source of protein while still relatively low in carb. Unlike meat, some of these vegetable based proteins do not contain complete proteins but if you are good about eating a variety of plant-based foods you can meet all of your essential amino acids needs.
A challenge that you may face is keeping your carbohydrates below 30 grams per day because by nature, these plant-based protein products contain 2–10 grams of carb per serving. But these products are also higher in dietary fiber (aka low-digestible carbs) which has a reduced effect on blood glucose levels (Grabitske, 2008). This may give you a little more flexibility with the number of carbs you can eat while still maintaining ketosis (e.g. up to 50 grams per day), but this will require tracking to be sure.
Dr. Stephen Phinney on plant-based low carb protein sources
Takeaway: It’s possible to be vegan and do nutritional ketosis but you will need to do some research, planning/calculating, and self-experimentation to make sure you are meeting all of your requirements. You may also want to include a multivitamin containing vitamin B12 and a vegetarian omega-3 supplement (such as flaxseed oil or algal DHA such as ‘Neuromins’) to ensure you are meeting all of your nutrient needs!
Grabitske HA, Slavin JL. Low-Digestible Carbohydrates in Practice. JAMA. 2008; 108(10):1677-1681.