I've been doing hardcore elimination diets for a year or two, experimenting.
I was never able to stick to ketosis because of the carb cravings. It took tremendous willpower and I could only do a couple of months without cheating.
Recently I decided to go back to ketosis. The carb cravings came roaring in after a few days. About to crack, I decided to try some scallops, to see whether they'd help. Boom - cravings gone, and stayed gone for several days (up to present).
Makes no sense. The ketosis carb cravings were supposed to be due to your body switching over to a new fuel source. So how can scallops eliminate them? Unless you're not really craving carbs at all, but instead some missing micronutrients.
Other things I've observed: 1. On a diet of meat, rice and fish, I have to eat scallops at least once per week or vitality begins to decline and food intake to increase. I assume this is for minerals. 2. Scallops cure muscle cramps during ketosis even when I exercise heavily (2-4 hour soccer games)
So this is all really pointing strongly towards a micronutrient answer, with the carb cravings during transition to ketosis really representing the body craving ?minerals? or something similar.
Background info: I've been doing elimination diet of just rice, water, ground beef, fish and scallops for some time. Recently I cut the rice and the ground beef, switched to solid beef and pork instead. The ground beef was too high fat, giving me mild stomach upset, and the rice slowed me down. I say "scallops" instead of "shellfish" because I find scallops the most palatable to eat plain, so all my data is from them.
EDIT: It appears nobody really knows the answer to the main question I was asking, about adaptation to ketosis and switching glucose sources as being the cause of supposedly unavoidable cravings. All three answers shed light on the mystery from different angles, which I appreciate, and all three will be useful to my life.
I'm choosing Melissa's for best answer because in the comments she stated that there wasn't any science directly addressing my question - which is enough to cause me to doubt the prevailing theory on the source of ketogenic carb cravings.
I do think micronutrients matter, particularly in ketosis. The iodine in the scallops can help support the thyroid, which is important since there is some evidence that low-carb can lead to low-thyroid issues (which may actually be more about low-iodine in the diet than the carb content). There are so many nutrients in shellfish though that it's hard to know though. Here is a chart from a book I have on amount of food you would have to eat to get the "brain selective nutrients":
well scallops and some seafood do have some carbs in them. They taste sweet. As do crab and shrimp to me (but without carbs). Perhaps the sweetness, carbs, minerals, and high tryptophan satisfy you? They certainly do for me, I much prefer simply cooked scallops without any sides than ground beef.
It could be that you're flushing out a lot of electrolytes with the water loss that goes with entering ketosis. The carb desire feeling could be your body trying to force you to hang onto those electrolytes. Scallops are really packed with minerals that you would be losing. http://nutritiondata.self.com/facts/finfish-and-shellfish-products/7741/2
Making sure you don't lose too many electrolytes is important in VLC. Your need for sodium goes up (!) in order to hold onto other minerals.
I missed your cramps mention earlier. That makes it even more likely that your problems were cause by electrolyte imbalance. You may want to add some supplemental sodium and magnesium.
Added: A good book for you (it's on my wish list) is The Art and Science of Low Carbohydrate Living: An Expert Guide to Making the Life-Saving Benefits of Carbohydrate Restriction Sustainable and Enjoyable http://www.amazon.com/gp/product/0983490708/ref=ox_sc_act_title_2?ie=UTF8&m=ATVPDKIKX0DER Several people here refer to it, and some of the text is viewable at amazon.
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