Atkins acknowledged himself that the very strict and very low level of carbohydrate intake he suggested was to make damn sure that even the most carb-sensitive person in the world would get into ketosis easily and quickly.
In fact, any diet that allows you to consume less than 100g of carbohydrate a day is considered a ketotic diet (i.e., puts you into ketosis). There are quite a few low carb plans out there that have a daily limit of between 40g and 60g carb a day (I'm thinking Charles Clark, the first two weeks of South Beach, Protein Power, etc).
3 shakes of Cambridge gives you around 42g of carb per day - which if you think of it in calorie terms is only 42 x 4 = 168 calores per day.
But what really pushes you over the edge into ketosis here is the fact that 3 shakes only gives you around 450 calories a day full stop. That means that your body is going to have to provide the other 1,500 or so it needs to keep you ticking over - which it does first by burning your glycogen stores (around 2,000 calories worth in there) and when they're gone, it has to burn your excess fat (hurrah!).
The vast majority of people will easily slide into ketosis with a daily intake of 42g. Those who are hypersensitive to carbs might have to avoid some of the Cambridge bars - the chewy ones like chocolate, caramel, malt toffee and chocolate orange (because they're higher in carbs) but most people are okay with the chocolate mint, cranberry and peanut ones because they contain the same amount of carbs (more or less) as the shakes.
Hope this puts your mind at rest.
