Fears over a vitamin c deficiency is one reason that people worry about when embarking on a ketogenic diet. However, it has been shown in several scientific studies that this fear is unfounded.
Here's why: First, many fresh, low carb vegetables such as peppers and dark leafy greens like kale and broccoli have loads of vitamin C. Plus, eating less carbohydrate results in needing less vitamin C. This is because glucose competes with Vitamin C for access to the same metabolic pathways in the body (Reference here).
So if your carbohydrate intake is high, you will have to increase your intake of vitamin C containing foods or natural supplements to get enough vitamin C to overcome the high blood sugar. It's only when eating the standard American diet which is high in carbohydrates and grain consumption that vitamin C needs are higher. Lowering your carb intake lowers the need to supplement with Vitamin C.
Because animals are able to make vitamin C internally, their flesh contains it. If you eat no carbohydrate at all, you can get enough vitamin C from lightly cooked meat and fat alone.
Don't believe it? Read this:
While studying the Inuit people in Alaska, anthropologist Vilhjalmur Stefansson documented the fact that the Inuit diet consisted of about 90% meat and fish. During his time there, he followed their custom, and he and the entire tribe would eat nothing but meat and fish for 6-9 months of each year. This was essentially a zero carb ketogenic diet. Stefansson survived on this ketogenic diet for 9 years while living with the Eskimo. When he returned to city life and described his experiences, doctors were amazed that his health had not suffered.
Stefansson himself wondered if his health had suffered during those years, so he agreed to an experimental study. He would live at the Bellevue hospital in New York City and eat nothing but fat and meat for an entire year. The doctors involved with this study came from Harvard, Cornell and other prestigious organizations, and they were convinced that he and another volunteer, Dr. Karsten Anderson, would develop health problems or at least vitamin deficiencies.
The volunteer's food intake was kept under close scientific scrutiny, so cheating was out of the question. The food that they ate was analyzed and the end of the study, the daily totals were averaged and noted:
At the end of the experiment, Stefansson and Anderson remained in perfect health - no vitamin deficiencies or serious health issues occurred. The results of the study were published by the Journal of Biological Chemistry in 1930. (Download from this site).
It's kind of ironic.. eating fruits and vegetables increases your carbohydrate intake, which increases the danger of a vitamin C deficiency. Good thing that fruits and vegetables have vitamin C included.
There's a great post here by the people at Break Nutrition on vitamin C needs for those on a ketogenic diet.