The brain doesn't need carbs in addition to ketones. What it needs is a small amount of glucose in addition to ketones. The body can make this glucose without any carbs being consumed at all. It breaks down either fat or protein to do this. (In cases of total starvation, it breaks down its own fat and muscle. What we are trying to do with the Atkins diet is to encourage our bodies to break down their own fat, but to use the protein we eat instead of breaking down our muscles.)

If someone consumed more protein than was needed for tissue repair, the body would break down the "extra" protein into glucose and the brain would prefer that glucose, instead of ketones, for fuel. If enough excess protein was eaten, the person would fall out of ketosis. But keep in mind that it has to be quite a bit of protein.