California law does not place any restrictions on the practice of holistic nutrition. Only those that wish to be a registered dietitian must first obtain state certification.
There is a law that requires disclosure, and posting of a sign. The sign "shall not be smaller than 8 1/2 inches by 11 inches and shall be legibly printed with lettering no smaller than 1/2 inch in length, except the lettering of the word “NOTICE” shall not be smaller than 1 inch in length." An example of this sign that can be printed for your use can be found here.
While you may practice nutrition consultation as it pertains to food, nutrition intake, meal planning, grocery list and recipe creation and nutritional assessment, you may not:
“…practice medicine or to undertake the diagnosis, prevention, treatment, or cure of any disease, pain, deformity, injury, or physical or mental condition and specifically does not authorize any person other than one who is a licensed health practitioner to state that any product might cure any disease, disorder, or condition.” - California Code, Business and Professions Code - BPC § 2068
California's nutrition law can be found in the California Business and Professions Code, sections 2585, 2586, 2586.2, 2586.4, 2586.6, 2586.8, 2053.6, and 2068.