What is a proper diet to go on with lithium?

There are no specific dietary requirements while taking Lithium. Generally you can eat what you like. However Lithium requires consistent monitoring to ensure you maintain the right balance of lithium in the blood to avoid the dangerous condition of lithium toxicity. Dietary changes, particularly those containing caffeine and salt, may affect lithium levels and increase your risk of developing lithium toxicity.

Salt consumption can cause fluctuations in serum lithium levels. While taking lithium, do not make sudden changes to your salt intake. A sudden decrease in sodium intake (a component of salt) may result in higher serum lithium levels, while a sudden increase in sodium might prompt your lithium levels to fall.

Caffeine might interact with serum lithium levels. You should maintain your caffeine levels at approximately the same level, as sudden changes in caffeine intake might cause a fluctuation in lithium levels. A drop in caffeine levels might lead to a corresponding increase in lithium levels. Conversely, a sudden increase in caffeine consumption may decrease your lithium levels.

Drink plenty of fluids, particularly during periods of prolonged or intense exercise. Dehydration may lead to high lithium levels, leaving you at risk for lithium toxicity.

To minimize your chances of gastrointestinal side effects, take lithium with food or milk. https://www.drugs.com/cg/lithium-toxicity.html