Author: Myron Yau, Nutritionist at HKBioTek
The recommended daily allowances (RDAs) of vitamin C for adult man and woman are 90 and 75mg respectively while the tolerable upper intake (UL) is 2000mg, at which the most common side effect is gastronintestinal disorders, like osmotic diarrhea and abdominal pain.
You may notice that there is a wide range between RDAs and UL, where the pharmaceutical dose of vitamin C supplements usually lies. Occasional ingestion of them with suggested dose shall not cause a big problem to your body. Since research shows that vitamin C absorption is saturable and dose-dependent, which means the degree of vitamin C absorption will be saturated at a certain level and excess vitamin C will just be excreted, toxicity is theoretically more likely when several large doses of the vitamin are ingested throughout the day than when the same amount is ingested as one single dose.
However, it may increase the risk of over-dosage together with dietary sources. In general, each serving of vitamin C supplements contains around 1000mg vitamin C. Together with citrus fruits, for example, an orange contains around 60mg vitamin C, around 120mg in kiwi, excessive ingestion of vitamin C may be resulted, especially for someone on diet who eat only fruits and vegetables to keep fit.
Prolonged overdose of vitamin C may increase the risk of kidney stone and iron toxicity in those with renal disease and disorders of iron metabolism respectively.
In conclusion, the correct logic is that immune system must be compromised if you have vitamin C deficiency, during which corresponding supplements may be very useful for compensation. At other times, occasional vitamin C supplementation may be better than usual supplementation which may lead to higher risk of some side effects.