Am I getting enough zinc and vitamin C? How can I tell?

Whether you get enough zinc and vitamin C is something you start to judge from your usual meals. Compare what you eat on ordinary days with the Thai Reference Daily Intakes (Thai RDIs): 11 mg of zinc and 100 mg of vitamin C a day. Blood tests have limits, especially for zinc. Zinc comes mainly from seafood and meat, and vitamin C from fruit and vegetables. If you want a proper assessment, ask a doctor or dietitian to look at your diet.

How can I tell whether I get enough zinc and vitamin C?

The most practical place to start is what you actually eat.

Write down a few ordinary days

Note everything you eat and drink for two or three ordinary days, including meals and drinks bought outside, rice and the main ingredients of each dish. Then compare the foods with the tables in this article. A rough record already shows whether seafood, meat and eggs appear most days, and how often you eat fruit and vegetables.

Compare with the Thai RDIs

The Thai RDIs are reference values for a rough comparison. Falling below the Thai RDI does not by itself mean you are not getting enough.

The Thai RDIs are single values for the general population aged 3 and over. The UK's NHS says people should be able to get all the zinc they need from their daily diet, and the NHS says the same for vitamin C, adding that vitamin C cannot be stored in the body, so it needs to be in the diet every day.

Food first

The ODS consumer fact sheets on zinc and vitamin C say most people in the United States get enough of both from what they eat and drink, and that people should get most of their nutrients from food and beverages.

Ask for a proper assessment

A doctor or dietitian can look at your food record, ask about your eating habits and decide whether anything else is worth checking.

How much zinc and vitamin C do Thai people get from food?

Thai researchers have measured what people eat in some areas. The figures describe groups, not any one person.

Studies at a glance

*The 2014 study's food database had zinc values for only about half of the foods, so the zinc figures count only part of what people ate.

Older adults in Chiang Mai

A 2025 study by Sathittrakun and colleagues at Chiang Mai University looked at the diets of 200 people aged 60 and over in San Pa Tong district. The median intake was 5.99 mg of zinc and 42.14 mg of vitamin C a day. The authors note that a single 24-hour recall may not capture day-to-day or seasonal changes in diet.

Office workers in Bangkok

A 2014 study from the Faculty of Public Health, Mahidol University, studied 215 office workers at four randomly chosen workplaces in Bangkok. Median vitamin C intake was 64 mg a day in men and 68 mg in women. The authors say the main limitation was under-reporting, a known problem with recalls and food records.

A 2018 study from the Faculty of Medical Technology, Mahidol University, divided 250 Bangkok workers into five groups by lifestyle, such as outdoor work and smoking. Median vitamin C intake in the groups ranged from 7 to 27 mg a day, based on one 24-hour recall, and the main sources were fruit and vegetables such as oranges, guava, papaya, tomatoes and cabbage.

Adults in Bangkok: a 1998 study

An earlier study from Ramathibodi Hospital used 3-day food records from 231 adults in Bangkok and nearby districts. Average zinc intake was 6.3 mg a day in men and 5.5 mg in women, and meat, fish, eggs and milk together supplied 42% of the zinc in their diets.

What these studies can and cannot tell you

Each study covered one area, and all relied on what people remembered or wrote down. They show patterns in groups and how much results depend on the method. They do not tell you your own intake; your own food record is the place to start.

What can a blood test show, and what can't it?

Zinc

The US NIH Office of Dietary Supplements (ODS) says serum or plasma zinc is what is typically measured, but both measures have important limitations. Levels vary with sex, age and whether the blood is taken in the morning or the evening, and they do not always match how much zinc a person eats. The ODS says doctors also consider factors such as eating too little.

Vitamin C

The ODS says the body tightly controls the vitamin C in its tissues and plasma, and that status is usually assessed by measuring plasma vitamin C, because other measures are harder to assess and less reliable.

Intake and blood levels do not always line up

In the 2018 Bangkok study, vitamin C intake and blood vitamin C were linked in only one of the five groups.

What this means in practice

A blood test is something a doctor decides on with you. For most people, the more useful first step is the food record.

Which foods are high in zinc and vitamin C?

Zinc

The ODS says oysters contain more zinc per serving than any other food, and that fruits and vegetables contain very little. The right-hand column compares each serving with the Thai RDI of 11 mg.

Vitamin C

The ODS says fruits and vegetables are the best sources of vitamin C. The right-hand column compares each serving with the Thai RDI of 100 mg.

Both tables come from the ODS, based on US Department of Agriculture data. Varieties and cooking in Thailand may give somewhat different amounts.

Cooking and vitamin C

The ODS says vitamin C dissolves in water and is destroyed by heat, so long storage and cooking can reduce it, and steaming or microwaving may lessen the loss. Many good sources, such as fruit, are usually eaten raw.

What might a day of meals look like?

Zinc and vitamin C add up across the day from different foods. The example below uses foods from the tables above.

Points to keep in mind

• Zinc comes mostly from animal foods. The ODS says vegetarians and vegans usually take in less zinc than people who eat meat.
• Vitamin C comes from fruit and vegetables, so eating them every day matters more than a large amount once in a while.
• Thai portions and dishes differ from the US servings in the tables, so use the example as a way to think about meals rather than an exact count.

Is there an upper limit?

Yes. The ODS gives US upper limits for adults of 40 mg of zinc and 2,000 mg of vitamin C a day, counting food and supplements together.

The ODS adds that the zinc in food is rarely as high as 50 mg.

When is it worth asking a doctor or dietitian about my diet?

Talk to a doctor or dietitian about your diet if, for example:

• you eat only a limited variety of foods; the ODS lists people who eat a limited variety of food among those more likely to get too little vitamin C
• you eat little or no meat, seafood or eggs; the ODS says vegetarians and vegans usually have lower zinc intakes
• you rarely eat fruit or vegetables
• you often skip meals or eat much less than before
• you smoke; the ODS says people who smoke have lower plasma vitamin C than people who do not

If you are pregnant, breastfeeding or take regular medication, talk to a doctor or pharmacist before taking dietary supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.