Water-soluble vitamins
Vitamins that dissolve in water (the B group and vitamin C), which the body stores little of and needs to receive regularly, with large losses when cooked in water.
The water-soluble vitamins include the eight vitamins of the B group (B1, B2, B3, B5, B6, B8, B9, B12) and vitamin C. Their solubility in water governs most of their behaviour: they circulate freely in the blood, are barely stored and any excess is eliminated in the urine.
This has a direct practical consequence: intakes need to be regular. Whereas the liver builds up reserves of fat-soluble vitamins for months, most water-soluble ones give only a few weeks of cover. The risk of overdose is low, however, because the excess is excreted, with the notable exception of B6, which is toxic at very high doses taken over a long period.
Vitamin B12 is the exception on both counts. It is stored in the liver for several years, and it is found almost only in animal products. This explains the twofold pattern seen in people who cut out these foods: no symptoms for a long time, then a deficiency that sets in once the reserves run out.
These vitamins are the most fragile in cooking. Heat, light and above all water carry them away: boiling vegetables can wash out a large share into the cooking water. Steaming, short cooking times and reusing the cooking liquid are the habits that really change your intake.