Vitamins are essential for the human body and can be found in various fruits, vegetables and other edibles but due to the fact that all of those food items can not be eaten raw and require some cooking so it is very unlikely that we are getting all the vitamins from the food after we have cooked it.
It all goes down to which method of cooking we use as different vitamins are lost from food due to longer exposure with light, air, temperature and water.
It is also necessary to consume fruits and veggies raw and in fresh form.
It is advised not to store fresh food items for long as it will only lower their nutrition level.
Although it is not advised to store fresh fruits and vegetables for long but if they are to be stored temporarily then make sure that they are well covered and are not exposed to light as well as air.
Also keep the stored veggies and fruits in refrigerator (with the exception of sweet potatoes and winter squash).
Another way to prevent from exposing to atmosphere is to cut larger pieces of fruits and vegetables so that only smaller cross sectional areas are exposed to air and light.
It is much better to steam cook or use microwave oven for cooking vegetables and fruits instead of boiling, deep frying or baking in oven.
This will help retain most of the minerals and vitamins in your food as steaming and microwave cooking minimizes cooking time and lowers the amount of nutrients being lost.
If frying or boiling is unavoidable then it is best not to remove skins of the fruits and vegetables and use very low quantity of oil and water for frying and boiling respectively.
During cooking keep the pan or pot covered so that essential nutrients do not get wasted through steam.
Serve the veggies and fruits when they become tender and crispy and avoid making them mushy or soggy.
This can be achieved by boiling the water for about a minute before you add vegetables or fruits and let the water simmer rather than boil vigorously.
Vitamins are water soluble which means if you soak fruits or vegetables in water for long then all the vitamins contained in them would be leached into that water.
The amount of vitamins that the food retains is mostly dependent on how long they were soaked in water.
So it is advised not to soak them for long.
The other way to retain the leached vitamins is not to drain the water which was used to boil the food, instead use it for making gravy, soup, stock or if possible serve the food without removing water as most of the vitamins and other essential minerals which your food contained have now been transferred and dissolved in water.
So make best use of that water.
Above all one should adopt not only healthy cooking practices for minimizing nutrients loss but it is also recommended to buy only fresh food and in a quantity that can be consumed in few days as longer storage time kills many vital nutrients.