Sources of Vitamin A
There are two dietary sources of vitamin A:
- Taking up retinoids (vitamers of vitamin A) directly, which occurs in animal based, non-vegan food.
- Converting a form of provitamin A into vitamin A. Provitamin A is abundant in lots of vegetables.
A quick note on supplements
Both forms can also be taken as a food supplement. Retinol overdose is harmful, especially during pregnancy. Provitamin A cannot be overdosed so easily since conversion to vitamin A stops with a high vitamin A blood level.
(Given the long discussion on the other answer I felt that this needed to be addressed, even if out of scope for the question.)
Provitamin A sources
The most common source of provitamin A is β-carotene. It occurs in all orange vegetables, such as carrots and sweet potatoes, but also in many others.
Other sources include α-carotene and γ-carotene. Not all carotenes are a source of provitamin A.
Provitamin A conversion
In a healthy human, provitamin A is converted into vitamin A. Conversion depends on the specific source of provitamin A, and other factors.
The actual question: Cooking and raw preparation
The bioavailable amount of vitamin A depends greatly on the preparation. In studies the following preparation methods of carrots improved β-carotene blood levels:
- Slicing, ideally homogenizing (e.g. blending)
- Extracting juice
- Preparation in fat
Effects of cooking
Cooking a carrot can have several effects:
- The carrot is usually sliced in smaller pieces.
- Fat is added.
- Carrot cells break and release β-carotene.
- The carrot is heated, and β-trans-carotene is converted to β-cis-carotene.
The first three of these increase the β-carotene blood level. The last one is potentially a problem, since β-cis-carotene is not converted to vitamin A as effectively as β-trans-carotene.
It seems like there is no decisive result yet whether the process of heating is beneficial or not. But certainly either option can provide you with sufficient amounts of provitamin A.
My recommendation: Grate your carrot and add some linseed oil for a great raw salad. Blend it for a great smoothie.
Main source: National Academies of Science - Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic,
Boron, Chromium, Copper, Iodine, Iron, Manganese,
Molybdenum, Nickel, Silicon, Vanadium, and Zinc, and the plentiful sources therein.