Two cooks can start from an identical basket of ingredients and produce meals that differ meaningfully in what the body actually takes up. The method is doing that work, not the shopping list.
Heat destroys some compounds and liberates others
Certain vitamins are sensitive to heat and degrade steadily the longer a food is held at temperature, which is why long simmering reduces the vitamin content of vegetables that were high in it when raw.
The same heat breaks down cell walls that would otherwise pass through the gut intact, releasing compounds that raw food holds onto and that the body cannot reach on its own.
So cooking is neither a loss nor a gain in general terms. It trades one category of nutrient for improved access to another, and which trade matters depends entirely on the ingredient.
Water carries nutrients out of the food
Water-soluble vitamins and minerals leach into any liquid the food is cooked in, and the longer the contact and the greater the volume of water, the more leaves the food itself.
Boiling and discarding the water therefore removes a portion of what was there, while steaming keeps the contact brief and braising keeps the liquid as part of the dish.
This is the mechanical reason a stew served with its cooking liquid delivers more than the same vegetables boiled separately and drained into a sink.
Fat governs whether some nutrients are absorbed at all
Several vitamins and plant pigments dissolve in fat rather than water, and they need dietary fat present in the same meal to be carried across the intestinal wall.
A vegetable rich in those compounds eaten entirely without fat delivers less of them than the same vegetable cooked in a modest amount of oil or ghee.
This is one of the quieter reasons traditional cooking so often pairs vegetables with a fat, since the combination was arrived at by taste long before the mechanism was described.
Structure decides how fast digestion happens
Grinding, blending and prolonged cooking all reduce the physical work the digestive system has to do, which speeds absorption and changes how quickly energy from a meal arrives.
A whole grain cooked until just tender behaves differently from the same grain milled to flour and baked, even where the composition on paper is unchanged.
Neither is inherently better, but the difference explains why identical ingredients in different forms do not leave a person feeling the same afterwards.
Time at temperature matters more than temperature alone
Brief exposure to high heat can be gentler on nutrients than a long spell at moderate heat, because total time is often the dominant factor in degradation.
Fast methods such as stir-frying combine short cooking, minimal water and added fat, which is why they preserve a good deal of what a long boil would remove.
Anyone eating to meet a specific clinical requirement should confirm the details with a qualified professional, since individual needs and absorption vary widely.