Is it true that leaf vegetables lose most of their nutritional value in cooking?

Yes, it encourages loss of all foods during cooking, not just leafy vegetables.

But this can be overcome by following the right cooking instructions.

Cooked vegetables do lose some nutrients.  Most of those are leached out to the water (if you boil them) and some are destroyed by heat, such as vitamin C.

On the other hand, vegetables contains a lot of nutrients bound to the cell walls, which humans cannot digest, and so the body is not able to absorb them.  This is especially true of the tougher greens such as kale, collards, spinach, chard, etc.

For example, spinach is rich in oxalates that will bind to valuable minerals in spinach such as iron and calcium.  So if you want that iron/calcium, you want to cook your spinach.

Tomatoes are rich in many nutrients but one is of particular interest to men's health - lycopene has been shown to reduce the risk of prostate cancer.  Although inconclusive at the moment there are also suggestions lycopene may also improve heart health. Heating tomatoes increases the levels of lycopene in tomatoes. But you should not each only cooked tomatoes since heating tomatoes does destroy other important vitamins (such as Vit C) so simply have a mixture of cooked and raw tomato products.

Also, beware of eating raw cruciferous vegetables.  A diet rich in raw cruciferous is very likely to cause thyroid problems.  They contain goitrogenic substances which block the production of thyroid hormones in your body.  Cooking cruciferous vegetables reduces the amount of goitrogenics by roughly 2/3.  These are some common cruciferous vegetables: arugula, broccoli, kale, cauliflower, cabbage, turnip, collard greens, bok choy, brussels sprouts, radish, rutabaga, and watercress.

So that raw kale smoothie you drink in the morning?  Wastes most of the nutrients of the kale compared to eating cooked kale.  But what about leaching nutrients out of the kale when cooking it?  Put it in soup, that way you get to consume all the goodies sucked out of kale.

It most probably true especially water soluble vitamin C

Mainly water soluble vitamins are lost through cooking when exposed to heat and water. since compared to other vegetables leafy vegetables are rich sources of calcium and iron which are not lost through cooking.Moreover the vitamin c value of leafy vegetables are less to other vegetables but still vitamin c helps in the better absorption of iron. So I would like to stick on to the fact that nutrient are lost through cooking.

Yes cooking destroys some of the nutrients like vitamin c that are heat soluble,some minerals are also lost but not all. Some important enzymes get denatured too and makes digestion difficulty.

Yes because too much cooking may lead to lose of nutritive contents of the vegetable

yes it's true because there is some mineral and vitamin destroyed in high temperature or in medium temperature for along time cooking .

Cooked vegetables do lose some nutrients. Most of those are leached out to the water (if you boil them) and some are destroyed by heat, such as vitamin C.

if it is over boiled or over fried to much it looses its nutritional values

YES It is true leafy vegetables mostly containing  water soluble vatamins are lost during cooking

No, that's not true! limited nutritional losses occur upon cooking. Most of nutrients' contents are similar in fresh and cooking vegetables; however research shows that they are partially destroyed in cooked food and this may reduce their bioavailibity in the human body. One example would be vitamin C. Also, fiber content for example, if measured in fresh and cooked vegetables, they are approximately similar but the body make no use of fiber from cooked vegetables since it's partailly destroyed.