Food for a one week-trekking ?

Have you got a guide book for the area? It should give you plenty of tips.

I have never trekked in Iceland but generally you will drink water from streams, usually you need to purify it but in some places it is perfectly clean as it is, a guidebook should say.

You can buy freeze dried food at supermarkets or outdoors shops.

Try cross posting this on the Scandinavia branch.

Freeze dried food is not different from pasta, cereals, ... These foods will bring you about 400 calories per 100 grs. That's also the case for jam, biscuits, chocolate, ...
You have to count on 3000 calories/day for a one-week trek, that means 750 grs of such food per day (not including low-calories food such as fruit or vegetable), or more than 5 kgs for one week.

You need a minimum of 3000 kilocalories per day like Willemspie wrote, men actually even more like 4000 kcal/day. As typical food like pasta, noodles, couscous, dried bread, canned tuna and jerky contain about 3000 kcal/kg, you need to supplement your diet with greasy foods like peanut butter, chocolate, potato chips, fatty salamis, greasy nuts (macadamia is best) etc, which have 4000-5500 kcal/kg making your load lighter. I have a rule of 800g of fairly high energy food per day minimum, but even then I tend to loose weight. If the weather is cool it is also possible to carry butter and hard cheese for about a week before they go bad. One thing to consider is cooking times, if you need to cook something for an hour instead of just bringing it to a boil (like couscous), you will have to carry also much more fuel.

One way to save weight is to eat a big pizza or a couple of big macs before hitting the trail, and if there is no danger of getting lost running out of food on the last day is not dangerous either. This practically takes one kg from the weight of the food required.