Food And Vitamin Loss Over Time

You know the claim on food value from the label that is stuck on it; let's pick a nutritious drink product and on that drink is some nutritional claim that states it has a shelf value of two years. After those two years that product still needs to contain 100% of the vitamin C level.
Vitamin deterioration
That does not mean that when the drink product was manufactured it contained 100% of vitamin C and after the two year has gone, it only contain half that level of vitamin C. What it means is that the manufacturer had to put an overage of that vitamin in the product because the vitamins will deteriorate over a period of time. Usually the higher the temperature the higher rate of deterioration, and the longer it is stored means something too.
Vitamin C is the most sensitive vitamin. The government does routinely inspect the food manufacturing companies with products that have this problem. What the food companies and food scientist does when the product is formulated they will add an overage of vitamins and minerals. It protects the product during the shelf life.
200% of vitamins in the product
So if they want the product to contain 100% of vitamin C after 2 years, then they have to put 200% of vitamins in the product, because of the deterioration that takes place. 2 years later it will have lost half of its value, and contain the claimed amount.
Food companies have documentation on all food products that they have manufactured. They know the exact deterioration rate on all their products, and therefore they can predict the deterioration rate of new and similar products. That means that a new nutritional beverage formula can be based on the level of a know products nutrition values.
Vitamin questions
That makes me ask my self. Can I trust that the food I buy from the store has the nutrition value that it claims? Can I feel sure that my body gets all the vitamins it must have to function? I say better be safe than sorry!
Thank you for your time.