Do tomatoes lose their nutritional value when they are made into tomato sauce? Do tomatoes lose their nutritional value when they are made into tomato sauce? What about canned tomato sauce?
Nutritional changes in cooking foods (including tomatoes) are not simple to answer:  cooking can reduce some nutrients, and it can make other ingredients more available.

Tomatoes are a case in point, as cooking has both effects.  According to as study in the Journal of Food and Agriculture, as reported in Cornell Chronical, both effects happen.   Vitamin C is reduced as it breaks down under heat, but certain lycoenes and anti-oxidants increased.


  Tomato samples were heated to 88 degrees Celsius (190.4 degrees
  Fahrenheit) for two minutes, a quarter-hour and a half-hour.
  Consistent with previous studies, vitamin C content decreased by 10,
  15 and 29 percent, respectively, when compared to raw, uncooked
  tomatoes. However, the research revealed that the beneficial
  trans-lycopene content of the cooked tomatoes increased by 54, 171 and
  164 percent, respectively. Levels of cis -lycopene (which the body
  easily absorbs) rose by 6, 17 and 35 percent, respectively; and
  antioxidant levels in the heated tomatoes increased by 28, 34 and 62
  percent, respectively.


See related:  Does cooking in a pressure cooker destroy nutrients?