Hi noorjahan,
I am not familiar with that Royal Jelly product, so I am not sure. My guess would be that the luke warm water helps it to dissolve, but heat would damage its nutritional value.
The RDA of Vit. D is 400IU. You should not need to supplement more as you get some Vit. D in enriched foods as well as naturally in other foods. You also get vit. D through exposure to sunlight.
Vitamin D is an essential nutrient. The Institute of Medicine recommends that children and adults up to age 50 get 200 international units (IU) of vitamin D daily. The recommendation for adults over age 50 is 400 to 600 IU daily. However, many health experts consider these recommendations to be too low. The American Academy of Pediatrics, for instance, now recommends that children and adolescents get 400 IU of vitamin D daily. Many experts now "unofficially" recommend that adults get as much as 1,000 to 2,000 IU of vitamin D daily. In addition, it's likely that the vitamin D guidelines will be revised upward.
Supplements can be a reasonable way to meet recommended levels — as long as you pay attention to how much you take. And be sure to tell your doctor about any supplements you take.
From the Mayo Clinic:
http://www.mayoclinic.com/health/vitamin-d-toxicity/AN02008Please use your best judgement.