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Since it’s summer time, you might assume you will get enough vitamin D from the sun. But that is likely not the case. Vitamin D insufficiency now is thought to affect 30 to 50 percent of the world’s population.

The main source of vitamin D for most people is the sun, but achieving adequate levels of vitamin D via sunlight can be difficult depending on your geographical location, skin color, and age. Even if you live in a warm climate, you may not make enough vitamin D from sunlight alone. In a study of Hawaii residents who reported an average of 29 hours of sun exposure per week, half had vitamin D levels below 30 ng/ml. The only way to know for sure if you’re getting enough vitamin D is to have a 25(OH)D blood test. The scientific evidence supports a favorable range of 25(OH)D levels between approximately 30 and 45 ng/ml.

Adequate stores of vitamin D help prevent rickets in children and osteoporosis in older adults. But, there are other critical health benefits, too. Vitamin D insufficiency is thought to be a key contributor to many diseases, including several cancers, diabetes, cardiovascular disease, depression, and autoimmune diseases. If you need further evidence of how important vitamin D is, consider this: Scientists have found that it has biological actions in almost every cell and tissue in the human body.

How much vitamin D do you need? First, I recommend taking vitamin D3 which is the natural form of the vitamin and a higher-quality supplement (补充) compared to vitamin D2. Take a consistent daily dose of vitamin D3 aiming to achieve a 25(OH)D level that is between 30 to 45 ng/ml. If you do not supplement, it makes sense to have your 25(OH)D levels tested; take a positive attitude to your health, because a long-standing insufficiency can be harmful. Importantly, more vitamin D is not necessarily better; a 25(OH)D above approximately 45 ng/ml does not provide any further benefit and may even be harmful. Visit DrFuhrman.com for more information on the importance of Vitamin D.

1.What does the report in paragraph 2 show?

A The main source of vitamin D is the sun.

B Most people cannot get enough vitamin D from the sun.

C There are many methods to know whether you get enough vitamin D.

D Levels of vitamin D vary between different geographical locations and age.

解析：选B。B 推理判断题。根据第二段的“but achieving adequate levels of vitamin D via sunlight can be difficult”“Even if you live in a warm climate, you may not make enough vitamin D from sunlight alone.”“an average of 29 hours of sun exposure per week”“half had vitamin D levels below 30 ng/ml”可知，通过阳光获得足够的维生素D可能很困难，即使你生活在温暖的气候中，你也可能无法仅从阳光中获得足够的维生素D，接着作者举例在一项针对夏威夷居民的研究中，这些居民报告平均每周暴露在阳光下29小时，但是其中一半的维生素D水平低于30ng/ml证明此观点。故选B。

2.What does the third paragraph focus on?

A Sun and vitamin D.

B The Health Benefits of Vitamin D.

C Vitamin D Fights Cancer.

D Are You Getting Enough Vitamin D?

解析：选B。B 段落大意题。根据第三段的“help prevent”“other critical health benefits”“how important vitamin D is”等可知，该段主要讲的是维生素D的健康益处。故选B。

3.What is recommended by the author?

A Vitamin D2 is recommended.

B 25(OH)D levels test isn’t necessary.

C We should always pay attention to our health.

D The more vitamin D, the better.

解析：选C。C 推理判断题。根据最后一段的“take a positive attitude to your health, because a long-standing insufficiency can be harmful.”对你的健康采取积极的态度，因为（维生素D）长期不足是有害的。即作者建议我们要积极主动地关注自己的身体健康。故选C。

4.Where can we most probably read the passage?

A In a science newspaper.

B In a geography magazine.

C In a medical book.

D On a health website.

解析：选D。D 推理判断题。根据全文内容及最后一段的“Visit DrFuhrman.com for more information on the importance of Vitamin D.”可知，本文最有可能出现在网站上，且是关于健康的。故选D。