Does low 25-hydroxyvitamin D mean calcium deficiency?

This is the 3154th article of Da Yi Xiao Hu

Osteoporosis is a common disease among the elderly, which can manifest as back pain or whole body bone pain, usually when turning over, sitting up, or walking for a long time. The pain worsens at night or during weight-bearing activities, sometimes accompanied by muscle cramps and limited movement, and is prone to fractures. When the elderly go to the hospital to check whether they have osteoporosis, the doctor will generally recommend blood tests for bone metabolism and nuclear medicine tests for bone density.

Aunt Wang is 75 years old this year. She has been feeling pain in both hips recently. She went to the hospital for tests and the results showed that her 25-hydroxyvitamin D was significantly reduced, and her bone metabolism and bone mass decreased. The hospital diagnosed her with "osteoporosis" and gave her oral vitamin D, sodium alendronate, calcium tablets and other medications. Aunt Wang looked at the test results and found that her blood calcium was within the normal range. Why did she still need to supplement calcium?

Calcium is the most common mineral element in the human body, also known as "bone mineral". Bones are mainly composed of collagen and calcium phosphate. Collagen is a protein that forms soft tissue, and calcium phosphate is a mineral that increases bone strength and soft tissue hardness. An adult's body contains about 1200g of calcium, accounting for 2% of body weight. Blood calcium and bone calcium are different. Blood calcium refers to free calcium in the blood. It mainly exists in plasma, so it is also called plasma calcium. It is supplied to various nerve and muscle tissues of the body through blood circulation to complete normal physiological reactions. In many cases, the levels of blood calcium and bone calcium are unbalanced. Therefore, although Aunt Wang's blood calcium is normal, her bone calcium is deficient. If you want to measure the calcium content of bones, you need to measure bone density and vitamin D levels.

Vitamin D is a fat-soluble steroid compound, and 1,25-dihydroxyvitamin D is its active form. Vitamin D obtained from food and vitamin D obtained through ultraviolet radiation on the skin (the ability of a 65-year-old's skin to produce vitamin D is approximately 25% of that of a 20- to 30-year-old) binds to vitamin D-specific binding proteins after entering the blood and is then transported to the liver. Under the action of 25-hydroxylase in the liver, 25-(OH) vitamin D is formed, which is further converted into biologically active 1,25-(OH) vitamin D under the action of la-hydroxylase in the kidneys to exert its biological function.

Recent studies have found that 1,25(OH) vitamin D, in addition to regulating the body's calcium-phosphorus balance, is also closely related to a variety of age-dependent chronic diseases such as hypertension, cardiovascular disease, and diabetes. There are currently many theories about the mechanism of aging, such as the oxidative free radical theory, the telomere loss theory, the immune system aging theory, the endocrine dysfunction theory, etc. Studies have now found that vitamin D is generally reduced in the elderly, which may have a certain effect on predicting aging. With the discovery of the role of vitamin D in the aging process, the relationship between vitamin D and aging and age-dependent diseases has gradually received attention.

The average human life expectancy is constantly increasing, leading to a continuous increase in the global elderly population. In 2017, the proportion of the population aged 60 and above in Asia was 12.2%, and it is expected to continue to rise in the next few decades, reaching about 24.2% by 2050. This proportion in China is even higher, at 16.2% in 2017 and will reach 35.2% in 2050. Aging is an inevitable process of the body and the root cause of geriatric diseases. The aging process of the body is more or less accompanied by vitamin D deficiency. So how can we, the elderly, reduce vitamin D deficiency?

Therefore, it is recommended that the elderly

1. Get more sun exposure. After the body is exposed to ultraviolet rays, cholesterol will be converted into vitamin D. Generally, each sun exposure time is about half an hour.

2. Many foods can supplement vitamin D, such as eating more fatty fish (tuna, eel, salmon, etc.), mushrooms, milk, peanuts, etc. If you want to supplement quickly, you can take cod liver oil and medicinal vitamin D orally.

Drugs used clinically to supplement vitamin D can be roughly divided into three categories:

The first type is ordinary vitamin D;

The second category is vitamin D analogs including calcifediol, calcitriol, etc.;

The third category is compound preparations containing ordinary vitamin D, such as calcium carbonate D3 tablets (Calcium Carbonate), alendronate sodium vitamin D3 tablets (Fimica), vitamin AD pills, etc.

Author: Department of Geriatrics, Shanghai Fifth People's Hospital, Fudan University

Zhang Xuemin Chief Doctor