Care of bone health is really important after bariatric surgery, particularly after gastric bypass. There is increased risk of fractures after bariatric surgery. Bones are living, dynamic organs in the human body and bone health is critically dependent on calcium, phosphorous, vitamin D and parathyroid hormone. This section describes, in some detail, the role of each of these elements and their interactions.
What is vitamin D?
Vitamin D occurs in two forms: vitamin D2 (called ergocalciferol) and vitamin D3 (called cholecalciferol). Both forms are equally effective and collectively are called vitamin D. Over 90% of vitamin D activity in the human body is provided by vitamin D3. Uniquely amongst the vitamins, D3 is made in the skin: by the action of Ultra-Violet B (UVB) rays of sunlight on a naturally occurring skin compound, called 7-dehydrocholesterol. In order to function effectively, vitamins D2 and D3 are chemically converted in the liver into a compound called 25-hydroxy-vitamin D (also called calcidiol), which is then further converted, in the kidney, into 1,25-dihydroxy-vitamin D (also called calcitriol). Calcitriol is the metabolically active form of vitamin D in the human body.
What are the functions of calcium and vitamin D in the human body?
Calcium is an important mineral in the human body. Calcium is a vital component of bones. Also, calcium is necessary for the normal functioning of nerves, muscles and the heart. Blood calcium levels are controlled by vitamin D and by the parathyroid hormone (PTH). If blood calcium levels drop, there is increased release of PTH, which stimulates the re-absorption of calcium stores from bones into the blood stream.
Vitamin D is critical for the absorption of calcium from the intestines. For dietary calcium to be absorbed across the lining of the intestine, a special calcium-transport protein is necessary. Vitamin D is essential for the activation (transcription) of the genes that encode the calcium-transport protein. Without vitamin D, only 10-15% of dietary calcium is absorbed. Vitamin D is needed also to regulate the excretion of calcium from the kidneys; without vitamin D, a large part of the body’s calcium store will be lost in urine. Thus, Vitamin D plays a vital role in maintaining healthy bones. Throughout life, there is constant re-modelling of bone that is dependent on well-controlled fluxes of calcium and phosphorous.
What are the normal requirements of vitamin D and calcium?
Vitamin D is measured in International Units (IU). The normal daily requirement of vitamin D in a healthy, young adult is 400-600 IU. The requirements are higher for the elderly, obese and those with intestinal, kidney or liver disease. The daily requirement of calcium is about 1000 mg. The calcium requirement is dependent on adequate intake in diet. In contrast, most of the vitamin D requirement is provided by natural synthesis in the skin. Few food items contain vitamin D: oily fish (e.g. salmon, mackerel, tuna, hilsa), egg yolk, cod liver oil and Shitake mushrooms are the limited dietary sources of vitamin D.
What is vitamin D deficiency?
Vitamin D is tested by measuring 25-hydroxy-vitamin D (calcidiol) in a blood sample. 25-hydroxy-vitamin D levels may be reported in terms of either nmol/L or ng/ml and the diagnoses, corresponding to various ranges, are shown in the table below:
How are calcium levels in the body tested?
A blood test for calcium reports on the level of calcium that is circulating in blood. Calcium is carried in blood in combination with proteins and also in a free state (called ionized calcium); commonly, total calcium is tested and is reported in terms of either mg/dl or mmol/L. In adults, the normal range of total calcium is 8.8-10.4 mg/dl or 2.2-2.6 mmol/L (the reference values may vary slightly amongst laboratories).
Normal calcium in a blood test does not indicate adequate dietary intake or satisfactory bone stores of calcium. When calcium intake is insufficient, there is increased release of parathyroid hormone, which maintains normal blood calcium levels by increasing the re-absorption of calcium from bones. Thus, there could be considerable depletion of bony calcium, despite a normal blood calcium level. Calcium in bones is measured by testing bone mineral density, using Dual energy X-ray Absorptiometry (DXA; often called DEXA scan).
Why is there a risk of calcium deficiency and vitamin D deficiency after bariatric surgery?
The main sites for absorption of dietary calcium are in the upper small intestine (called the duodenum and jejunum). After a gastric bypass, food will not pass through the duodenum and the upper part of the jejunum; hence, calcium will not be properly absorbed. Calcium tablets (1000-1500 mg) should be taken regularly after gastric bypass surgery. With extra calcium intake, enough calcium should be absorbed from the lower part of the small intestine. Calcium carbonate is easily available in chewable forms and is better tolerated immediately after surgery; it should be taken with meals to improve absorption. Calcium citrate is the preferred for long-term usage because it is better absorbed in the absence of gastric acid.
There is a high incidence of vitamin D deficiency in the general population, irrespective of bariatric surgery. The main cause of vitamin D deficiency is inadequate exposure to ultra-violet B rays of sunlight because of darkly pigmented skin, clothing, atmospheric pollution and long winter months (in countries like the UK). Obese individuals need 2-3 times the normal daily requirement of vitamin D and are likely to have undetected vitamin D deficiency prior to bariatric surgery.
What are the consequences of calcium deficiency and vitamin D deficiency?
The main impact of calcium or vitamin D deficiency is on bones. In adults, vitamin D deficiency causes improper mineralization of bones, resulting in a condition called osteomalacia. Typical symptoms of osteomalacia are bony pain, mainly in the hips, pelvis, ribs and feet, and muscular aches. Also, deficiency of calcium and vitamin D can cause demineralization of bones, resulting in osteoporosis, with increased risk of fracture.
What tests are required to detect calcium or vitamin D deficiency?
Following gastric bypass and sleeve gastrectomy, blood tests for calcium, vitamin D and parathyroid hormone (PTH) should be done at regular interval. Raised PTH levels indicate a condition called secondary hyperparathyroidism, in which normal levels of blood calcium are maintained by increasing re-absorption of calcium from bone. Dual energy x-ray absorptiometry (DXA) may be done to keep check on bone mineral density.
What is the treatment of vitamin D deficiency?
Vitamin D deficiency is usually treated with Vitamin D3 (cholecalciferol). A blood test is then repeated to ensure that the 25-hydroxy-vitamin D level has come within the normal range.