The importance of vitamin D cannot be stressed enough. While you are very likely to be mindful of its benefits in making your bones strong and reducing the risk of osteoporosis, its possible role in regulating immunity and preventing chronic diseases is something that most people don’t realize.
You need vitamin D to absorb calcium. But this vitamin is also crucial in reducing inflammation. It also helps the body produce proteins that are naturally antibiotic and help fight infections.
Vitamin D deficiency has been linked to conditions such as allergies, asthma, heart disease, inflammation, depression, complications associated with pregnancy and auto-immune disorders such as thyroid and inflammatory bowel syndrome.
Studies show that poor vitamin D status also increases your risk of developing metabolic syndrome, an umbrella term of conditions that further increase the risk of other health complications.
What is metabolic syndrome?
Metabolic syndrome is group of factors that increases your risk of many serious health conditions. If you are diagnosed with any three of more of the following conditions, you have metabolic syndrome:
- High blood pressure (levels greater than 130/85 mmHg)
- High blood sugar levels and/or insulin resistance (fasting blood glucose greater than 100 mg/dL)
- Excessive fat around the abdomen (waist circumference more than 40 inches in men and more than 35 inches in women)
- High levels of triglycerides in the blood (levels greater than 150 mg/dL)
- Low levels of HDL (high density lipoprotein levels less than 40 mg/dL)
While old age is one major risk factor, metabolic syndrome can be triggered by obesity, unhealthy diet of high fat and refined sugar, sedentary lifestyle, smoking and insulin resistance. Postmenopausal women are also at a high risk.
The good news is you can prevent and manage metabolic syndrome by making dietary and lifestyle modifications such as losing weight, avoiding processed food and sugar, eating healthy diet (fresh, wholesome foods), engaging in regular exercise and quitting smoking.
At this point it is important to understand that insulin resistance plays a key role in metabolic syndrome. In fact, some experts also use the term “insulin resistance syndrome” to define this disorder.
What is insulin resistance?
When you eat food and after it is digested, your blood glucose levels begin to rise. Beta cells of the pancreas release a hormone called insulin that allows cells to absorb this circulating sugar. Cells use this glucose to create energy. Insulin not only helps regulate sugar levels in the blood but also controls how the body stores excess sugar as fat. Let’s see how this works.
Insulin signals the cells in your liver, skeletal muscles and fat tissues to take the glucose from the bloodstream. What if you already have more glucose than your cells need? Insulin signals the liver to store this extra sugar as glycogen, which is released when you need additional energy, for example during periods between the meals or when you are physically active.
In people with insulin resistance (IR), the cells develop resistance to the hormone insulin. This leads to an increase in their blood sugar levels, which in turn stimulates the pancreas to secrete more insulin to keep glucose levels under control. Initially, insulin resistance produces no symptoms but over time, the pancreas gives up and stops making enough insulin, resulting in higher levels of glucose in the blood. This situation is called pre-diabetes and can progress in to fully blown diabetes.
Studies show that IR is associated with major risk factors of metabolic syndrome.  It increases inflammation in the blood vessels and raises your risk of high blood pressure, stroke, type 2 diabetes, PCOS and non-alcoholic fatty liver disease.
Now, there is compelling evidence that vitamin D deficiency may also play an important role in the development of metabolic syndrome, insulin resistance and diabetes.
Vitamin D’s role in metabolic syndrome and diabetes
Two recent studies suggest that vitamin D deficiency is a major risk factor for metabolic syndrome. [2-4].
According to the findings of an ongoing Chinese study (Beijing Child and Adolescent Metabolic Syndrome study), Chinese adolescents and young adults with low vitamin D levels were more likely to have metabolic syndrome than those without deficiency. The study also found that participants with obesity, high triglycerides and type 2 diabetes have significantly lower levels of vitamin D. All these conditions belong to the cluster of risk factors for metabolic syndrome.
The study concluded that, “Given this association between low vitamin D levels and MS, the role of vitamin D supplementation in Chinese youths needs further examination, particularly in those at risk for MS.” 
A 2017 Japanese study found that higher levels of vitamin D in the blood is associated with reduced risk of developing metabolic syndrome. 
Researchers in this field aren’t yet certain as to how vitamin D may influence this risk. However, some studies suggest the following mechanisms to explain the association of vitamin D deficiency with an increased risk of metabolic syndrome:
- Vitamin D is fat soluble and in obese individuals it could be stored in the adipose tissue. This might reduce the levels of vitamin D in their blood.
- Metabolic syndrome is closely associated with inflammation and vitamin D reduces inflammation.
- Vitamin D deficiency may increase the risk of IR
- Vitamin D regulates insulin section and also influences insulin sensitivity. Beta cells of pancreas, fat tissue and skeletal muscles have receptors for vitamin D. This highlights the role of vitamin D in the proper functioning of beta cells, and also in the uptake of glucose by the cells of skeletal muscle and fat tissue.
- Vitamin D deficiency may be associated with elevated blood pressure as it is known to supress the renin-angiotensin- aldosterone system, a hormone system that controls blood pressure.
- Elderly; and this is important as the elderly are at a high risk of developing vitamin D deficiency. 
- In patients with psychotic disorders. In fact, patients with depression, dementia, autism and schizophrenia have been found to have low levels of vitamin D 
- In post-menopausal women.  
Vitamin D deficiency and increased risk of obesity
Is it possible that the underlying reason for obesity is vitamin D deficiency? Can it cause weight?
According to some studies, people with higher levels of fat around their abdomen are more likely to have lower levels of vitamin D than those without. However, it is not entirely clear whether vitamin D deficiency makes the body prone to storing excess fat or whether high fat levels reduces the bio-availability of vitamin D in the blood. Rachida Rafiq, a researcher for one of the studies, pointed to “a possible role for vitamin D in abdominal fat storage and function." 
There is some evidence that babies born to mothers with vitamin D deficiency are at a higher risk of developing obesity in childhood and in later years.
Vitamin D deficiency and increased risk of type 2 diabetes
Poor vitamin D levels have been linked to the risk of developing type 2 diabetes. It is because vitamin D is involved in how your body uses glucose, through its action on vitamin D receptors present on beta cells of the pancreas. Beta cells release insulin in response to the glucose present in the bloodstream. Vitamin D receptors are also expressed on the cells of your skeletal muscles and adipose tissue. Insulin unlocks these cells to take in glucose for energy.
Vitamin D exerts most of its action due to its action on vitamin D receptors or VDRs. Almost every cell in the body has these receptors. Vitamin D binds to these receptors and activate them to express genes with varied roles and functions. For example, your immune cells have these VDRs. Vitamin D gets attached to these receptors on the immune cells to express genes that produce proteins with antibiotic properties. This is how healthy levels of vitamin D help you fight infections. One of the most important roles of vitamin D that you might be familiar with is to absorb calcium. This is also done through its action on VDRs present in your intestines. This is also how vitamin D helps improve the function of beta cells of the pancreas, and also how it makes liver and muscle cells more sensitive to insulin, thus improving insulin sensitivity.
Research indicates that people who have better levels of vitamin D may have reduced risk of developing type 2 diabetes.  It could be because vitamin D may help improve insulin sensitivity in the cells and reduce inflammation. Does this mean vitamin D supplements may help?
Can vitamin D supplements help?
Whether or not vitamin D supplements can reduce risk components of metabolic syndrome remains unclear. We need further studies to state with absolute certainty that vitamin D supplements may be helpful in preventing metabolic syndrome. But there is a solid evidence that vitamin D deficiency will increase your risk of conditions such as high blood sugar, insulin resistance and high blood pressure that form an important part of the umbrella term metabolic syndrome.
As discussed above, certain populations such as older adults, post-menopausal women and patients with dementia and depression have low levels of vitamin D and are at an increased risk of metabolic syndrome, indicating that improving vitamin D status may help reduce the risk of metabolic syndrome in these populations.
Some studies suggest that vitamin D supplementation could be effective in improving blood glucose levels in patients with type 2 diabetes. 
A meta-analysis study suggests that vitamin D supplements may help reduce insulin resistance, especially when high doses of vitamin D are given at a time and for short periods. 
This 2018 study found that type 2 diabetic patients with a healthy vitamin D status (>30 ng/ml) have lower HbA1c level, which is the amount of glucose attached to haemoglobin. Low HbA1c reflects that the diabetes is well-managed. 
There is a substantial amount of research that boosting your vitamin D levels may be helpful in auto-immune disorders, cardiovascular disease and eye disorders (such as dry eyes, myopia age-related macular degeneration and diabetic retinopathy).
Your skin makes vitamin D when it is exposed to sunshine. Some foods such as eggs, outdoor grown mushroom and fatty fish also contain vitamin D but generally not enough to help you maintain sufficient levels that are needed to for good health and to prevent the risk of various chronic diseases.
Unfortunately, today’s lifestyle is a major reason why you are not getting enough sun-exposure to make all the vitamin D you need. Whether you are spending too much time working indoors or slathering too much sunscreen, the fact is we are not getting enough sun. However, sensible sun exposure - catching sunrays at the right time and for the right amount of time (for the skin to turn slightly pink) - is important for making healthy vitamin D levels.
Some factors increase your risk of vitamin D deficiency such as:
- Geographic location (people living in northern latitudes are at an increased risk)
- Darker skin tone
- Excessive use of sunscreen
- Liver and kidney disorder
If sensible sun exposure is not possible, it is best to take vitamin D supplements. And if you are already taking vitamin D supplements, it is best if you also take magnesium and vitamin K2.
While you need magnesium to help the body properly use vitamin D that you obtain through sun exposure or from supplements, you need vitamin K2 to guide calcium to go to places like bones where the mineral is required. Vitamin K2 also prevents calcium from entering areas where it may cause harm. For example, you don’t want calcium to accumulate in to your muscles, arteries and kidneys. And this is where vitamin K2 helps as the vitamin activates proteins that keep calcium from getting into arteries. What’s more, both magnesium and vitamin K2 work in their unique ways in reducing the risk of diabetes.
- Cho et al. Insulin Resistance and Its Association with Metabolic Syndrome in Korean Children. BioMed Research International. 2017
- Al-Dabhani K et al. Prevalence of vitamin D deficiency and association with metabolic syndrome in a Qatari population. Nutr Diabetes 2017
- Al-Khalidi B et al. Standardized serum 25-hydroxyvitamin D concentrations are inversely associated with cardiometabolic disease in U.S. adults: a cross-sectional analysis of NHANES, 2001-2010. Nutr J. 2017
- Diaz et al. Vitamin D Status Is Associated with Metabolic Syndrome in a Clinic-Based Sample of Hispanic Adults Metab Syndr Relat Disord. 2016
- Fu et al. Vitamin D levels are associated with metabolic syndrome in adolescents and young adults: The BCAMS study. Clin Nutr. 2018
- Akter et al. Serum 25-hydroxyvitamin D and metabolic syndrome in a Japanese working population: The Furukawa Nutrition and Health Study. Nutrition. 2017
- Wang et al. Inverse Relationship between Metabolic Syndrome and 25-Hydroxyvitamin D Concentration in Elderly People without Vitamin D deficiency. Scientific Reports. 218
- Yoo et al. Association Between Vitamin D Insufficiency and Metabolic Syndrome in Patients With Psychotic Disorders. Psychiatry Investigation 2018
- Schmitt EB et al. Vitamin D deficiency is associated with metabolic syndrome in postmenopausal women. Maturitas. 2018
- Srimani et al. Prevalence and association of metabolic syndrome and vitamin D deficiency among postmenopausal women in a rural block of West Bengal, India. PLoS One. 2017.
- European Society of Endocrinology. "Larger waistlines are linked to higher risk of vitamin D deficiency: Higher levels of belly fat are associated with lower vitamin D levels in obese individuals, according to data presented in Barcelona at the European Society of Endocrinology annual meeting, ECE 2018." ScienceDaily. 2018.
- Park et al. Plasma 25-hydroxyvitamin D concentration and risk of type 2 diabetes and pre-diabetes: 12-year cohort study. PLoS One. 2018
- Wu et al. Vitamin D supplementation and glycemic control in type 2 diabetes patients: A systematic review and meta-analysis. Metabolism. 2017
- Li X et al. The Effect of Vitamin D Supplementation on Glycemic Control in Type 2 Diabetes Patients: A Systematic Review and Meta-Analysis. Nutrients. 2018
- Alkhatatbeh MJ et al. Association between serum 25-hydroxyvitamin D, hemoglobin A1c and fasting blood glucose levels in adults with diabetes mellitus. Biomed Rep. 2018