Adequate vitamin levels D have long been touted as critical for optimal function. As noted in the talk above, there are VDR (Vitamin D Receptors) literally all throughout our body which demonstrates a need for it on a systemic level. Considered a fat soluble vitamin, many would appropriately state the case that due to the variety of functions and systems vitamin D plays a role in, it actually resembles a hormone more than a vitamin.
The optimal way to generate vitamin D (as well as a list of other non vitamin D related benefits) is by way of intelligent sun exposure (not burning the skin). The safe, optimal amount of exposure varies individually depending on skin type, location, time of day and year. Your ability to naturally generate D is also contingent upon having adequate levels of cholesterol as it serves as a building block for sun derived D formation. Just like all hormones, if we block cholesterol production, we weaken our ability to produce hormones, and thus deficits from low T to decreased immune function can occur or worsen.
If for some reason, generating D through intelligent sun exposure is not an option, and you are looking to boost or maintain levels, supplementing is the next best option, as dietary sources are generally pretty weak. Optimal level (we generally aim for 60-80ng/mL) and dosing is individually specific and based on ability to metabolize vitamin D, as well the goal (i.e. maintenance, raise baseline level, or enhance brain, mood, immune & gut function). Higher vitamin D levels have been associated with a 50% reduction in respiratory infections like the flu, and reduce the risk of getting the flu by 10% (the same success as many flu vaccines, including this year). In some cases it has even been shown to enhance immune function when supplementing on top of already higher levels.
New medical research supports a change in the reference range for Vitamin D. It has been determined that Vitamin D levels below 50ng/mL are sub-optimal. Hypovitaminosis D increases the risk of infection, cancer, autoimmune disease, hypertension, arteriosclerosis, diabetes, and/or insulin resistance, musculoskeletal pain, epilepsy, and migraine.
(Vitamin D levels are easy to assess and can be done with a simple blood test; 25-OH-vitamin D, or 25-hydroxy-vitamin D. Incorporating this measurement is the most intelligent way to monitor and ensure adequate levels, leading to optimal dose calibration.)
Study after study has shown D3 (cholecalciferol) to be substantially more efficacious at boosting levels due to its higher rate of absorption and bioavailability in comparison to D2 (ergocalciferol). (If someone prescribes or suggests D2, I would question them as to why D2 instead of D3.)
The evidence continues to mount in favor of including K2 if supplementing with D3. Another fat soluble vitamin, K2 is not only anti-inflammatory, but ensures proper anatomical distribution (i.e. to bone rather than arteries or soft tissue) of Ca (Ca absorption increasing congruent with Vitamin D levels). Being that these are both fat soluble vitamins (A, D, K, & E), they either need to be an emulsified form (preferred emulsifier being MCT oil), or you need to take them with a fat.
(For sake of convenience and maximum absorption, this is why we use a D3/K2 formula emulsified in MCT oil. Having each drop/serving be 1000iu D3 & 30mcg K2 also allows you to calibrate dosing based on need & objective.)
MK-7 v MK-4
The other caveat with K2 is making sure you get the superior form. MK-7 or menaquinone 7 is the longer lasting, natural form (derived from natto; fermented soy), while MK-4 (menaquinone 4 or menatetrenone) is synthetic and has a shorter half life, meaning it does not stay around in the body long enough to do the intended job.
The other way K2 is produced is by certain strains of gut flora or probiotic. This is hard to measure and a still evolving understanding, but we have found a handful of studies naming Lactococcus Lactis (subspecies: cremoris) as the K2 producing strain. The exact science and rate of K2 production by this strain is not fully understood and accurately quantifiable at this point, so if supplementing with D3, based on current understanding, including K2 in the form of MK-7 appears to be the way to go, especially if at risk for CAD (coronary artery disease).