Why Vitamin D3 and K2 Are So Important for Infants and Children

By: Sabrina Granniss, IBCLC

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5 min

Vitamins play an essential role in every function of our body and overall health. However, the dynamic duo of vitamin D3 and K2 steals the spotlight, working in perfect harmony to promote healthy bones, cardiovascular health, and a robust immune system. While it is important to include a wide range of foods in our diet to get the nutrients that we need, supplements can help fill in the gaps, making sure we get enough of each vitamin and mineral required to promote healthy bodies and boost our levels at times when we need extra support. 

Fat-soluble vs. water-soluble vitamins

The way the body stores and processes vitamins can be broken down into two main groups. 

  • Water-soluble vitamins are dissolved in water. They are absorbed more quickly than fat-soluble vitamins, and any extra of the vitamin will leave the body through your urine. Water soluble vitamins include the majority of vitamins, including your B vitamins and vitamin C.
  • Fat-soluble vitamins are dissolved in fat. They are absorbed by the fat in your body and stored there and in your liver. They get carried through the body by the bloodstream to where they are needed. There are four fat-soluble vitamins, which are vitamin A, Vitamin E, Vitamin D, and Vitamin K. Because they are stored, they can build up since they do not leave the body quickly like water-soluble vitamins.

The magic duo of D3 and K2

Both vitamins D and K have their benefits, but if they are not taken together, you may not get the benefits you are striving for. Vitamin D3 and K2 work together for healthy bones, cardiovascular health, and strong immune system function. The fact is, they work better together than taken separately. (5) Without vitamin K2, vitamin D3 may not be able to be used in the body, which can lead to other health concerns. Using a supplement that has both vitamin D3 plus vitamin K2 eliminates the worry.


Vitamin D3 helps the body effectively absorb calcium and directs it from your intestines to your blood. This is where vitamin K2 jumps into action. Vitamin K2 directs that calcium into your bones. Without enough vitamin K2, calcium can build in the arteries and not reach your bones where you want it. (2)

Vitamin D3 with K2 for infants and children

A study of 107 healthy Caucasian women in a 9-month follow-up study found that their exclusively breastfed infants received <20% of the daily recommended amount of vitamin D during the first year of life. (3) 


The body absorbs about 10%-15% of calcium through diet. With sufficient vitamin D status, this increases to 30%-40%. (4) If a woman doesn’t have sufficient vitamin D levels, this can also affect her baby’s vitamin D status.


In one study, as much as 50% of infants younger than 5 days old were shown to be deficient in vitamin K. (6)


Breastmilk does contain vitamin D, but it may not be enough to raise the baby's levels. Often, vitamin D levels in babies are low because of the mother's insufficient vitamin D status. If a mom chooses not to give her baby a vitamin D3 supplement, she can take 6400 IU/day, which has been shown to be sufficient to raise her baby’s levels. (7) Otherwise, it is recommended to supplement the baby directly with 400 IU/day of vitamin D3. (8)


Although infant formula is fortified with vitamin D, your baby still may not be getting the Recommended Daily Allowance, especially in the first few weeks of life. Once the baby takes 32 ounces of formula daily, they should be given a Vitamin D3 supplement. Babies fed both breastmilk and formula, known as combo feeding, are also best protected by taking a vitamin D3 with a K2 supplement. 

Benefits of vitamin D3

  • Healthy bones and skeletal development
  • Development of healthy teeth
  • Healthy cardiovascular system
  • Maintain bone density
  • Helps maintain the body’s proper phosphorus and magnesium levels
  • Reduces the risk of high blood pressure
  • Reduces the risk of insulin resistance and diabetes

Forms of vitamin D

D2 - Has a shorter half-life compared to vitamin D3. It also doesn’t raise the levels of the active form of vitamin D in our body. Plants make vitamin D2. In supplements, it is often sourced from yeast exposed to UV light. It is cheaper to produce than vitamin D3.


D3 - We absorb vitamin D3 directly from sun exposure through our skin and foods like fatty fish, egg yolks, beef liver, and microalgae. (9) In one study, infants were given either a supplement of vitamin D2 or D3 for 3 months. The group with vitamin D2 had an increase of 4ng compared to infants given vitamin D3, who had an increase of 9ng. This meant that 75% of infants in the D2 group had sufficient levels after supplementation compared to 96% of infants in the vitamin D3 group. (10)

Benefits of vitamin K2

  • Helps blood clot, preventing newborn hemorrhaging
  • Bone mineralization
  • Healthy teeth formation
  • Aids brain development and a healthy nervous system
  • Healthy joints
  • Promotes healthy weight
  • Reduces respiratory tract infections when it  is taken with vitamin D3

Forms of vitamin K

K1 - shorter half-life, so levels fluctuate more in the body. Not the preferred form for supplementation.


K2 - To add to the complexity of choices, K2 may be in the form of MK-4 or MK-7. Vitamin K2 as MK-7, compared to K1, has a longer half-life and has been shown to raise Vitamin K serum levels in the body. (1) 


Taking a vitamin D3 + K2 from MK-7 supplement regularly will keep your vitamin K levels stable for longer. MK-4 is less well absorbed or bioavailable to the body than MK-7. In studies, MK-4 seemed less predictable than MK-7. It didn’t raise everyone’s vitamin K levels at the same time, even when it was being taken regularly. 


Only some vitamins work together, like vitamin D3 and K2, yet finding a high-quality vitamin supplement containing both is challenging. When taken together, you can be confident that your body can utilize calcium correctly, leading to strong bones, teeth, and a healthy cardiovascular system. 


Legendairy Milk Baby Vitamin D3 & K2 Liquid Drops supplement are a synergistic blend of plant-sourced Vitamin D3 and K2 MK-7 that work better together than alone. The vitamin D3 is sourced from certified organic, non-GMO, vegan algae. The vitamin K2 is sourced naturally from fermented chickpeas. The product is certified USDA Organic, non-GMO, and free from preservatives, additives, artificial flavors, colors, and common allergens.


Babies grow quickly, and they must have enough vitamin D3 and K2 starting right after birth to support their rapidly developing body, skeletal system, healthy gums and teeth, and a strong immune system. It is always recommended to check with your doctor or care provider before beginning a new supplement.

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