The Truth About The 6 Most Common Nutrient Deficiencies, According To A Functional Medicine Expert

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It’s no secret that our bodies require fuel to function. But it’s not enough to just drink water and eat three meals a day. Looking at it on a deeper level, our bodies actually need a specific ratio of various nutrients in order to thrive. Everything from our hormones, cognitive function, energy levels, mood, blood sugar, and more relies on these nutrients to keep your body running the way it should. But how do we know which nutrients we should prioritize? After all, there are a lot. While the specific nutrients you should focus on can vary based on your unique health case, these are the most common deficiencies I see in my telehealth functional medicine clinic, and the ones that can have the greatest impact on your health if left unchecked.


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1. Vitamin D

Vitamin D, often recognized as the "sunshine vitamin," is a crucial nutrient that acts more like a hormone within your body than a vitamin by regulating hundreds of important pathways in your body. In fact, it’s one of the few things that every single cell of your body requires to function how it should. Primarily known for its role in maintaining optimal bone health, it facilitates the absorption of essential minerals like calcium and phosphorus and plays a vital role vitamin D plays an important part in immune function, assisting the body in fending off infections and reducing inflammation. Its influence extends to supporting muscle strength, regulating cell growth, mood balance, and cardiovascular health. Needless to say, deficiencies in this nutrient only spell trouble for your health long-term.

Symptoms and side effects of deficiency: Due to its profound influence on multiple areas of your health, studies have linked deficiencies in Vitamin D to a wide range of health problems, including:

Recommended levels and daily intake: In functional medicine, we consider anywhere between 60 and 80 ng/mL to be ideal. According to the National Institutes of Health, (9) you should be getting 15 mcg or 600 IU of Vitamin D per day - or anywhere between 2,000 and 6,000 IU per day if you are taking supplements to correct deficiencies. 

What to look for in a supplement: Since Vitamin D is fat-soluble, you’ll want to look for a supplement that is paired with another fat-soluble vitamin to take advantage of vitamin synergy. For example, most vitamin D supplements - like my supplement The D3-K2 - combine these two nutrients together as they enhance absorption of one another.

Foods high in Vitamin D: Vitamin D is primarily found in animal-based foods and is particularly abundant in seafood in dairy products.

  • Cod liver oil: 440 IU (1 teaspoon)
  • Sardines: 164 IU (3 ounces)
  • Salmon: 400 IU (3 ounces)
  • Mackerel: 400 IU (3 ounces)
  • Tuna: 228 IU (3 ounces)
  • Raw grass-fed milk: 98 IU (1 cup)
  • Caviar: 33 IU (1 cup)
  • Organic eggs: 1 large: 41 IU (10 percent Daily Value)
  • Mushrooms: 1 cup: 2 IU (1 percent Daily Value)

For a complete guide to Vitamin D, check out my article here.

2. Iodine

Iodine is considered an “essential nutrient”, and because your body doesn’t produce it on its own, you have to get it through food, drink, or supplementation. Iodine is a trace mineral that’s found naturally on earth and in certain foods but the tough thing about iodine is that it’s not found in all that many common foods, making deficiencies in this nutrient extremely common.

The problem is, iodine is necessary for thyroid health since it is actually part of your thyroid hormones themselves. For example, you may have heard of “T3” and “T4” thyroid hormones; well, “3” and “4” stand for the number of iodine atoms it takes to produce each of these hormones. When you don’t get enough iodine, it can influence your thyroid hormone synthesis, which is a problem because the thyroid is largely in control of your metabolic activity and can lead to downstream metabolic health problems.

Symptoms and side effects of deficiency: Two of the biggest side effects of severe iodine deficiency is the development of hypothyroidism and the presence of a goiter - an enlarged thyroid gland characterized by swelling or a lump in the front of your neck. Because of iodine’s close connection to thyroid health, symptoms of iodine deficiency often coincide with symptoms of thyroid problems:

  • Fatigue 
  • Weight Gain / weight loss resistance
  • Sensitivity to cold
  • Dry Skin
  • Brittle nails
  • Hair loss
  • Brain fog
  • Swelling
  • Mood swings
  • Depression
  • Anxiety

Recommended levels and daily intake: Since everyone’s biochemistry is unique, your iodine levels can vary but they should be (10) in the 100–199 mcg/L range with a recommended daily intake of 150 mcg of iodine per day for adults.

What to look for in a supplement: You’ll want to take your thyroid hormones into consideration before supplementing with iodine as studies have found (11) that excess iodine also contributes to thyroid problems just as much as an iodine deficiency does.

Foods high in iodine: Other than fortified food products, iodine is mainly found in sea vegetables and seafood.

  • Cod: 158 mcg (3 ounces)
  • Seaweed: 116 mcg (2 tablespoons)
  • Oysters: 93 mcg (3 ounces)
  • Egg: 26 mcg (1 cooked)
  • Iodized salt: 45 mcg (1 gram)
  • Beef liver: 14 mcg (3 ounces)

Read my article here for a complete guide to iodine, its relationship to your thyroid health, and the best iodine supplements.

3. Iron

Iron is a mineral that’s found on earth and in certain foods. Our bodies don’t make iron, so we have to get it through the foods we eat. Iron plays a significant role in your health as it is a key component of hemoglobin - the protein in red blood cells responsible for carrying oxygen from the lungs to different parts of the body. It is also involved in the production of certain enzymes and is necessary for proper growth and development. While deficiencies in iron can be minimal, if left unchecked iron deficiency can continue to get worse and can result in iron-deficiency anemia.

Symptoms and side effects of deficiency:

  • Extreme fatigue
  • Weakness
  • Pale skin
  • Chest pain, fast heartbeat or shortness of breath
  • Headache, dizziness or lightheadedness
  • Cold hands and feet
  • Inflammation or soreness of your tongue
  • Brittle nails
  • Unusual cravings for non-nutritive substances, such as ice, dirt, or starch
  • Poor appetite, especially in infants and children with iron deficiency anemia

Recommended levels and daily intake: Since women are more likely than men to struggle with iron deficiency - for many reasons, one of them being menstruation - they require more iron. In fact, the daily recommended intake for iron is only 8 mg for men compared to 18 mg for women and 27 mg for pregnant women. According to the National Institutes of Health, (12) levels below 30 mcg/L indicate iron deficiency whereas anything lower than 10 mcg/L indicates iron-deficiency anemia.

What to look for in a supplement: For an iron supplement you’ll want to look for brands that prioritize sourcing and purity with their ingredients while taking into consideration the recommended dosage based on your starting levels and whether or not you are looking to maintain iron levels or correct a deficiency.

Foods high in iron: There are two types of iron - heme and non-heme. Heme iron has higher bioavailability and is found in animal foods whereas non-heme is found in plant-based foods.

  • Oysters: 8 mg (3 ounces)
  • White beans: 8 mg (1 cup)
  • Beef liver: 5 mg (3 ounces)
  • Lentils: 3 mg (½ cup)
  • Spinach: 3 mg (½ cup)
  • Kidney beans: 2 mg (½ cup)
  • Sardines: 2 mg (3 ounces)

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4. Magnesium

As the fourth most abundant mineral (13) in your body, magnesium is crucial to accomplishing at least 300 important biochemical reactions. Alongside calcium, sodium, and potassium, magnesium is considered an electrolyte that is needed for energy production, protein synthesis, muscle and nerve function, regulation of blood sugar levels, and the formation of bones and DNA. Magnesium is also necessary for maintaining a steady heartbeat and supporting the immune system. Most deficiencies come from a poor diet or gut problems that interfere with magnesium absorption.

Symptoms and side effects of deficiency: Magnesium deficiency can affect multiple areas of your health, but these are the most common problems I see associated with low magnesium levels.

Recommended levels and daily intake: In general, you want your magnesium levels (18) to be anywhere between 1.8 to 2.6 mg/dL. For the average adult male, a daily recommended intake (13) of 400-420 mg is ideal, with women requiring a daily intake between 310-320 mg.

What to look for in a supplement: There are many different forms of magnesium, each with their own advantages. Magnesium oxide is the form most commonly found in supplements, but it is not nearly as easily absorbed as other forms. Magnesium citrate is a good option for its laxative properties, whereas magnesium glycinate is excellent for its calming effects, and magnesium threonate has shown promise for its neurological support.

If you are looking to correct nutrient deficiencies, I recommend The Magnesium from my supplement collection since it is formulated with carefully curated magnesium compounds backed by research and studies for their enhanced bioavailability – specifically Albion chelated magnesium plus Magtein™ (magnesium L-threonate), the only form of magnesium proven in animal studies to cross the blood-brain barrier.

Foods high in magnesium: Magnesium is abundant in many common plant-based foods, making this one of the easier nutrients to meet the recommended daily intake.

  • Spinach: 157 mg (1 cup)
  • Swiss chard: 154 mg (1 cup)
  • Dark chocolate: 95 mg (1 square)
  • Pumpkin seeds: 92 mg (⅛ cup)
  • Almonds: 80 mg (1 ounce)
  • Black beans: 60 mg (½ cup)
  • Avocado: 58 mg (1 medium)
  • Salmon: 53 mg (1 filet)
  • Kefir: 50 mg (1 cup)
  • Figs: 50 mg (½ cup)
  • Banana: 32 mg (1 medium)

5. Vitamin B12

Vitamin B12 is a water-soluble vitamin and an essential nutrient, meaning your body requires it for proper functioning but cannot produce it on its own, so it must be obtained through the diet or supplements. Vitamin B12 is involved in several key processes in the body including, red blood cell formation, nervous system function, DNA synthesis, and the maintenance of healthy cells.

Symptoms and side effects of deficiency: Since Vitamin B12 plays such a significant role in your neurological and cardiovascular health, symptoms can manifest primarily in these areas.

What to look for in a supplement: While B12 is super important, there are so many different types of B vitamins necessary for your health, making it important to get a well-rounded amount of each. I recommend looking for a B-complex vitamin containing methylated B vitamins, like my supplement The Methylator, that contains activated forms of B vitamins like B9 L-Methylfolate (L-5-MTHF), B6 Pyridoxyl-5-Phosphate (P5P), and B12 versions (such as Adenosyl B12, Cyano B12, Hydroxycobalamin B12, or Methyl 12).

Recommended levels and daily intake: According to most researchers, your levels should be (23) between 200 or 250 pg/mL and it is recommended to get approximately 2.4 mcg of Vitamin B12 per day. 

Foods high in B12: Vitamin B12 is naturally found in animal products such as meat, fish, poultry, eggs, and dairy products, but is difficult to find in plant-based foods unless they are fortified with B12.

  • Beef liver: 70 mcg (3 ounces)
  • Clams: 17 mcg (3 ounces)
  • Salmon: 2.6 mcg (3 ounces)
  • Tuna: 2.5 mcg (3 ounces)
  • Beef: 2.4 (3 ounces)

6. Vitamin A

Vitamin A is essential for a strong, balanced immune system. Some researchers believe this has to do with our dendritic cells; our alarm cells of the immune system that can send out a “red alert” to stimulate immunity, or a “calm down” message that tones down excessive immune reactions that can damage the body. The “calm down” message makes use of vitamin A.

Symptoms and side effects of deficiency: 

What to look for in a supplement: Because of the poor absorption rate of plant-based sources of Vitamin A, you’ll want to look for a supplement that utilizes Vitamin A sources from fish liver oil or retinyl palmitate.

Recommended levels and daily intake: About 900 mcg of Vitamin A is recommended (29) for males per day and 700 mcg is recommended for women.

Foods high in Vitamin A: ​​Plant beta-carotenes, a precursor to vitamin A, are found in sweet potatoes and carrots but the conversion rate to the usable retinol is very weak at only 3 percent. (30) Therefore, animal sources are going to provide the highest bioavailability to help overcome deficiencies.

  • Beef liver: 6,582 mcg (3 ounces)
  • Sweet potato: 1,403 (1 whole)
  • Carrots: 459 (½ cup)
  • Egg: 75 mcg (1 large, hardboiled)
  • Salmon: 59 mcg ( 3 ounces)
  • Tuna: 20 mcg (3 ounces)

The Takeaway

The truth is, these nutrient deficiencies only give us a snapshot into the vast amount of deficiencies we face as a society. Other common deficiencies I see in my telehealth clinic include Vitamin E, Vitamin K2, and other nutrients like collagen, selenium, and zinc to name a few. Lab work is the best way to determine if you have any deficiencies so that you can make make necessary dietary changes and target your supplements to your individual needs.

As one of the first functional medicine telehealth clinics in the world, we provide webcam health consultations for people around the globe. 


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Dr. Will Cole, DNM, IFMCP, DC is a leading functional medicine expert who consults people around the globe, starting one of the first functional medicine telehealth centers in the world. Named one of the top 50 functional and integrative doctors in the nation, Dr. Will Cole provides a functional medicine approach for thyroid issues, autoimmune conditions, hormonal imbalances, digestive disorders, and brain problems. He is the host of the popular The Art Of Being Well podcast and the New York Times bestselling author of Intuitive Fasting, Ketotarian, The Inflammation Spectrum and the brand new book Gut Feelings: Healing the Shame-Fueled Relationship Between What You Eat and How You Feel.

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