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:

• Autoimmune disease (1)
• Cancer (2)
• Cardiovascular disease (3)
• Cognitive impairment and dementia (4) 
• Depression (5)
• Inflammation (6)
• Sleep problems (7)
• Weak bones (8)

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)