The Difference Between Fat-Soluble and Water-Soluble Vitamins
“Do you take vitamins?” “Do you buy them at a vitamin store?”
The word “vitamin” has become a catch-all term used to refer to almost any dietary supplement. But the fact is that vitamins are very different from other categories of supplements like herbs, amino acids and enzymes. For one thing, Americans often lack sufficient vitamin intake. Research shows that only 10% of Americans eat a good diet,[i] [ii] and even those that do may receive insufficient amounts of vitamin D, vitamin E and choline.[iii] Clearly, a need for vitamin supplementation exists.
For many, this need is met with the use of a multivitamin supplement. In fact, a survey by the Council for Responsible Nutrition revealed that multivitamins are the most popular supplement, with 56% of U.S. adults using them.
Of course there are many other types supplements that contain vitamins, including sports nutrition products, and a broad range of specialty formulas. Naturally, it's important to understand the role that each vitamin performs in order to choose the right ones for any given dietary supplement formulation; and the first thing we need to understand is the difference between fat-soluble and water-soluble vitamins.
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What Are Water-Soluble Vitamins?
Water-soluble vitamins are those vitamins that can dissolve in water. Water-soluble vitamins are carried to the body's tissues but are not stored in the body. Consequently it is less likely to reach toxic levels of these vitamins. Water-soluble vitamins are found in plant and animal foods or dietary supplements and must be taken in daily. Vitamin C and members of the vitamin B-complex are water-soluble.
What Are Fat-Soluble Vitamins?
Fat-soluble vitamins are those vitamins that can dissolve in fats and oils. Fat-soluble vitamins are absorbed along with fats in the diet and can be stored in the body’s fatty tissue. This means that it may be possible to reach toxic levels of these vitamins—although it would require an excessively high intake in order to do that. Fat-soluble vitamins come from plant and animal foods or dietary supplements. Vitamins A, D, E, and K are fat-soluble.
Functions of Water-Soluble Vitamins
Vitamin C is best known for its role in the synthesis of collagen, a connective tissue protein used as a structural component of blood vessels, tendons, ligaments, and bone. This water-soluble vitamin is also needed for the synthesis of: 1) the neurotransmitter norepinephrine, which performs critical brain function including an effect on mood, and 2) the amino acid carnitine, which is essential for the transport of fat into cellular mitochondria, where the fat is converted to energy or ATP.[iv]
Another significant function of vitamin C is the important role it plays as an antioxidant, protecting vital molecules in the body from damage by free radicals and reactive oxygen species. These molecules include proteins, lipids (fats), carbohydrates, and nucleic acids (DNA and RNA). Vitamin C also plays a complementary role with other antioxidants, such as vitamin E, helping to regenerate them from their oxidized from back into their reduced (active) form. [v] [vi]
Vitamin C also plays a profound role in the health of the immune system, stimulating the production and function of white blood cells, including leukocytes, neutrophils, lymphocytes and phagocytes.[vii] [viii] [ix] [x] [xi] [xii] [xiii] In addition, research has demonstrated that supplemental vitamin C promotes healthy serum levels of antibodies[xiv] [xv] and C1q complement proteins.[xvi] [xvii] [xviii]
Each of the B vitamins has their own functions to serve in the body, but in general they may be considered to play a role in energy metabolism and helping to promote homeostasis when the body is under stress. The use of the entire B-Complex is recommended since the individual B vitamins affect one another’s absorption, metabolism, and excretion.[xix]
Each of the B vitamins is converted into coenzymes in the body. These B vitamin coenzymes are involved, directly or indirectly in energy metabolism. Active individuals with poor or marginal nutritional status for a B-vitamin may have decreased ability to perform exercise at high intensities. Exercise stresses metabolic pathways that depend on thiamine, riboflavin, and vitamin B-6. Consequently, the requirements for these vitamins may be increased in athletes and active individuals.[xx]
The B-complex vitamins are intimately involved in the function of the nervous system,[xxi] and so can play a role in helping to counter some of the negative effects of stress. In fact, the ability of humans to respond to stresses can be influenced by nutritional status—including the status of key B vitamins.[xxii] In one study, vitamin B1 (thiamine) and vitamin B6 (pyridoxine) together were found to be especially necessary for workers whose activity is associated with nervous-emotional stress.[xxiii] Similar results were seen in a previous study.[xxiv]
A substantial body of scientific evidence suggests that generous intakes of three B vitamins may help promote cardiovascular health in the United States. The particular B vitamins involved are folic acid, vitamin B6 and vitamin B12. Research indicates that these vitamins help promote healthy levels of homocysteine, the amino acid byproduct of metabolism. This is important since keeping homocysteine levels in check are important for maintaining cardiovascular health. This has been shown in numerous studies using vitamin B6, vitamin B12 and folic acid; either individually or in combination.[xxv] [xxvi] [xxvii] [xxviii] [xxix] [xxx] [xxxi] [xxxii] [xxxiii]
Functions of Fat-Soluble Vitamins
Vitamin A plays an important role in vision, bone growth, reproduction, gene expression (i.e., the process by which the information coded in genes (DNA) is converted to proteins and other cellular structures), cell division, and cell differentiation (i.e., the process during which a cell becomes part of the brain, muscle, lungs, blood, or other specialized tissue).[xxxiv] [xxxv] [xxxvi] [xxxvii] [xxxviii] Vitamin A helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses.[xxxix] [xl] [xli] [xlii] [xliii] [xliv]
Vitamin A is available in two basic forms. The first is preformed vitamin A, or retinol—an active and usable form of this nutrient. The other is beta-carotene, which can be converted into retinol in the body. About 26% of vitamin A consumed by men and 34% of vitamin A consumed by women as beta-carotene carotenoids in the United States.[xlv]
Vitamin A toxicity is caused by over consumption of preformed vitamin A (retinol), not beta-carotene. Since preformed vitamin A is rapidly absorbed and slowly cleared from the body, toxicity may result acutely from high-dose exposure over a short period of time or chronically from a much lower intake.[xlvi] Toxicity is most likely to occur with long-term consumption of preformed vitamin A in doses exceeding 25,000 to 33,000 IU/day. In January 2001, the Food and Nutrition Board (FNB) of the Institute of Medicine set the tolerable upper intake level (UL) of vitamin A intake for adults 10,000 IU/day of preformed vitamin A.[xlvii]
Vitamin D is the “sunshine vitamin”, so coined because exposure to the sun’s ultraviolet light will convert a form of cholesterol under the skin into vitamin D. This nutrient is best known for its role in helping to facilitate the absorption of calcium and phosphorus (as well as magnesium), and so helping to promote bone health.[xlviii] In addition, vitamin D: 1) supports the normal proliferation of cells and stimulates the differentiation of cells (specialization of cells for specific functions),[xlix] 2) is a potent immune system modulator,[l] 3) plays a role in insulin secretion under conditions of increased insulin demand,[li] and 4) may be important for supporting the renin-angiotensin system’s regulation of blood pressure.[lii]
Outright vitamin D deficiency is present in 41.6 percent of the U.S. population,[liii] while vitamin D insufficiency (i.e., lacking sufficient vitamin D) is present in 77 percent of the population.[liv] If you are deficient in vitamin D you will not be able to absorb enough calcium to satisfy your body’s calcium needs.[lv] It has long been known that severe vitamin D deficiency has serious consequences for bone health, but other research indicates that lesser degrees of vitamin D deficiency are common and increase the risk of osteoporosis and other health problems.[lvi] [lvii]
From a scientific perspective, vitamin E is not just one compound (as is the case with other vitamins), but rather a family of eight isomers. These include alpha-, beta-, gamma-, and delta-e tocopherols, and are called isomers. In this context, the term isomer refers to compounds with the same structural formula but different spatial arrangements of atoms.[lviii] The other four isomers are alpha-, beta-, gamma-, and delta- tocotrienols (a family of natural compounds related to tocopherols). However, according to the Food and Nutrition Board, Institute of Medicine, vitamin E is defined as one specific compound, alpha-tocopherol. The reason for this is that alpha-tocopherol is the only form of vitamin E that is actively maintained in the human body and found in the largest quantities in blood and tissues.[lix] Consequently, alpha-tocopherol is the only form that meets the latest Recommended Dietary Allowance (RDA) and Daily Value (DV) for vitamin E. Nevertheless, the other tocopherols and tocotrienols do have value to human health.
The primary function of vitamin E is as an antioxidant. Since lipids (fats) are part of all cell membranes, fat-soluble vitamin E is ideally suited to quench free radicals that would otherwise compromise the integrity of cell membranes. Vitamin E also protects LDL cholesterol from oxidation, which is important since oxidized LDLs have been implicated in the development of cardiovascular diseases.[lx]
In addition to its role as an antioxidant, vitamin E has also been shown to: 1) affect the expression and activities of molecules and enzymes in immune and inflammatory cells, and 2) inhibit platelet aggregation and to enhance vasodilation (which promotes circulation).[lxi] [lxii]
According to the National Health and Nutrition Examination Survey (NHANES) 2003-2006, Americans’ average dietary intake of vitamin E from food (including enriched and fortified sources) is 6.9 mg/day (10.35 IU). This intake is well below the current Daily Value of 15 mg/day (22.5 IU). This means that more than 90% of Americans do not meet RDA for vitamin E.[lxiii]
Vitamin K, a fat-soluble vitamin, was first known for its essential role in the functioning of several proteins involved in blood clotting or coagulation (which is important to prevent excessive bleeding after an injury).[lxiv] In fact, the “K” in vitamin K is derived from the German word “koagulation.” Now, vitamin K is understood to offer additional benefits, including its role vitamin-K dependent proteins which are necessary for bone mineralization (e.g., helping to keep calcium in the bone).[lxv] Likewise, vitamin K has been shown to help promote healthy arteries.[lxvi] [lxvii]
The two naturally occurring forms of vitamin K are K1 and K2. Plants synthesize phylloquinone, which is also known as vitamin K1. Friendly intestinal bacteria synthesize a range of vitamin K2 forms collectively referred to as menaquinones.[lxviii]The bone health and arterial health contributions are specific to vitamin K2.
Knowing the fundamental differences between the various water-soluble and fat-soluble vitamins is a great way for brand owners and potential brand owners to stay informed, increase their authority, and – most importantly – increase their ability to meet their customers' needs.
[i] Report Card on the Quality of Americans’ Diets. Nutrition Insights, INSIGHT 28. USDA Center for Nutrition Policy and Promotion. December 2002.
[ii] Diet Quality of Americans in 2001-02 and 2007-08 as Measured by the Healthy Eating Index-2010. Nutrition Insight 51. USDA Center for Nutrition Policy and Promotion. April 2013.
[iii] United States Department of Agriculture. Center for Nutrition Policy and Promotion/ Comparison of Nutrient Content of each 2010 USDA Food Pattern to Nutritional Goals for that Pattern. Retrieved August 19, 2014 from http://www.cnpp.usda.gov/Publications/USDAFoodPatterns/ComparisonofNutrientContentofeach2010USDAFoodPatterntoNutritionalGoalsforthatPattern.pdf.
[iv] Carr AC, Frei B. Toward a new recommended dietary allowance for vitamin C based on antioxidant and health effects in humans. Am J Clin Nutr. 1999;69(6):1086-1107.
[v] Carr AC, Frei B. Toward a new recommended dietary allowance for vitamin C based on antioxidant and health effects in humans. Am J Clin Nutr. 1999;69(6):1086-1107.
[vi] Bruno RS, Leonard SW, Atkinson J, et al. Faster plasma vitamin E disappearance in smokers is normalized by vitamin C supplementation. Free Radic Biol Med. 2006;40(4):689-697.
[vii] Prinz W, Bortz R, Bregin B, Hersch M. The effect of ascorbic acid supplementation on some parameters of the human immunological defence system. Int J Vitam Nutr Res. 1977;47(3):248-257.
[viii] Vallance S. Relationships between ascorbic acid and serum proteins of the immune system. Br Med J. 1977;2(6084):437-438.
[ix] Kennes B, Dumont I, Brohee D, Hubert C, Neve P. Effect of vitamin C supplements on cell-mediated immunity in old people. Gerontology. 1983;29(5):305-310.
[x] Panush RS, Delafuente JC, Katz P, Johnson J. Modulation of certain immunologic responses by vitamin C. III. Potentiation of in vitro and in vivo lymphocyte responses. Int J Vitam Nutr Res Suppl. 1982;23:35-47.
[xi] Jariwalla RJ, Harakeh S. Antiviral and immunomodulatory activities of ascorbic acid. In: Harris JR (ed). Subcellular Biochemistry. Vol. 25. Ascorbic Acid: Biochemistry and Biomedical Cell Biology. New York: Plenum Press; 1996:215-231.
[xii] Levy R, Shriker O, Porath A, Riesenberg K, Schlaeffer F. Vitamin C for the treatment of recurrent furunculosis in patients with imparied neutrophil functions. J Infect Dis. 1996;173(6):1502-1505.
[xiii] Anderson R, Oosthuizen R, Maritz R, Theron A, Van Rensburg AJ. The effects of increasing weekly doses of ascorbate on certain cellular and humoral immune functions in normal volunteers. Am J Clin Nutr. 1980;33(1):71-76.
[xiv] Prinz W, Bloch J, Gilich G, Mitchell G. A systematic study of the effect of vitamin C supplementation on the humoral immune response in ascorbate-dependent mammals. I. The antibody response to sheep red blood cells (a T-dependent antigen) in guinea pigs. Int J Vitam Nutr Res. 1980;50(3):294-300.
[xv] Feigen GA, Smith BH, Dix CE, et al. Enhancement of antibody production and protection against systemic anaphylaxis by large doses of vitamin C. Res Commun Chem Pathol Pharmacol. 1982;38(2):313-333.
[xvi] Haskell BE, Johnston CS. Complement component C1q activity and ascorbic acid nutriture in guinea pigs. Am J Clin Nutr. 1991;54(6 Suppl):1228S-1230S.
[xvii] Johnston CS, Cartee GD, Haskell BE. Effect of ascorbic acid nutriture on protein-bound hydroxyproline in guinea pig plasma. J Nutr. 1985;115(8):1089-1093.
[xviii] Johnston CS, Kolb WP, Haskell BE. The effect of vitamin C nutriture on complement component C1q concentrations in guinea pig plasma. J Nutr. 1987;117(4):764-768.
[xix] Whitney E, Rolfes S. Understanding Nutrition, Ninth Edition, Belmont, CA: Wadsworth/Thomson Learning; 2002.
[xx] Manore MM. Effect of physical activity on thiamine, riboflavin, and vitamin B-6 requirements. Am J Clin Nutr 2000 Aug;72(2 Suppl):598S-606S.
[xxi] Whitney E, Rolfes S. Understanding Nutrition, Ninth Edition, Belmont, CA: Wadsworth/Thomson Learning; 2002.
[xxii] Sauberlich HE. Implications of nutritional status on human biochemistry, physiology, and health. Clin Biochem 1984; 17(2):132-42.
[xxiii] Bondarev GI, Martinchik AN, Khotimchenko SA, et al. [Correlation of the actual vitamin B1, B2 and B6 consumption with the biochemical indices of their body allowance] Korreliativnaia vzaimosviaz' fakticheskogo potrebleniia vitaminov B1, B2 i B6 s biokhimicheskimi pokazateliami obespechennosti imi organizma. Vopr Pitan 1986; (2):34-7.
[xxiv] Bogdanov NG, Bondarev GI, Piatnitskaia IN, et al. [Vitamin status of diamond cutters] Vitaminnyi status rabochikn, zaniatykh promyshlennoi obrabotkoi almazov. Vopr Pitan 1984; (2):28-31.
[xxv] Bjorkegren K, Svardsudd. Elevated serum levels of methylmalonic acid and homocysteine in elderly people. A population-based intervention study., J Intern Med 1999; 246(3):317-24.
[xxvi] Rasmussen K, Moller J, Lyngbak M. Within-person variation of plasma homocysteine and effects of posture and tourniquet application. Clin Chem 1999; 45(10):1850-5.
[xxvii] Kunz K, Petitjean P, Lisri M, et al. Cardiovascular morbidity and endothelial dysfunction in chronic haemodialysis patients: Is homocyst(e)ine the missing link? Nephrol Dial Transplant 1999; 14(8):1934-42.
[xxviii] Alpert MA, Homocysteine, atherosclerosis, and thrombosis. South Med J 1999; 92(9):858-65.
[xxix] Bellamy MF, McDowell IF, Ramsey MW, et al. Oral folate enhances endothelial function in hyperhomocysteinaemic subjects. Eur J Clin Invest 1999; 29(8):659-62.
[xxx] Woodside JV, Young IS, Yarnell JWG, et al. Antioxidants, but not B-group vitamins increase the resistance to low-density lipoprotein to oxidation: a randomised, factorial design, placebo-controlled trial. Atherosclerosis 1999; 144(2):419-27.
[xxxi] Bronstrup A, Hages M, Pietrzik K. Lowering of homocysteine concentrations in elderly men and women. Int J Vitam Nutr Res 1999; 69(3):187-93.
[xxxii] Suliman ME, Divino Filho JC, Barany P, et al. Effects of high-dose folic acid and pyridoxine on plasma and erythrocyte sulfur amino acids in hemodialysis patients. J Am Soc Nephrol 1999; 10(6):1287-96.
[xxxiii] Mansoor MA, Kristensen O, Hervig T, et al. Plasma total homocysteine response to oral doses of folic acid and pyridoxine hydrochloride (vitamin B6) in healthy individuals. Oral doses of vitamin B6 reduce concentration of serum folate. Scand J Clin Lab Invest 1999; 59(2):139-46.
[xxxiv] Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academy Press, Washington, DC, 2001.
[xxxv] Gerster H. Vitamin A-functions, dietary requirements and safety in humans. Int J Vitam Nutr Res 1997;67:71-90.
[xxxvi] Futoryan T, Gilchrest BE. Retinoids and the skin. Nutr Rev 1994;52:299-310.
[xxxvii] Hinds TS, West WL, Knight EM. Carotenoids and retinoids: A review of research, clinical, and public health applications. J Clin Pharmacol 1997;37:551-8.
[xxxviii] Ross AC, Gardner EM. The function of vitamin A in cellular growth and differentiation, and its roles during pregnancy and lactation. Adv Exp Med Biol 1994;352:187-200.
[xxxix] Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academy Press, Washington, DC, 2001.
[xl] Ross AC. Vitamin A and retinoids. In: Modern Nutrition in Health and Disease. 9th Edition (edited by Shils ME, Olson J, Shike M, Ross AC). Lippincott Williams and Wilkins, New York, 1999, pp. 305-27.
[xli] Ross AC, Stephensen CB. Vitamin A and retinoids in antiviral responses. FASEB J 1996;10:979-85.
[xlii] Semba RD. The role of vitamin A and related retinoids in immune function. Nutr Rev 1998;56:S38-48.
[xliii] Ross DA. Vitamin A and public health: Challenges for the next decade. Proc Nutr Soc 1998;57:159-65.
[xliv] Harbige LS. Nutrition and immunity with emphasis on infection and autoimmune disease. Nutr Health 1996;10:285-312.
[xlv] Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academy Press, Washington, DC, 2001.
[xlvi] Ross AC. Vitamin A and retinoids. In: Shils M, Olson JA, Shike M, Ross AC. ed. Modern Nutrition in Health and Disease. 9th ed. Baltimore: Lippincott Williams & Wilkins; 1999:305-327.
[xlvii] Food and Nutrition Board, Institute of Medicine. Vitamin A. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, D.C.: National Academy Press; 2001:65-126.
[xlviii] Holick MF. Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis. Am J Clin Nutr. 2004;79(3):362-371.
[xlix] Holick MF. Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis. Am J Clin Nutr. 2004;79(3):362-371.
[l] Lin R, White JH. The pleiotropic actions of vitamin D. Bioessays. 2004;26(1):21-28.
[li] Zeitz U, Weber K, Soegiarto DW, Wolf E, Balling R, Erben RG. Impaired insulin secretory capacity in mice lacking a functional vitamin D receptor. FASEB J. 2003;17(3):509-511.
[lii] Li YC, Kong J, Wei M, Chen ZF, Liu SQ, Cao LP. 1,25-Dihydroxyvitamin D(3) is a negative endocrine regulator of the renin-angiotensin system. J Clin Invest. 2002;110(2):229-238.
[liii] Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res. 2011;31(1):48-54.
[liv] Ginde AA, Liu MC, Camargo CA Jr. Demographic differences and trends of vitamin D insufficiency in the US population, 1988-2004. Arch Intern Med. 2009;169:626-32.
[lv] Holick MF. Vitamin D: A millennium perspective. J Cell Biochem. 2003;88(2):296-307.
[lvi] Heaney RP. Long-latency deficiency disease: insights from calcium and vitamin D. Am J Clin Nutr. 2003;78(5):912-919.
[lvii] Zittermann A. Vitamin D in preventive medicine: are we ignoring the evidence? Br J Nutr. 2003;89(5):552-572.
[lviii] Seager SL, Slabaugh MR. Chemistry for Today: General, Organic, and Biochemistry, Fourth Edition. Pacific Grove, California: Brooks/Cole; 2000:364.
[lix] Traber MG. Utilization of vitamin E. Biofactors. 1999;10(2-3):115-120.
[lx] Traber MG. Vitamin E. In: Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, eds. Modern Nutrition in Health and Disease. Philadelphia: Lippincott Williams & Wilkiins; 2006:396-411.
[lxi] Food and Nutrition Board, Institute of Medicine. Vitamin E. Dietary reference intakes for vitamin C, vitamin E, selenium, and carotenoids. Washington D.C.: National Academy Press; 2000:186-283.
[lxii] Traber MG. Does vitamin E decrease heart attack risk? summary and implications with respect to dietary recommendations. J Nutr. 2001;131(2):395S-397S.
[lxiii] Fulgoni VL 3rd, Keast DR, Bailey, Dwyer J. Foods, fortificants, and supplements: where do Americans get their nutrients? J Nutr. 2011;141(10):18-47-1854.
[lxiv] Brody T. Nutritional Biochemistry. 2nd ed. San Diego: Academic Press; 1999.
[lxv] Shearer MJ. The roles of vitamins D and K in bone health and osteoporosis prevention. Proc Nutr Soc 1997;56(3):915-937.
[lxvi] Geleijnse JM, Vermeer C, Grobbee DE, et al. Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: The Rotterdam Study. J Nutr 2004;134:3100-5.
[lxvii] Beulens JW, Bots ML, Atsma F, et al. High dietary menaquinone intake is associated with reduced coronary calcification. Atherosclerosis 2008;203(2):489-93.
[lxviii] Shearer MJ. Vitamin K. Lancet. 1995;345(8944):229-234.