My quest to understand vitamins concludes with this post. In reading The Vitamins by Combs and McClung, I sought answers to popular questions on the topic. What I found was that we are asking the wrong questions. It’s not about natural or synthetic. It’s about bioavailability and biopotency. It’s not about deficiency diseases. It’s about maintaining vitamin stores. It’s not about curing or preventing chronic diseases. It’s about maintaining a healthy balance. This week it’s not about fortification. It’s about the food matrix and biopotency.
Now, to answers about the last two questions I asked:
7.What is the difference in nutritional value between intrinsic and fortified products? Words matter. We all want to know how effective a vitamin is when it gets into our body. Will it help me achieve a healthy lifestyle? Will it protect me from bone fractures, cancer, or other worrisome illnesses? Will it perform good things on its own, or does it need to work with other nutrients? What do we call our expectations for nutrient performance? I called it ‘nutritional value’ in my teaching career and in questions 7 and 8. Note, I prepared all questions before I opened the book. Combs and McClung call it ‘biopotency.’ In these answers I will use biopotency.
Biopotency is “the intrinsic activity of a vitamer.” The term relates to a vitamer’s performance in cellular physiology and metabolism. Note, vitamers are different forms of molecules with vitamin activity but different biopotencies. Further complicating matters is that the vitamer must reach its target site in the body. That is where bioavailability comes in. Vitamers can be unstable in foods before eating. During digestion, the food structure or matrix either enhances or diminishes vitamer absorption. Remember that niacin is bound in corn and is not absorbed in the gut. Home processing by soaking and cooking corn in lime releases niacin for absorption. Some vitamers are more readily absorbed than others. Once absorbed, the vitamer is ready for transport to the target site.
Biopotency of folate vitamers presents an interesting lesson in vitamin nutrition. Leafy green vegetables have high levels of folates. Boiling of greens drives folates into the water, making it unavailable to us. Antifolates in the food matrix bind to certain folate vitamers preventing absorption. Other vitamers become unstable in the acidic conditions of the gut. Patients with enteric diseases are least able to absorb useable forms of folate. The presence of vitamin C during digestion enhances folate absorption. FDA mandated fortification of cereal-grain products with folate in 1998. The goal was to prevent neural tube defects in babies leading to birth defects. In the past two decades, this mandate has proved very successful. Folic acid is the most bioavailable of all folate vitamers. It is the form present in foods fortified with folate.
Folic acid is the most recent success story for fortification. Fortification of cereal grains with niacin helped to eradicate pellagra in the US. It was also effective at eliminating other vitamin deficiency diseases. Italy used a different strategy that was as successful. FDA approved vitamin D as an optional ingredient to fortify milk and milk substitutes. Enrichment of milk does lead to increases of vitamin D uptake in adults (1,2). The matrix in intact ingredients can either enhance or diminish vitamin bioavailability. The same effect goes in whole foods.
8.What types of compounds enhance or reduce the nutritional value of a vitamin? I answered parts of this question above and in last week’s post. The issues were bioavailability and biopotency. Nutrient antagonists and enhancers are more prevalent for minerals than for vitamins. Vitamin stability includes more than protective or antagonistic compounds. Microenvironments in foods and the digestive tract also affect stability. Storage of fresh foods and exposure to heat results in large losses of vitamins. Home cooking and heat processing are very destructive. Vitamins are susceptible to acidic and basic environments. They oxidize, break down when heated, and can become inactive when modified by enzymes. Metals can either enhance or degrade vitamin activity.
Vitamins are complex chemicals that play important roles in our bodies. Nutritionism is more than studying nutrients and their role in the body. It is also about understanding compounds that interfere or interact with vitamins. How stable are vitamins during cooking, processing, or storage of foods? How stable are they during digestion?
A few examples illustrate dependence of vitamins on their external environment. Riboflavin is very sensitive to light. It undergoes rapid degradation in glass containers. Cooking in open pots or interaction with sodium bicarbonate also degrades this molecule. Pyridines interfere with niacin biopotency. Zinc enhances production of niacin vitamers. Bacterial thiaminases, present in acidic environments, break down the vitamin. Flavonoids, tannins, heme, and excess alcohol interfere with proper absorption of thiamin. A reduced folate carrier enhances absorption of thiamin in the gut. Vitamin C is susceptible to oxidation. This reaction is reversible under the right conditions. Vitamins A and D oxidize in the presence of reactive oxygen species (ROS). The presence of iron and zinc enhance vitamin D function. Lead inhibits the vitamin.
Vitamin E protects other vitamins from oxidation in our bodies. The main function of Vitamin E is to prevent oxidative damage in cell membranes. Vitamin E is a very effective antioxidant in living systems by sucking up damaging ROS. Vitamin E patrols cell membranes to halt oxidation of fatty acids and proteins. The vitamin becomes oxidized in the process. Vitamin C helps regenerate vitamin E back to its active state as an antioxidant. Vitamin E also supports bone health, eye health, and a healthy pregnancy. The most effective vitamer of vitamin E is α-tocopherol. Many reactions in the body generate ROS. Containing these oxidative products by tocopherols helps prevent chronic disease. Vitamin E mediates cardiovascular disease, many cancers, and inflammatory responses. Some fat in the gut helps absorption of fat-soluble vitamins. Low-fat meals inhibit vitamin E absorption during digestion.
The most important concept in understanding vitamins is how they work together. It is tempting to assign a specific duty to each vitamin, but our metabolism is not that simple. In that zone between sufficiency and subclinical deficiency vitamins depend on each other for biopotency. Not only do vitamins work together to protect our organs, the levels of each are important. Low vitamin stores of one vitamin, may inhibit proper function of another vitamin. Stores too high for one vitamin can compromise metabolic function for another one. Our health depends on all vitamins functioning at optimal capabilities. Self-administered supplements of single vitamins can upset this delicate balance. These supplements can interfere with metabolic health.
Take home lesson. Dietitians and nutritionists want us to eat healthy. Current guidelines emphasize fruits, vegetables, whole grains, fish and lean meat. Nutrition professionals encourage us to do more home cooking to improve healthiness of our diets. Food scientists conclude that many consumers do not eat a healthy diet. They believe that fortification improves vitamin status of those eating unhealthy diets. Dietitians and nutritionists blame ultra-processed foods as the reason many diets are unhealthy.
Intrinsic ingredients and whole foods both enhance and inhibit vitamin bioavailability and biopotency. Vitamin fortification helps the general population consume adequate levels of these nutrients. Losses of vitamins occur during storage of fresh foods and heating during cooking or processing. The microenvironment of foods and in the gut also affect vitamin stability. When present at adequate levels, vitamins work together to preserve metabolic health. Imbalances of vitamins can upset those fine cellular mechanisms. So how do we determine the nutritional value of a specific vitamin in a specific food? It’s complicated. It’s all about the vitamer’s stability, its bioavailability, and its biopotency. The % Daily Value is the best estimate.
The Vitamins by Combs and McClung is an excellent reference and textbook. It helped give me a refresher course on the topic and clear up some misperceptions on my part. I hope that I provided an accurate assessment on vitamin controversies appearing on the internet. The book is very technical and represents the height of Nutritionism. I support the nutritional sciences. I bemoan the criticism that many self-educated nutritionists level at the discipline. Combs and McClung represent the pinnacle of understanding in the field. I can only hope that scientific inquiry will guide future discussions in nutrition.
Next week: Two versions of the ultra-processed foods debate
(1)Bahareh, N and TR Neyestani, 2022. The effects of vitamin-fortified foods on circulating 25(OH)D concentrations in adults: a systematic review and meta-analysis. British Journal of Nutrition 127:1821-1838.
(2)Itkonen, ST, M Erkkola and CJE Lamberg-Allardt, 2018. Vitamin D fortification of fluid milk products and their contribution to vitamin D intake and vitamin D status in observational studies—a review. Nutrients 10:1054.