Riboflavin (vitamin B2) is one of those vitamins we don’t hear much about in petfood production. Unlike with several other vitamins, the requirement for dogs and cats has been relatively well researched, it hasn’t been implicated in unfounded controversy and petfood manufacturers fortify foods to the necessary level with relative ease. However, this lack of “noise” in the vitamin aisle shouldn’t be confused with a lack of importance, because riboflavin is positioned at the very heart of healthy metabolism.
Riboflavin derives its name from two significant facets of its molecular structure: a sugar (ribose) and a three-ring isoalloxazine structure (flavin). The flavin component fluoresces to a bold yellow color when exposed to ultraviolet light. This was in part how the compound was first discovered and has been exploited ever since to measure riboflavin levels. The ribose portion is where the molecule is phosphorylated to produce the functional co-enzymes.
Riboflavin must be phosphorylated to a co-enzyme form before it can exert its effects in the dog or cat. This is a two-step process that first requires the enzyme flavokinase to produce riboflavin-5’-phosphate (also known as flavin mononucleotide, or FMN), and then pyrophosphorylase catalyzes it to flavin adenine dinucleotide (FAD). The process is under the control of thyroid hormone and is what allows the FMN and FAD co-enzymes to participate in intermediary metabolism and numerous oxidation-reduction reactions. These reactions include the conversion of vitamin B6 to its active co-enzyme and the metabolism of folic acid, pyridoxine, vitamin K, niacin, vitamin D, reduced glutathione and vitamin C.
Because of this broad range of metabolic interactions, riboflavin deficiencies are seldom singular in nature but rather manifest as multi-nutrient deficiencies. Signs of riboflavin deficiency include weakness, fatigue, cataracts, dermatitis, alopecia, anemia, impaired reproduction and growth, nervous system deterioration and, in extreme circumstances, death.
Ingredient sources richest in riboflavin include meat, milk, eggs and yeast products. Rendered protein meals are a marginal and inconsistent supply. Vegetable matter can be a modest source while grains are relatively poor, with most of the riboflavin concentrated in the bran and germ fractions.
Natural sources of riboflavin in animal and plant tissues are mostly the co-enzymes FMN and FAD. To be absorbed, the nucleotides must be hydrolyzed from the ribose sugar. Conversely, in milk and the industrially produced vitamin, the riboflavin is in the free form (not phosphorylated), which allows for absorption as ingested. The absorption of riboflavin in the small intestine requires a sodium ATP-ase energy-dependent active transport system. Absorption can be affected by the animal’s nutritional status and is improved when accompanied by a meal.
Diet composition can also influence absorption. For example, dietary levels of copper, zinc, iron, nicotinamide, ascorbic acid and tryptophan have been reported to influence bioavailability. In the circulatory system, riboflavin is transported bound to proteins (e.g., albumin) and immunoglobulins. There has been no upper (toxic) level of riboflavin established, and excess dietary intake beyond the animal’s need is quantitatively excreted in the urine.
Riboflavin was first synthesized in 1935. While early commercial production depended entirely on chemical methods, today most of the approximately 2,400 tons of worldwide annual production relies on more cost-effective biosynthetic techniques. These industrial fermentation processes use organisms such as the filamentous fungi Ashbya gossypii grown on an oil-rich medium, yeasts like Candida famata and gram-positive bacteria such as variants of Bacillus subtilis that overproduce riboflavin.
Following a few clean-up steps, the net result of riboflavin production is a bright sun-yellow to golden powder. It is most commonly used in petfood vitamin premixes as an 80% active ingredient; however, there are other options available depending on application (60%, 90% and 96% riboflavin).
Free riboflavin is susceptible to degradation from UV light exposure. Not that it applies to petfood, but the classic example is the loss of riboflavin in milk stored in clear glass containers. Sun drying of fruits and vegetables has also been blamed for degradation. This can be exacerbated with the addition of sodium bicarbonate in an effort to retain visual freshness. In essence, riboflavin losses are worsened with extreme shifts in pH—acidic or basic.
As one of the water-soluble vitamins, riboflavin can be lost through leaching in high-moisture processes. In animal-based proteins, it is relatively stable through thermal processing and storage. It is also considered to be stable in baked and wet foods. However, the levels contributed by most petfood ingredients are not adequate to support the entirety of canine or feline requirements. So supplementation is generally necessary.
When considering supplemental sources, one must understand that while riboflavin is relatively stable compared to vitamins like A and C, there can still be small amounts of riboflavin sacrificed during petfood production. For example, retention in extruded foods is good with nearly 75% surviving through drying, but this can be compromised with extreme extrusion and drying temperatures. In addition, in a dry petfood on the store shelf, riboflavin can be lost at about 2% per month.
Riboflavin is one of the many water-soluble B vitamins that are constantly turned over by the body with little to no stored reserves. Thus, it must be replenished daily by the diet. While riboflavin is a part of many ingredients used to make petfoods, some supplementation is generally required to overcome ingredient variation, processing and storage losses. Providing this extra riboflavin from any of the biosynthetic sources available in the market is a safe, consistent and effective way to meet the dietary needs for this key vitamin at the heart of metabolism and health.
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