Bonita Applebaum replied: "Hair loss or thinning of the hair is a common problem of female guinea pigs that have been repeatedly bred. These sows tend to lose hair with each successive pregnancy. Hair loss is frequently noted among juvenile guinea pigs by weakened state at or around the time of weaning. "Barbering" also results in hair loss. This vice (bad habit) occurs when guinea pigs habitually chew on the haircoats of guinea pigs that are lower in the social "pecking order." Younger guinea pigs in particular, can lose substantial amounts of hair as a result of this activity. Hair can also be lost because of fungal disease and external parasite infestations. But if you believe it's a Vitamin C deficiency.. Good-quality food and fresh, clean water must be readily available at all times. Commercially available pelleted chows provide all of the essential nutrients, as long as the pellets are fresh and wholesome when offered. Some guinea pig owners are tempted to feed rabbit pellets, assuming that they are roughly equivalent to guinea pig pellets, but this is not so. Unlike most mammals (including rabbits), guinea pigs require a high level of the vitamin, folic acid. Unlike rabbits, guinea pigs cannot manufacture their own vitamin C and must, therefore, receive it from an outside source. Interestingly, people and our primate relatives share this dependence on vitamin C from the food we consume. Pellets milled for guinea pigs take these special requirements into consideration and are appropriately fortified with these 2 nutrients, among many other essential ones. Guinea pig chows generally contain 18-20% protein, 16% fiber and about 1 gram of vitamin C per kilogram of ration. Even when the fresh pellets are properly stored in a cool, dry place, about half of the vitamin C content is degraded and lost within 6 weeks of manufacture. Therefore, the diet should be supplemented with vitamin C as follows: 200 milligrams of ascorbic acid (vitamin C) should be added to about 1 qt of drinking water, made up fresh every 12 hours, or a single guinea pig should be offered one handful of kale or cabbage or one-quarter of an orange daily. Researchers are not in agreement on the advisability of adding other items to the balanced ration (pelleted chows). We recommend that fresh greens, hay and small amounts of fruit be offered daily with several precautions: These items should not exceed 10-15% of the daily diet Furthermore, the fresh items must be thoroughly washed to avoid pesticide residues and possible bacterial contamination. Guinea pigs cannot manufacture vitamin C and must receive an adequate supply of it from outside food sources. Vitamin C deficiency results in scurvy, which is characterized by inappetence, swollen, painful joints and ribs, reluctance to move, poor bone and teeth development, and spontaneous bleeding from the gums and into muscle. Adequate levels of vitamin C are always included in the formulation of pelleted diets for guinea pigs. Often, however, handling and improper storage (exposure to light, heat and dampness) of the feed pellets results in loss of vitamin C. Therefore, even guinea pigs fed presumably reliable pelleted diets may develop scurvy if the diet's vitamin C content has been reduced or lost. A veterinarian should be consulted if this disease is suspected so that the diagnosis can be confirmed. The veterinarian will prescribe a program of vitamin C supplementation (via food or water or injection) to reverse the signs"