When you're already plunking down five dollars or more for a smoothie, it can be hard to justify the additional cost of the various supplements and mix-ins offered on the menu. But with names like "recharge," "immune booster" and "cold fighter," the pricey additions can be hard to resist.

Tara Gidus, R.D., a sports nutritionist and team dietitian for the Orlando Magic, helps weigh in on what's really going into drinks—and whether the benefits justify the claims.

Cold Fighters (vitamin C, zinc, echinacea): As cold and flu season kicks into high gear, it's natural to seek out supplements that might help you dodge such germs. Many smoothie chains will offer cold-fighting or immune-boosting blends containing ingredients such as vitamin C, zinc and echinacea. Mega doses of vitamin C have not been proven to stave off colds, although some studies have found that taken on a regular basis the vitamin may reduce the length or severity of symptoms. "Since most smoothies are made with fruit, they already contain a good dose of vitamin C," Gidus says. "But since it's a water-soluable vitamin" (meaning the body can rid itself of any excess), "taking a boost of it may help and won't hurt." Zinc may help boost immunity and lessen a cold's severity—if you start taking it as soon as symptoms appear. But evidence for echinacea is less conclusive. "The medical science isn't that strong, and there are concerns about dosages and purity of the supplements you're getting," Gidus says.

Energy Enhancers (B vitamins, ginseng, ginkgo biloba): A double espresso can give you a quick jolt of caffeine-fueled energy, but many smoothie add-ins purport to do the same, without the caffeine buzz. B vitamins are one common supplement. "We know that B vitamins are an important part of the body's energy metabolism pathway," Gidus says. "But does more give you more energy? Probably not." Ginseng may perk you up a bit, and some research suggests that ginkgo biloba may improve thinking and memory.

Flax or Chia Seeds: Either of these trendy add-ins are going to give a smoothie a dose of Omega-3s—the good-for-you fatty acids that reduce inflammation, may lower the risk of heart disease and increase brain function. "In addition to Omega-3s, these seeds will add some fiber and a little protein," says Gidus, "so I would say they are definitely worth it."

Probiotics: These healthy bacteria can help keep the digestive system in good shape and may also help boost immunity. But if your smoothie is yogurt-based, you're probably getting a good dose already.

Protein: If your smoothie already contains a protein source -- such as peanut butter or yogurt -- an additional protein boost is probably unnecessary. That said, protein powders such as soy or whey (which is milk-based) can sometimes be a worthwhile addition to fruit smoothies. "The protein helps slow down how quickly all that fruit sugar is absorbed into your system," Gidus says. "So you stay fuller longer and are less likely to experience the sugar high and subsequent crash."

Spirulina: This blue-green algae is rich in antioxidants, like beta-carotene and vitamin E, that help cells fend off free-radical damage. "It will provide good nutrients, but be warned that it has a strong taste that may not be masked by your other smoothie ingredients," Gidus says.

Sally Wadyka is a Boulder, Colorado-based journalist who writes about nutrition, health and wellness.