Regular vitamin C supplementation shortens the duration of a cold by 8 percent in adults, according to the 2013 Cochrane review by Harri Hemila and Elizabeth Chalker, pooling 31 comparisons covering 9,745 cold episodes. It does not reduce how often you get one. Across 29 trials and 11,306 participants, incidence was unchanged.
Eight percent of a seven day cold is about half a day. That is the honest size of the effect, and it is the number that most cold season advice quietly skips over. The one exception in that review is interesting: in five trials of people under short term extreme physical stress, mostly marathon runners and soldiers on subarctic exercises, vitamin C roughly halved cold incidence. So the nutrient does something. It just does not do much for a person sitting in an office in November.
vitamin C from food, and where it actually is
The common claim that oranges are the vitamin C food is wrong by a wide margin. Per 100 g, raw red bell pepper carries about 128 mg of vitamin C, guava about 228 mg, and kiwifruit about 93 mg. An orange sits around 53 mg. A single medium red pepper will clear 150 mg, which is well past the 75 mg daily recommendation for adult women.
The old tip about pairing vitamin C with iron is real and worth keeping, but the mechanism is narrower than it sounds. Ascorbic acid improves absorption of non-heme iron, the form found in plants such as spinach, lentils and pumpkin seeds. It does relatively little for heme iron from meat, which is already absorbed efficiently. So squeezing lemon over spinach is useful. Squeezing lemon over steak is just nice.
probiotics: a real effect, an unclear recipe
The Cochrane review on probiotics for preventing upper respiratory infections, updated by Hao and colleagues in 2015, found probiotics better than placebo at reducing the number of people with at least one acute infection, and reduced antibiotic use. The authors rated the evidence quality as low, mainly because trials used different strains, doses and durations.
That matters practically. “Eat probiotics” is not a dosage. The trials that showed benefit generally used specific strains, often Lactobacillus and Bifidobacterium species, at counts in the billions of organisms per day, for months. A serving of Greek yogurt may contain live cultures, or it may have been heat treated after fermentation and contain none. Check the label for “live and active cultures.”
Some of the classic recommendations do not survive scrutiny either. Miso soup is fermented, but boiling it kills the organisms, so miso stirred into hot broth at serving temperature retains more than miso simmered for ten minutes. Most supermarket pickles are vinegar brined rather than lacto fermented, which means no live cultures at all. Look for refrigerated pickles with no vinegar in the ingredient list. Kefir is the most reliably potent of the common options, typically carrying a wider range of strains than yogurt.
hydration, and the eight glasses number
The advice to drink eight 8 ounce glasses of water a day has no clear origin in the research literature. Heinz Valtin traced it in a 2002 review in the American Journal of Physiology and could not find supporting evidence.
What exists instead is the Institute of Medicine’s 2004 adequate intake figure: about 2.7 liters of total water daily for adult women and 3.7 liters for men. Total water includes everything, coffee and tea and the water in food, and roughly 20 percent of intake typically comes from food. The panel’s headline conclusion was that most healthy people meet their needs by drinking to thirst.
Being adequately hydrated does not make you less likely to catch a virus. It does make being sick less miserable, and it matters more if you have a fever, since fever increases fluid losses.
chicken soup is not a placebo, but it is not medicine
There is a small amount of real work here. A 1978 study in Chest by Saketkhoo and colleagues measured nasal mucus velocity after participants drank hot water, cold water or chicken soup, and found chicken soup produced the largest improvement in nasal clearance. A 2000 paper in Chest by Stephen Rennard found that chicken soup inhibited neutrophil migration in vitro.
Neither of those is evidence that soup shortens a cold. What they suggest is that hot, salty, savory liquid temporarily loosens congestion, which is a genuine symptom benefit and not nothing when you cannot breathe through your nose.
If you are making it from a whole bird, simmer gently rather than boiling. A hard boil emulsifies fat into the liquid and clouds the broth. Three to four hours at a bare simmer extracts collagen from connective tissue, which is what gives good broth its body. The frequent claim that bone broth is a meaningful source of dietary collagen for your joints is far weaker than its popularity suggests, and the amino acid profile of gelatin is incomplete. Make it because it tastes good and it is warm.
the two things with the strongest evidence
Neither is nutritional. Hand hygiene has a larger and more consistent effect on respiratory infection transmission than any food, and adequate sleep matters more than any supplement. A 2009 study in the Archives of Internal Medicine by Sheldon Cohen inoculated 153 volunteers with rhinovirus and found those sleeping less than seven hours a night were roughly three times more likely to develop a cold than those sleeping eight or more.
That is a bigger effect size than anything in the vitamin C literature, and it costs nothing. It is also the advice people least want to hear, which is probably why the kefir gets more attention.