Garlic has been used for centuries as both a food ingredient and a medicine.
In fact, eating garlic can provide a wide variety of health benefits (1Trusted Source).
This includes reduced heart disease risk, improved mental health and enhanced immune function (2Trusted Source, 3Trusted Source, 4Trusted Source, 5Trusted Source, 6Trusted Source).
This article explains how garlic is particularly protective against the common cold and the flu.
Garlic Can Boost Immune Function
Garlic contains compounds that help the immune system fight germs (5Trusted Source, 6Trusted Source).
Whole garlic contains a compound called alliin. When garlic is crushed or chewed, this compound turns into allicin (with a c), the main active ingredient in garlic (7Trusted Source).
Allicin contains sulfur, which gives garlic its distinctive smell and taste (8).
However, allicin is unstable, so it quickly converts to other sulphur-containing compounds thought to give garlic its medicinal properties (5Trusted Source).
These compounds have been shown to boost the disease-fighting response of some types of white blood cells in the body when they encounter viruses, such as the viruses that cause the common cold or flu (5Trusted Source, 9Trusted Source).
Garlic can be crushed, chewed or sliced to produce allicin, which is thought to give garlic its immune-boosting properties.
Can Garlic Help Prevent Colds and The Flu?
Garlic has shown promise as a treatment for preventing colds and the flu.
Studies have shown that garlic reduces the risk of becoming sick in the first place, as well as how long you stay sick. It can also reduce the severity of symptoms (9Trusted Source, 10Trusted Source).
One study gave 146 healthy volunteers either garlic supplements or a placebo for three months. The garlic group had a 63% lower risk of getting a cold, and their colds were also 70% shorter (11Trusted Source).
Another study found that colds were on average 61% shorter for subjects who ate 2.56 grams of aged garlic extract per day, compared to a placebo group. Their colds were also less severe (9Trusted Source).
If you often get sick with a cold or flu, eating garlic can help reduce your symptoms or prevent your illness entirely.
However, a review of the evidence found that many of the studies investigating the effects of garlic on the common cold were of poor quality (12Trusted Source).
It's also unknown if you need to take garlic constantly, or if it also works as a short-term treatment when you start getting sick.
Regularly eating garlic may help prevent the common cold or the flu. If you do get sick, eating garlic can reduce the severity of your symptoms and help you recover faster.
How to Maximize the Benefits of Garlic
The way garlic is processed or prepared can really change its health benefits.
The enzyme alliinase, which converts alliin into the beneficial allicin, only works under certain conditions. It can also be deactivated by heat.
One study found that as little as 60 seconds of microwaving or 45 minutes in the oven can deactivate alliinase, and another study found similar results (13Trusted Source, 14Trusted Source).
However, it was noted that crushing garlic and allowing it to stand for 10 minutes before cooking can help prevent the loss of its medicinal properties.
The researchers also stated that the loss of health benefits due to cooking could be compensated for by increasing the amount of garlic used.
Here are a few ways to maximize the health benefits of garlic:
Crush or slice all your garlic before you eat it. This increases the allicin content.
Before you cook with your crushed garlic, let it stand for 10 minutes.
Use a lot of garlic — more than one clove per meal, if you can.
Ensure whole garlic is crushed, chewed or sliced before it's eaten. Let crushed garlic stand for 10 minutes before you cook it.
Another easy way to increase your garlic intake is by taking a supplement.
However, be cautious, as there are no regulated standards for garlic supplements.
That means the allicin content and quality can vary, and so can the health benefits.
Powdered garlic is made from fresh garlic that has been sliced and dried. It does not contain allicin, but is said to have allicin potential.
Powdered garlic is processed at low temperatures, and then put inside capsules to protect it from stomach acid.
This helps the enzyme alliinase survive the harsh environment of the stomach so that it can convert alliin to the beneficial allicin in the intestine.
Unfortunately, it is unclear how much allicin can be derived from powdered garlic supplements. This varies greatly depending on the brand and preparation (15Trusted Source, 16Trusted Source).
Aged Garlic Extract
When raw garlic has been sliced and stored in 15–20% ethanol for over 1.5 years, it becomes aged garlic extract.
This type of supplement does not contain allicin, but it does retain the medical properties of garlic. Many of the studies showing benefits against colds and the flu used aged garlic extract (2Trusted Source, 9Trusted Source, 17Trusted Source).
Garlic oil is also an effective supplement, and is made by infusing raw garlic into cooking oils. You can add it directly to your meals, or take it in capsules.
However, it's worth noting that animal studies have shown that garlic oil can be toxic to rats at higher doses and in certain conditions (18Trusted Source).
Homemade garlic oil has also been linked with several cases of botulism, so if you're going to make your own, make sure to use proper preservation methods (19Trusted Source, 20Trusted Source, 21Trusted Source).
Common types of garlic supplements include powdered garlic, aged garlic extract and garlic oil. Aged garlic extract may be the best type.
How Much Garlic Should You Eat Per Day?
The minimum effective dose for raw garlic is one segment (clove) eaten two to three times per day.
You can also take an aged garlic supplement. In that case, a normal dose is 600 to 1,200 mg per day.
High intakes of garlic supplements can be toxic, so don't exceed the dosage recommendations.
You can get a benefit from garlic by eating 2-3 garlic cloves per day. Supplement doses range from 600 to 1,200 mg per day.
Other Tips to Boost Immune Function
Here are 5 more ways to boost immune function and help you avoid colds and the flu:
Take a probiotic: Probiotics can promote a healthy gut, enhance your immune system and reduce your risk of infection (22Trusted Source, 23Trusted Source, 24Trusted Source, 25Trusted Source).
Eat a healthy, balanced diet: Your whole diet is important. Getting a balance of important nutrients will make sure your immune system stays in good shape.
Don't smoke: Cigarette smoke can weaken your immune system and make you more prone to infection (26Trusted Source, 27Trusted Source, 28Trusted Source).
Avoid excess alcohol: Excess alcohol is thought to damage your immune system and make you more susceptible to infections (29Trusted Source, 30Trusted Source, 31Trusted Source).
Take a zinc supplement: Take zinc lozenges or syrup within 24 hours of the start of a cold, as this may reduce the duration of the cold (32Trusted Source).
A healthy diet and lifestyle are essential for keeping your immune system in good shape.
Take Home Message
Studies show that garlic can help fight colds and the flu. It can reduce your chances of catching an illness, and help you recover faster.
To maximize these benefits, it is best to consume raw garlic or aged garlic extract.
At the end of the day, garlic is both tasty and super healthy. Then there are many other great reasons to include it in your diet.