We’ve all heard it before, “you should drink orange juice when you are sick because it will boost your immunity.” But is that really true?

Myth 1: Orange juice boosts your immune system

It turns out that the opposite is likely true. Sugar actually signals your immune system to take a nap. Oranges and orange juice are not very similar when it comes to nutrition — oranges are reasonably healthful, while orange juice is a lot more similar to soda than it is like an orange. As with soda, OJ is very high in the sugar fructose.

White blood cells (leukocytes) are the business end of your immune system. They eat (phagocytosis) the invaders (pathogens) in your body — keeping you well. You can measure how much they are eating — how well they are keeping you — by something called the leukocyte index, or LI. The higher the LI the more invaders are being destroyed and the more resistant you are to infection.

According to research in the American Journal of Clinical Nutrition, a funny thing happens when you consume most kinds of carbs — your LI goes down. In other words carbs make your immune system sleepy. They’ve tested different kinds of carbs and fructose is the most powerful suppressor of your LI — it reduces your immune system’s ability to eat invaders by over 41 percent. It takes more than five hours for your immune system to return to normal.

Alternative: Drink some water, have chicken soup, maybe some tea, and try eating an orange instead of drinking the sugar contained in the five or six oranges that go into a small glass of orange juice.

Myth 2: Juice is a good source of vitamin C

It turns out that even though juice might contain a lot of vitamin C, the high sugar content actually prevents your body from absorbing that vitamin C.

Glucose (carbohydrates in your blood) competes for the same receptors in your body as vitamin C uses — only one of them can win. The higher your blood sugar levels, the less vitamin C your cells can take up. In addition, higher blood-sugar levels impair re-absorption of vitamin C by your kidneys. So drinking juice increases the amount of vitamin C you need while simultaneously making it more difficult to absorb.

Alternative: Try eating whole fruits — no syrup, no added sugar, just the fruit itself.

Myth 3: You can trust food labels

It actually turns out that labeling laws allow for some amazing things when it comes to processed food. For example, the words “fat free” — actually, the food that is legally labeled “fat free” — might be pure fat. Pam fat-free cooking spray is a perfect example of this.

Grab a can of “Original” Pam, turn it around and look at the nutrition “facts” label and you’ll see: “calories 0,” and “Total fat 0g”. But if you keep reading down to the ingredients list, the first ingredient is “Canola oil,” and there is essentially nothing else in the can other than the propellant that makes it shoot out.

By definition, oil is pure fat. That’s just what oil is — fat and nothing else. So, how are these nutrition “facts” possible? Does this mean that putting oil inside of a can magically turn into a fat-free substance?

It turns out that if a serving of food has less than half a gram of fat in it, then you may legally call it fat free. So, Pam just makes the serving size a one quarter of a second spray (I time myself), or less than one third of a gram. So, there isn’t a half a gram of anything, so even as pure fat, it can legally be called “fat free.”

Alternative: Be skeptical of health claims on the label. If a food has a health claim on its packaging, it’s probably bad for you. Read the label, the ingredient label, and think about what they’re saying. It’s your body and your health, so you can’t delegate responsibility to a label.

Josef Brandenburg is a Washington, D.C.-area certified fitness expert with 11 years of experience. In 2004, he started The Body You Want personal training fitness program, which specializes in weight loss and body transformations for busy people. Read more about The Body You Want at josefbrandenburg.com.