While some scientists believe that lack of sunshine and a consequent deficiency of vitamin D cause winter colds and flu, a good case could also be made for sugar as the main culprit. Winter holidays seem always to involve tempting sugary treats; yet sugar can paralyze the immune system.

A study published in the American Journal of Clinical Nutrition in 1973 was the first to measure sugar’s effect on the immune system. These researchers found find that consuming 100 grams of sugar (equal to 20 teaspoons of table sugar or 2 pieces of pumpkin pie) as glucose, fructose, sucrose, honey or orange juice in one sitting had a devastating effect. Immune suppression began about 30 minutes after ingestion, reached a peak between 1 and 2 hours, and lasted up to 5 hours. (The same amount of starch did not have this immune-inhibiting effect).

Subsequent research found the following effects of sugar on the ability of the immune system’s white blood cells to destroy bacteria:

With no sugar consumption, each white blood cell was able to destroy 14 bacteria
After consuming 6 teaspoons of sugar: 10 bacteria
After 12 teaspoons of sugar: 5.5 bacteria
After 18 teaspoons of sugar: 2 bacteria
After 24 teaspoons of sugar: 1 bacterium

So just 10 teaspoons of sugar could temporarily cut your immune strength in half!

Oh, but you say, I don’t eat much sugar. . . . Well, the average American eats more than 22 teaspoons of added sugar daily—probably a lot more than they realize, especially if they eat processed food, which is loaded with hidden sugar. And during holidays, it becomes almost mandatory to consume your share of excess sugar.

Each of the following supply about 10 teaspoons of sugar: a piece of pie; a cup of rice pudding; less than a cup of ice cream; 7 pitted dates; 5 small, dried figs; 2 cups of orange juice; 3/4 cups raisins; a 12-oz soda; a cinnamon bun; 2 ounces of hard candy; 2½ ounces of fudge; 2 Fig Newtons or ginger snaps.

Does this mean we can never have a sweet treat, even on Thanksgiving or Christmas?

No. Even though your immune cells may take a hit, one piece of pie on Thanksgiving isn’t going to kill you or even make you sick. What’s dangerous is making sweet treats a temporary way of life—something that’s all too easy to do over the holidays. Given sugar’s addictive power, the more you eat, the more you crave.