Sugar And Immune Function
Chester J. Zelasko, Ph.D. | March 11, 2008

The Bulletins in February explored many of the myths of weight loss and fitness. I invited readers to send additional myths they would like me to check out, and you did! One myth required more explanation than I could fit in a Bulletin. Because of the importance of the topic, I decided to examine the research behind the myth and write about it in this Newsletter.

The question: “I’ve read that for every three grams of sugar a person eats or drinks, immune function will be decreased by 50% for up to six hours. Is this true or is it a myth?”

I reviewed a variety of Internet websites, and this comment seems always to come back to an article written by Dr. Michael Murray, a naturopath who has written a number of health books. In his article, Murray reports the methods and most of the results of a study published in 1973 by researchers at Loma Linda University (Murray was not one of the researchers). In short, the study examined the response to a 100-gram load of various types of sugar including glucose, fructose, sucrose, honey, and pasteurized orange juice. Immune function was reduced--specifically, the ability of white blood cells (neutrophils) to destroy bacteria was compromised for up to five hours.

Up to this point, the information is accurate. However, Murray reports that the decrease in immune function was 50% after two hours and had not returned to normal by five hours. That’s not accurate. He also reports that starch did not have an effect on immune function, and that’s not accurate. Further, it’s what he doesn’t say about the research that’s important in interpreting the results. Let’s take a look at the research in question.

The Study
The purpose of the study was to see if phagocytosis, a function of the immune system, would be compromised in response to the ingestion of sugar (1). Further, the researchers chose to examine a variety of popular sweeteners to see if the various types of sweeteners had different effects. An important goal of the research was to see if the immune systems of diabetics would be compromised by eating too much sugar on a regular basis.

The researchers drew blood samples, then administered one of the sugars to 10 healthy subjects; they drew blood again at 30 minutes, one hour, two, three, and five hours after ingestion. They exposed all the blood samples to a bacterium, and then measured how many bacteria were consumed by the white blood cells at each time point. They termed this the Phagocytic Index (PI).

The Results
Dr. Murray reported that the decrease in immune function was more than 50% at two hours, which is not what the published data indicates. The greatest decrease was 43.5% for sucrose (table sugar); all other forms of sugar were less than that at two hours. Starch also decreased the PI at two hours by 10.2%. While the PI at the five-hour mark had recovered to within 15% of initial levels for all other forms of sugar, starch continued to decline to a PI of 13.4%.

One of the interesting aspects of this study was the response to fasting. Subjects underwent a 60-hour fast--that’s right, over two days--and then had PI measured in the same way as the subjects eating the sugar. While blood sugar levels decreased as would be expected, the PI increased by over 50%. While done on a limited number of healthy subjects, fasting may be one way to enhance immune activity. Certainly, you don’t want to fast without discussing it with your physician, especially if you’re a diabetic. But this may be a partial explanation of why chronic under-eating seems to increase lifespan in animals: it may be that the immune systems of the animal are enhanced.

The Problems
The way I see it, these are the problems with the study:

The number of subjects was low: only 10 healthy subjects completed the study. In another part of the study, seven diabetic subjects were tested in response to a 100-gram load of glucose. Interestingly, they were the only subjects who actually had a decrease in PI over 50% after two hours. Still, testing only 17 subjects is atypical and doesn’t yield enough information to base recommendations for populations of healthy or diabetic people.

The amount of sugar was extreme for a single meal: 100 grams of sugar translates to 3.3 ounces of sugar or 400 calories just from sugar. Put in terms we can relate to, that would be like drinking 2.8 cans of soft drink, 3.8 cups of orange juice, or 26.7 teaspoons of table sugar--that’s over half a cup of sugar. All at once. Even for a real sweet tooth, that’s more than a typical person would eat at one time. The sweeteners should have been dosed in increments starting at 25, 50, 75, and finally 100 grams. That would provide more information about what people do in the real world.

The researchers didn’t use whole foods as a control. There’s a good reason for that. I calculated how much sugar it would take in the form of cookies to reach 100 grams of sugar, and it would take eight Double Stuf Oreos. But then you’d have all the fat and other carbs along for the ride, as well as a bit of protein, to complicate the results. However the average person is more likely to eat eight cookies at one time than a half cup of sugar by itself.
There was no replicating study and no follow-up articles. In my review of the research since the article was published in 1973, the study hasn’t been replicated or verified by any other researchers, and that’s almost unheard of in the world of research. There were nine researchers who were authors of this study. In checking each author, not one of them ever pursued this type of research again and not one of them ever wrote an article on another aspect of the study. In fact, this line of research has never been pursued by anyone else. As a former college professor who knows firsthand the publish-or-perish pressure of universities, that speaks volumes to me. If anything, research has not confirmed the effects of that much sugar on the immune system: in a 2002 review article, researchers didn’t even include carbohydrate intake when they examined a variety of factors that could inhibit immune function (2). Of the 13 factors discussed, when it came to diet, only the under- or over-consumption of vitamin E and zinc were deemed important enough to include.

Bottom Line
The question raised by the reader--whether sugar compromises the immune system--is more myth than fact. We just don’t know, based on research, whether a typical amount of sweetener a person would eat will affect the immune system in any way.

But that doesn’t mean I think you should eat as much sugar as you want. We have too many refined carbohydrates in our diet, and we should all eat less of them. This is especially true if you’re a diabetic. There’s no question in my review of the research that intake of sugars and refined carbohydrate must be controlled by diabetics because it will compromise their immune systems. But there’s no reason to deny your sweet tooth completely, either. The key, as always, is portion control.

  1. Sanchez, A et al. Role of sugars in human neutrophilic phagocytosis. Am J Clin Nutr. 1973; 26:1180-1184.
  2. Calder, PC and Kew, S. The immune system: a target for functional foods? British Journal of Nutrition. 2002; 88:S165–S176.
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