Losing weight is associated with a very slight loss in immune function; but losing weight often (i.e. 'weight cycling' or yo-yo dieting) means a measurable reduction in natural immunity, according to a recent study summarized here.

 Introduction

The obesity epidemic is widely recognized. It's been reckoned that half the women in Western countries are currently attempting, or have recently attempted, to lose weight. And, although weight loss is known to improve the outlook for people with cardiovascular disease and diabetes, among other conditions, several studies have found an increased mortality rate with weight loss. Why should this be so?

Repeated cycles of loss and gain are common consequences of failed weight loss attempts. Scientists in Seattle have done a study to see whether this pattern of weight loss/gain might influence immune function in the body; they've reported their findings in the Journal of the American Dietetic Association.

What was done

It's well known that intentional weight loss is linked with a decrease in immune function in lab tests, due to lessened effectiveness of the 'natural killer cells' (NK cells) that normally attack cancer and virus-infected cells. The scientists wondered if repeated intentional weight loss might be linked with more pronounced decreases in immune functioning.

Postmenopausal women aged 50-75 were recruited if they had a BMI between 25 and 40 (i.e. they were overweight to severely obese) and were otherwise healthy. They were currently weight-stable. A questionnaire was used to get their history of intentional weight loss.

Their NK cell cytotoxicity (a measure of the cells' effectiveness) was determined, using four different 'concentrations', to obtain an overall percentage of target cells killed; the target cells used were leukemia cells. Analyses were then done to look for linkage between the NK cell cytotoxicity (as a marker for the body's immunity) and intentional weight loss attempts. In addition to cytotoxicity, NK cells were also studied for their number and different types.

What was found

There were 114 women with an average age of 61 and an average BMI of 30 in the study. Two-thirds of them reported intentionally losing weight (10 pounds [4.5 kilos] or more), at least once.

Average NK cell cytotoxicity in those women who had lost 10 pounds at least once was significantly lower than those who had never lost at least 10 pounds - 25% vs. 31%.

The average NK cell cytotoxicity decreased as the frequency of weight losses increased; for women who had lost 10 pounds once, the difference to 'never lost' was about 2%, while for women who had lost at least 10 pounds 5 times or more, the difference was over 7%.

Finally, for women who remained 'weight stable' for a period, NK cytotoxicity was higher with the increased duration of this stability; weight stability for 5 years or more was associated with a 10% higher NK cytotoxicity than in those women who were weight stable for 2 years or less. (By 'weight stability' the scientists meant staying within 10 pounds of their current weight.)

The numbers and types of NK cells gave similar results, i.e. they were decreased in those women who had frequent intentional weight losses.

What this means

This study shows that frequent intentional weight loss or possibly 'weight cycling' (yo-yo dieting) may have long-term adverse effects on immune function. Longer periods of weight stability are linked to better immune function. We all know how important immunity is - it's not just the body's defense against viral infections, but it's also vital for battling bacterial infections and cancer.

The answer is, don't yo-yo diet! If you need to lose weight, get your plan approved by a dietician and a physical therapist or trainer, and then go to it. Just make sure that the weight stays off, and you don't get on that dangerous 'weight cycle'.

Source

• Frequent intentional weight loss is associated with lower natural killer cell cytotoxicity in postmenopausal women: possible long-term immune effects. ED. Shade, CM. Ulrich, MH. Wener,  et al., J Am Diet Assoc, 2004, vol. 104, pp. 903--912

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