Exercise, Lymphokines, Calories, and Cancer

Exercise and cancer: linkages with obesity?

This review examines possible protection against various types of tumors from both occupational and leisure-time physical activity. A growing number of well-controlled studies suggest that both hard physical work and an active leisure reduce the overall incidence of cancer. Benefit is demonstrated most clearly for colonic tumors; here, benefit persists after control of the data for body mass index, and a likely mechanism is the speeding of colonic transit, with a resulting decrease in exposure to carcinogens. There are occasional reports of protection against tumors of the breast, reproductive tract, and other body organs. In some of these latter sites, potential mechanisms of protection include a suppression of sex hormone secretion and reduction of body fat depots, where androgens are converted to toxic estrogen derivatives. While vigorous athletic participation may well modulate hormone output, a reduction of body fat is a more likely explanation of any benefit from more moderate physical activity. Protection against cancer is unlikely to become a major argument for an active lifestyle, but it is nevertheless a useful byproduct of physical activity undertaken with other health objectives.

Effect of amount and type of exercise on experimentally induced breast cancer

It appears that exercise derived by treadmill running can enhance or inhibit the development of mammary cancer depending on the intensity and duration of the activity. The effects of timing of exercise relative to the phases of carcinogenesis and the frequency of exercise on the tumorigenic response in the mammary gland have yet to be studied. What is needed are further investigations of a spectrum of exercise conditions that exert differential effects on measures of physical fitness, energy intake, body composition, and/or the efficiency of utilization of carbohydrate, fat, and protein as energy substrates. This should facilitate identification of critical relationships between physical activity and the risk for cancer. This goal can best be achieved initially through carefully controlled laboratory studies using appropriate experimental systems. There are several mechanisms by which exercise could alter the course of tumor development. A goal in this field should be to first characterize the influence on tumorigenesis of amount and type of exercise that have differential effects on metabolism and thereafter proceed to investigate the basis for the effects observed. Experiments should be designed to dissociate effects of exercise related to local and/or systemic changes induced by skeletal muscle contraction from those attributable to changes in energy expenditure. Ultimately, investigations of the type proposed should allow for determination of whether the quantity and quality of exercise needed to attain health-related benefits for chronic diseases such as cancer differs from what is recommended for fitness benefits. This should also permit formulation of a specific set of recommendations about the amount and type of physical activity that in concert with appropriate dietary practices can significantly reduce the risk for cancer.

Voluntary exercise and experimental mammary cancer

The results of these studies indicate that voluntary activity suppresses the development of chemically and virally induced primary mammary tumors in rats and mice fed high-fat diets. These diets were chosen to mimic the current U.S. fat consumption of approximately 40% of calories as fat. It remains to be seen if activity exerts a similar suppressive effect on animals fed their customary low-fat diet (10% calories as fat). In general, the activity profiles of the female Fischer F-344 and Sprague-Dawley rat and the C3H/o mu j mouse exhibited a similar pattern with an early peak followed by a gradual plateau over time. The effects of activity on body fat composition showed a trend toward a decreased percent of body fat when compared to sedentary animals but a statistically significant decrease was found only in the F-344 female rat. In the DMBA model, carcinogen dose did alter outcome parameters. For example, time to first tumor was extended under low- but not high-DMBA conditions, and, conversely, tumor multiplicity was significantly decreased in the high- but not low-DMBA group. In the NMU model, an inverse association was found between the amount of activity and tumor incidence. A similar association was not found with the DMBA model. The reason for this is uncertain, but further analysis in terms of other parameters such as total tumor number may shed more light on this discrepancy. The suppressive effect of activity on the MMTV-induced mouse mammary tumor is of particular interest since it raises the possibility that activity may exert effects on the process of provirus insertion, and/or oncogene activation--an area of great potential promise in cancer prevention. Activity appeared to enhance the volume and to a lesser degree the number of metastatic foci in the lungs of F-344 retired breeders under high-fat but not medium-fat conditions. In addition, the most active animals in the high-fat group exhibited the greatest volume of metastases. These results, together with those in the NMU model, point to the critical importance of the quantity of voluntary activity an animal engages in and its relation to both primary and secondary cancer prevention. They imply that beyond a certain point of either frequency or intensity, the beneficial effect of exercise may be nullified by competing deleterious effects. The metastases study has also brought to light the importance of dietary fat as a potential intervening variable.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of voluntary exercise and/or food restriction on pancreatic tumorigenesis in male rats

Studies were undertaken to evaluate the effects of caloric restriction on voluntary exercise in a rat model of pancreatic cancer. Suckling male Lewis rats were initiated with 3 doses (30 mg/kg body weight) of the pancreatic carcinogen azaserine. At 28 days of age they were weaned to one of four experimental protocols; namely, sedentary/ad libitum fed, voluntary exercise/ad libitum fed, sedentary/food restricted, and voluntary exercise/food restricted. Voluntary exercise was provided by free access to running wheels fitted with odometers. Food restriction was intended to be mild, at less than 10% reduction of ad libitum intake. Putative pre-neoplastic pancreatic foci were identified microscopically at 4 months post-azaserine treatment. As previously shown, food restriction led to increased wheel activity, but the increased activity could not be maintained beyond the first half of the post-initiation treatment phase. Additionally, the extensive wheel running occurred only when available food was restricted by greater than 10%. Exercise per se had a lesser effect compared to food restriction on pancreatic tumorigenesis.

Energy balance, body size, and cancer

Increased energy intake and physical inactivity have been shown to heighten the risk of breast, large bowel, and other cancers. Large body size and fatness, as measured by adult stature, body weight and body mass indices, are positively related to a variety of cancers, including breast, colorectum, prostate, endometrium, kidney, and ovary, as well as to total cancer incidence or mortality in many investigations, although conflicting reports exist. Adult weight gain has also been specifically implicated in a few etiologic studies of breast and large bowel cancer. Furthermore, increased birthweight and childhood stature have been linked to increased risk of leukemia, lymphoma, osteogenic sarcoma, and central nervous system malignancies between infancy and young adulthood. Greater body weight also adversely affects breast cancer survival. These findings are complementary and support a role for positive energy balance in promoting human carcinogenesis. Potential mechanisms are discussed.

Endurance Training in Middle-Aged Male Physicians

In brief: Vigorous exercise has been associated with reduced cardiovascular risk factors and a lower incidence of coronary heart disease. To determine the effects of endurance training on middle-aged men, a group that is at risk for heart disease, two groups of Physicians were surveyed: 1,269 runners (members of the American Medical Joggers Association who had been running at least 10 miles per week) and 683 nonrunners (members of the AMA). These groups were chosen because they are similar in age, sex, life-style, profession, socioeconomic position, and knowledge about health. Results showed that the runners had lower body weight and reduced cardiovascular risk factors, and that they smoked fewer cigarettes. However, prospective studies are needed to determine both the risks and benefits of endurance exercise.