Last updated: 15-Jan-2007
Newsletter Excerpts
The Clinical Burden of Obesity
Obesity is a well-established risk factor for many significant diseases, including cardiovascular disease, stroke, and diabetes. This is because, among other things, obesity leads to adverse metabolic changes such as elevated blood pressure and cholesterol, as well as resistance to insulin (this collection of symptoms has been labelled the “metabolic syndrome”).
In addition to an obvious connection with accidental falls, being overweight also increases the prevalence of certain cancers, kidney failure, asthma, arthritis and other musculoskeletal disorders, gallbladder disease, hormonal and reproductive problems, sleep apnea, impaired immune function, and blindness. There is also a well-known connection between obesity and depression, and a growing awareness of the risk of dementia.
Recent work by Canadian researchers Katzmarzyk and Janssen (Canadian Journal of Applied Physiology, 2004, Vol. 29) summarized the best current understanding of the clinical burden of obesity, as seen in the table below. The authors itemized the types of diseases most often associated with obesity (BMI ≥30.0), as well as the relative risk of disease, 95% confidence interval, and population attributable fraction (PAF). The PAP is the proportion of the disease in the exposed group attributable to the risk exposure.
A relative risk of 1.4 means the factor is associated with a 40% increase in risk.
(PreventCancer.ca Newsletter, Issue 1)
Is Vegetable and Fruit Consumption Protective Against Cancers and Cardiovascular Disease?
Historically, the answer to this question has been ‘Yes’ with the outstanding question being to what degree.
In the early 1980s, Doll and Peto (The Causes of Cancer, 1981, Oxford: University Press) estimated that approximately 35% of cancers were attributable to diet (excluding alcohol usage). A major report endorsed by the American Cancer Society concurred with this result, concluding that 20-40$ of cancers are directly related to dietary choices. (World Cancer Research Fund, Food, Nutrition and the Prevention of Cancer: A Global Perspective, London 1997). However, some authorities have been more cautious concerning the relationship between diet and cancer, suggesting that reduction in cancer rates of about 30% might be possible over 20 years through a combination of primary prevention factors (including diet, obesity and physical activity) (Adami et al, European Journal of Cancer, 2001, Vol. 37 (Suppl 8).
The most recent research has endorsed the more cautionary approach, especially in reference to vegetable and fruit consumption. Hung and colleagues recently published (Journal of the National Cancer Institute, 2004, Vol. 96) the results of a major prospective cohort study assessing the protective role of vegetable and fruit consumption on cancers and cardiovascular diseases. This study was based on over 110,000 individuals followed for up to 22 years. After adjusting for total caloric intake, age, smoking status, alcohol use, personal history of hypertension, hypercholesterolemia, diabetes, and (for women only) family history of breast cancer, menopausal status, and use of hormone replacement therapy, they found no protective effect associated with either vegetable or fruit consumption on cancers.
Similar results are found in three articles published in the February 8, 2006 issue of the Journal of the American Medical Association. Based on a study of postmenopausal women with an 8.1 year follow up, researchers found that a dietary intervention includi9ng reduced total fat intake and increased intakes of vegetables, fruits and grains did not significantly reduce the risk of breast cancer, colon cancer, coronary heart disease, stroke or cardiovascular disease. The authors did note that the 8.1 year follow-up period may not be sufficient to detect significant differences.
Although the debate continues concerning cancer and dietary patterns, unhealthy eating does appear to be associated with increased risk of other chronic diseases such as heart disease, stroke, and diabetes.
In developed countries, the WHO reported that an estimated 28% of ischemic heart disease and 18% of ischemic strokes can be attributable to low vegetable and fruit intake (The World Health Report 2002. Reducing Risks, Promoting Healthy Life. Geneva: WHO; 2002).
The research by Hung et al referred to earlier actually confirmed these results with respect to the association between vegetable and fruit consumption and cardiovascular disease. While they found no association between increased consumption of vegetables and fruit and a reduced risk of cancers, they did find that vegetable and fruit intake was inversely associated with the risk of cardiovascular disease. Participants eating at least five or more servings of vegetables and fruit per day had a 28% lower risk of cardiovascular disease than those who consumed fewer than 1.5 servings daily.
(PreventCancer.ca Newsletter, Issue 1)
The Clinical Burden of Physical Inactivity
In addition to its contributing role in obesity onset, physical inactivity generates its own independent health burden.
The table below, based on work by Katzmarzyk and Janssen (Canadian Journal of Applied Physiology, 2004, Vol. 29) summarizes the Canadian data for the types of diseases more often associated with physical inactivity. Physical inactivity was defined as using less than 12.6 kJ/kg of body weight per day in physical activities. Included in this table are the relative risk of disease, 95% confidence interval and population attributable fraction (PAF). The PAF is the proportion of the disease in the exposed group attributable to the risk exposure. A relative risk of 1.3 means that the factor is associated with a 30% increase in the risk of the disease.
In developing these results, the authors noted that since studies on physical inactivity usually adjust for obesity as a confounding variable, the disease risk and health care costs [of physical inactivity] can be considered to be independent of the effects of obesity.
(PreventCancer.ca Newsletter, Issue 2)
Is Weight Loss Harmful?
Assessing the health benefits associated with weight loss is a controversial exercise. A number of studies have indicated that fluctuations in weight may actually increase mortality compared with maintaining a steady weight. Furthermore, an inappropriate focus on weight loss may increase the risk of eating disorders. Finally, a number of studies suggest that a slight degree of overweight (as currently defined) may actually be protective in individuals with chronic conditions such as heart failure.
A key dividing point revealed in the literature is whether the weight loss is intentional or unintentional. Astrup (Obesity Reviews, 2003, Vol. 4) notes that this is “an important methodological problem, because some of the subjects may have suffered from an underlying disease process both causing weight loss and leading to an increased mortality rate.” Results from early studies assessing weight loss in the general population found equivocal associations between weight loss and mortality. These mixed results were based largely on observational studies that were not specifically designed to test the hypothesis that intentional weight loss reduces mortality.
Gregg et al. (Annals of Internal Medicine, 2003, Vol. 38) assessed the relationship between intentional versus unintentional weight loss and increased mortality in the general population. They found that intentional weight loss is associated with a 24% lower mortality rate (compared to those who reported not trying to lose weight or who did not experience weight loss). On the other hand, unintentional weight loss was associated with a 31% higher mortality rate, i.e. these later patients had other serious diseases that caused the weight loss.
Perhaps surprisingly, individuals who tried but were unsuccessful in losing weight also experienced a somewhat lower mortality rate. The authors note that the “most plausible explanation for our finding that attempted weight loss was independently associated with reduced mortality is that weight loss attempts are a marker for healthy behaviours.” Further encouragement from this study was that the best result (a 30% reduction in mortality) was observed in those individuals with a modest intentional weight loss of 1-9 kgs.
(PreventCancer.ca Newsletter, Issue 2)
Breastfeeding May Reduce Childhood Obesity
Breastfeeding would appear to be related to obesity control in children.
The practice of breastfeeding has recently been shown to have a protective effect against childhood obesity. One study showed that the prevalence of obesity in breastfed children was 2.8%, compared to 4.5% in children who had never been breastfed (Kries, et al. British Medical Journal, 1999, vol. 319). Since the risk of adult obesity may be twice as high in obese children compared with non-obese children, encouraging breastfeeding may also be an effective way to prevent adult obesity.
A recent review found nine adequately designed, large-scale epidemiological studies confirming the significant protective effect of breastfeeding against childhood obesity (Arenz et al. International Journal of Obesity, 2004, Vol. 28).
Although still controversial, possible explanations for this inverse association include hormonal and behavioural mechanisms and differences in macronutrient intake.
General caloric and protein intake are higher in bottle-fed infants than in those who are breast-fed. Higher plasma-insulin concentrations in bottle-fed infants could lead to the early development of adipocytes (fat cells) and increased fat deposition. Breastfed infants, on the other hand, may experience inhibited adipocyte differentiation due to the bioactive factors present in breast-milk.
Metabolic imprinting, a phenomenon involving a lifelong predisposition to certain diseases as a result of early nutritional experience, may be an additional explanation for the increased risk of obesity in formula-fed infants.
Breastfeeding, when possible, may prevent obesity and offers no adverse effects.
(PreventCancer.ca Newsletter, Issue 3)
Do Commercial and Organized Self-Help Weight Loss Programs Work?
A 2005 review by Tsai et al (Annals Internal Medicine, Vol. 142) assessed the available evidence on the effectiveness of such programs as Weight Watchers, Jenny Craig, LA Weight Loss, Take Off Pounds Sensibly (TOPS), Overeaters Anonymous, etc. They found three randomized, controlled trials of the Weight Watchers program, but no such evaluations of the other major commercial weight loss programs.
The authors of this review conclude that:
“Weight Watchers is the only commercial weight loss program whose efficacy has been demonstrated in a large, multisite, randomized, controlled trial. It produces a mean loss of approximately 5% of initial weight, which may be sufficient to prevent or ameliorate weight-related health complications. Weight Watchers is moderately priced but is still beyond the financial reach of many persons. TOPS and Overeaters Anonymous are important options for such persons, despite the lack of documented efficiency.”
(PreventCancer.ca Newsletter, Issue 3)
The Independent Relationship of Excess Weight and Physical Inactivity to Premature Mortality from Cancers and Cardiovascular Disease
In 2004, Hu and colleagues (New England Journal of Medicine, Vol. 351) reported the results of a 24 year follow-up period for 116, 564 women in the US. When these women entered the study in 1976, they were between the ages of 30 and 55 and were free of known cancers or cardiovascular disease. After adjusting for age, smoking status, parental history with respect to coronary heart disease, menopausal status/hormone use, and alcohol consumption, they identified the relationship between BMI and the relative risk of premature death from all causes, as shown on Figure 1. A relative risk of 1.4 means the factor is associated with a 40% increase in risk. Body mass index (BMI) is derived by dividing weight in kg. by height in meters squared.
One of the results from this study is that women with a BMI of between 25.0 and 26.9, usually classified as part of the overweight group, do not have a statistically elevated risk of premature mortality. This is consistent with information from other research.
The recent understanding that obesity is an independent risk factor for diseases such as heart failure is also significant. In other words, the increased risk of accumulating extra weight through body fat cannot be accounted for entirely by comorbidities such as high blood pressure and high cholesterol. In fact, Hu et al. established that both increased weight and reduced physical activity are “strong and independent predictors of death.” This means that an individual who is overweight and physically active still has a significantly higher risk of death from all causes compared with an individual of healthy weight who is physically active. This relationship holds for the risk of death specifically from cardiovascular diseases, as well as for death from cancers and death from all causes. The concept of activity or fitness mitigating the effects of obesity has thus been seriously called into question. Being overweight is manifestly risky, no matter how fit you are. The relationship between obesity and physical fitness as identified by Hu et al. is noted on Table 1. Based on their large prospective study, Hu et al. found that women with a (health) BMI of <25.0 but were inactive had a 55% increase in all-cause mortality compared to women with a BMI of <25.0 who engaged in moderate to vigorous physical activity at least 2.5 hours per week. For cardiovascular disease and cancers, premature mortality increased by 89% and 32%, respectively. While the study by Hu et al. is based on results for females, similar relationships exist for men. In fact, their work confirms the results found in numerous smaller studies. Katzmarzyk and co-authors (Obesity Reviews, 2003, Vol. 4) reviewed the approximately 170 articles |
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published prior to February of 2003 and, based on their meta-analysis, determined that “both physical activity and adiposity are important determinants of mortality risk.” Furthermore, “physically active individuals have a lower risk of mortality by comparison to physically inactive peers, independent of level or adiposity.” |
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(PreventCancer.ca Newsletter, Issue 4)