Ecological approach to investigations of noncommunicable health challenges: cancers, diabetes mellitus and obesity at the world population level

Abstract

Globally, cancers, diabetes mellitus and obesity have emerged as the major health and development challenges which are responsible for millions of deaths annually. Population-based prevention strategies have been advocated and adopted as a public health approach. However, unfortunately, no country has achieved their expected results in the past 30 years. An important way to control cancers, diabetes mellitus and obesity is to focus on reducing the risk factors associated with these non-communicable diseases. However, previous case or cohort studies into the risk factors associated with the three epidemics have controversial findings which may be the results of circumstantial study designs. It may be necessary to use broadly based ecological study to obtain new insights into the associations between risk factors and epidemic at population level. The international health organizations, such as the WHO and the International Diabetes Federation (IDF) monitor and publish country specific health data in relation to the cancer, diabetes and obesity. These data have been helping governments, policy-makers, funders and researchers track and investigate the priorities of health research and development based on public health needs and ensure that funds and resources are used to meet the priorities. In terms of tracking and investigating the risk factors of the epidemics, ecological studies have several advantages in study designs over case or cohort researches: 1) More risk factors can be included in the data analysis. 2) Cumulative/ prolonged effects of risk factors on epidemics can be considered in the studies through backdating the risk data. 3) The data on risk variables used in ecological studies are objective because they are collected independent from epidemiological data. In patient-based surveys or anonymised clinical records people with any disease tend to exaggerate negative life events in comparison to people with average or good health. For instance, obese people may misinform how much sugar they have consumed trying to appear more cautious in their dietary choices than they really are. With the advantages of ecological studies, this thesis seeks to show that reduced natural selection, nutrition/diet and birth behaviour may be independent predictors of the modern noncommunicable epidemics. To achieve this, we collected and analysed data from 191 countries across over 30 years in ten investigations: Natural selection is considered a force of evolution that adapts populations to their environments. However, humans manipulated their environments and supplemented natural properties of their bodies by medical procedures and technologies, so that natural selection no longer is a force of adaptation. Its operation as a force differentiating reproductive success of individuals has been seriously relaxed. This allows practically any person to pass their genes to the next generation, thus leading to accumulation of deleterious mutations whose effects are controlled by artificial means. In Investigations 1-3, it is proposed that modern humans may not be naturally well adapted to the current environment because their survival capacity and “fitness” have been maintained by application of high levels of medical services, nutrition and public health advocacy. The studies were conducted through analysing correlations between relaxed natural selection indexed by the Biological State Index (Is) with incidence rates of cancers and Type 1 diabetes mellitus, and prevalence rates of sex-specific obesity. Meat has been advocated as one of the major contributors to obesity prevalence because it contains high energy component of fat. It is a fact that selective breeding, butchery and cooking which aim for leanness (more protein) have minimized the fat intake in our daily diet. However, meat is still reported as a contributor to body weight increase significantly because of its protein content. Investigation 4 hypothesized that meat protein in modern diet may have been providing energy surplus to our daily life which contributes to obesity. The hypothesis was examined through analysing the correlations between obesity prevalence and total meat and meat protein consumption respectively. Both meat and sugar (sucrose) in our daily diet contain the slower digested component and cause insulin resistance. However, it is widely accepted that sugar has been a major contributor to obesity. The role of meat in this regard has not been widely recognised. Investigation 5 compared the use of sugar and meat to predict obesity prevalence worldwide showing that meat availability predicts increase of obesity to the same extent as sugar availability. Red meat and processed meat have been proposed as the major predictors of prostate cancer, but those studies are circumstantial, and the findings are controversial. Total meat (flesh) has not been associated with prostate cancer. Investigation 6 postulated that total meat (flesh) may be an independent predictor of prostate cancer. This postulation was examined using country specific data, from a global perspective, that population with more total meat consumption, may have higher incidence rate of prostate cancer, with empirical, macro-level data collected from the major international organizations. Gluten has been considered as the trigger of a number of diseases. Worldwide, incidence of gluten-related diseases is increasing. Wheat, the storage proteins, is the main source of gluten, but the adverse effects of wheat on obesity have not been tested. Investigation 7 analysed and compared the associations between obesity prevalence and wheat, rice and maize, and identified that wheat is the hidden risk factor of obesity. Contrarily, consumption of maize and rice showed the protective role in obesity prevalence. Therefore, the adverse effects of wheat on increasing body weight may have been covered by maize and rice when cereals consumption is advocated as the healthy diet component. Previous studies into the relationship between low parity and risk of cancers revealed that the decreasing number of children born into a family was associated with the risk of cancers of the mother and a few other cancers of family members. However, these studies did not identify that parity may be the most influential predictor of breast cancer and ovarian cancer. Neither did these studies show that greater parity has the protecting effects on developing site cancers of family members. Investigation 8 hypothesized that greater family size may protect the whole family from developing cancers. The hypothesis was examined through analysing relationships between total fertility rate, indexing family size and incidence rates of male and female cancers. Investigations 9 and 10 analysed and compared the contributing effects of multiple risk factors of female breast cancer and ovarian cancer and identified that low parity (indexed by birth rate) may be the most influential risk factor of female breast cancer and ovarian cancers respectively. The information gathered from the ten studies reveals that 1) Reduced natural selection may be the significant predictor of cancer, Type 1 diabetes and obesity; 2) Meat consumption may be the risk predictor of obesity and prostate cancer; 3) Wheat may be a hidden contributor to obesity prevalence worldwide. 4) The number of children born into a family may be the strong predictor of female breast cancer and ovarian cancer and it may be associated with the cancer risk of all family members. In general terms, the investigations presented in this thesis show that “ecological analyses” of worldwide data confirm known relationships between some risk factors and incidence/prevalence of non-communicable diseases and can reveal new, hitherto unknown relationships, that are interpretable in the context of human biology.