before becoming life-long habits (see Chapter 3). Smoking is a good example. According to Viner (2012, p. 8):

[T]he tracking history of smoking is of initiation and increasing use in adolescence, followed by relative stability from the late teens onwards. Compared with other forms of drug use, cigarette smoking shows the least decline in young adulthood. Those who start early in adolescence, e.g., less than 14 years, are more likely to become lifelong smokers.

Comparing health behaviors across high-income countries is difficult due to a scarcity of data and extensive challenges with measurement. In contrast with such health indicators as blood pressure or serum lipid concentrations, which are easy to measure in standardized units across countries, health behaviors are rarely examined in a uniform fashion, even within the same country, and in some countries they are not measured at all. For example, a survey about tobacco use can ask about cigarette smoking only or can also include cigar and pipe smoking or the use of smokeless tobacco. Respondents can be asked how many cigarettes they smoke each day or whether they smoke “currently,” have “ever smoked,” have ever smoked 100 cigarettes, or have smoked in the past 30 days.

Similarly, questions about physical activity can differentiate between leisure-time “exercise” and physical activity, between regular and episodic physical activity, between levels of exertion, and between duration and frequency of activity per day, per week, or per month. They can measure sedentary behavior, moderate activity, and intense physical activity. They can document types of activity, such as sitting, walking, cycling, running, or gardening. No single measure is the established standard and, unlike smoking, no single measure is the strongest predictor of premature death or morbidity or has the strongest link to obesity.

Questions about diet are perhaps the most varied. They can address food groups (e.g., fruits and vegetables, grains, meats), specific nutrients (e.g., saturated and unsaturated fats, trans fats, sodium, calcium), and caloric intake. A proper dietary history involves numerous questions (Paxton et al., 2011) and can produce different results depending on how the data are collected (Erinosho et al., 2011). Nutrition science has yet to identify the most important dietary predictors of longevity or of specific disease outcomes, such as cancer or myocardial infarctions. Few countries include sufficient questions on population surveys to evaluate the national diet, and very few questions are asked in common across countries, confounding efforts to make meaningful or accurate comparisons. Often, the only common data across countries are governmental statistics on national caloric expenditures and food consumption divided by the population size to derive per capita estimates.

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