High-fat and low-fat (behavioural) phenotypes: biology or environment?

It is now widely accepted that obesity develops by way of genetic mechanisms conferring specific dispositions which interact with strong environmental pressures. It is also accepted that certain dispositions constitute metabolic risk factors for weight gain. It is less well accepted that certain patterns of behaviour (arising from biological demands or environmental influences) put individuals at risk of developing a positive energy balance and weight gain (behavioural risk factors). Relevant patterns of behaviour include long-lasting habits for selecting and eating particular types of foods. Such habits define two distinct groups characterized as high-fat (HF) and low-fat (LF) phenotypes. These habits are important because of the attention given to dietary macronutrients in body-weight gain and the worldwide epidemic of obesity. Considerable evidence indicates that the total amount of dietary fat consumed remains the most potent food-related risk factor for weight gain. However, although habitual intake of a high-fat diet is a behavioural risk factor for obesity, it does not constitute a biological inevitability. A habitual low-fat diet does seem to protect against the development of obesity, but a high-fat diet does not guarantee that an individual will be obese. Although obesity is much more prevalent among HF than LF, some HF are lean with BMI well within the normal range. The concept of 'different routes to obesity' through a variety of nutritional scenarios can be envisaged, with predisposed individuals varying in their susceptibility to different dietary inputs. In a particular subgroup of individuals (young adult males) HF and LF displayed quite different profiles of appetite control, response to nutrient challenges and physiological measures, including BMR, RQ, heart rate, plasma leptin levels and thermogenic responses to fat and carbohydrate meals. These striking differences suggest that HF and LF can be used as a conceptual tool to investigate the relationship between biology and the environment (diet) in the control of body weight.