The incidence of overweight and obesity has increased over recent decades in developed countries. In 2008, the World Health Organization (WHO) estimated that over 1.4 billion adults were overweight and, of these, 200 million men and nearly 300 million women were obese. Moreover, more than 40 million children under the age of five were overweight in 2010 (World Health and Organization, 2012). In adults, overweight and obesity conditions are usually classified accord- ing to the body mass index (BMI), defined as an individual’s weight in kilograms
divided by the square of the height in meters. A BMI value 25 kg/m2corresponds
to overweight and a BMI 30 kg/m2corresponds to obesity. BMI provides a useful
population-level estimation of overweight and obesity in adults. However, it is the same for both sexes and ages and so it should be considered as an approximate guide since it may not reflect the same degree of fatness in different individuals (World Health and Organization, 2012).
1. Introduction
The balance between energy intake and energy expenditure is the basis of weight management throughout life (Lawson et al., 2004). Energy intake is regu- lated by several mechanisms including hormones, body-fat storages, gut peptides and post ingestion factors (Frary and Johnson, 2004). On the other hand, energy expenditure is explained by the resting metabolic rate, the thermogenic effect of food and voluntary physical activity. A longstanding positive energy balance re- sults in overweight and obesity. Although abnormalities in any of the regulation mechanisms can lead to weight fluctuations, evidence strongly suggests that die- tary and activity patterns are the primary causes of the weight gain in industrial societies (Willett, 1998). Evidence is provided e.g. by the dramatic changes in the prevalence of overweight in individuals migrating from countries with low adiposity to industrialized countries (Willett, 1998), where the consumption of energy-dense foods rich in fat, salt and sugars has been increasing. In parallel, there has been a decrease of physical activity as a consequence of the sedentary lifestyle, changing modes of transportation and urbanization. Since obesity is a risk factor for cardio- vascular diseases, diabetes mellitus, liver and gall bladder disease, and is associ- ated with an increased risk of mortality (Ogden et al., 2007), the obesity epidemic has induced new studies aiming at identifying environmental factors that could play a role in the energy balance.
The development of obesity has recently been associated with the gut micro- biota composition, in particular with the increased capacity of the microbiota to harvest energy from the diet (Turnbaugh et al., 2006). The finding that germ-free mice were apparently protected against diet-induced obesity (Bäckhed et al., 2004) has however been contradicted (Fleissner et al., 2010). These effects were shown to be highly dependent on the type of high-fat diet fed to the germ-free mice, and were also found to be linked to differences in energy expenditure. Stud- ies on energy balance and energy-restricted diets administered to overweight and obese individuals have associated the caloric and nutrient intake with the fecal microbiota composition (Angelakis et al., 2012). The weight loss in a group of 12 obese individuals assigned to either a fat-restricted or a carbohydrate-restricted diet during the course of one year was associated with an increase in the relative abundance of Bacteroidetes, while that of Firmicutes decreased (Ley et al., 2006b). Moreover, a 10-week energy-restricted diet in adolescents brought about increased counts of B. fragilis and decreased counts of Clostridium coccoides and B. ongum in their fecal samples (Santacruz et al., 2009). Similarly, obese adolescents with weight loss above 4 kg had reduced counts of bacteria within the E. rectale-C.
coccoides group and increased numbers of the Bacteroides/Prevotella group, after
a 10-week energy-restricted diet (Nadal et al., 2009).
Diet modification with an energy restriction plan is the most common treatment for moderate obesity. However, most people are unable to make the lifelong die- tary changes needed for weight management (Bäckhed et al., 2004), thus yielding limited and transient weight loss. Weight loss programs using very low energy diets (VLED) combined with exercise and behavioural changes have provided an alternative treatment for severely obese patients, by producing greater weight loss than the conventional diet and avoiding the excess loss of lean body mass
1. Introduction
(Pekkarinen, 1999). VLED are defined as diets providing a maximum of 800 kcal with high quality protein (Mustajoki and Pekkarinen, 2001). Their major advantage is the rapid weight loss. No studies on the gut microbiota of individuals in a VLED plan have hitherto been published.
Bariatric surgery has been increasingly employed in humans as a treatment for severe obesity and has been reported to have an impact on the fecal microbiota composition of obese patients. Bacteroides/Prevotella group abundance was reported to increase three months after the bypass implantation, being highly dependent on the caloric intake. However, no change in the proportion of Firmicu- tes/Bacteroidetes ratio was observed in the same study (Furet et al., 2010). A different study observed higher proportions of -Proteobacteria, Fusobacteria and
Akkermansia in the fecal microbiota of gastric bypass individuals, which differed
from those of both obese and lean subjects (Zhang et al., 2009). The lean individ- uals had increased proportions of Lachnospira as compared to the obese and gastric bypass individuals. Moreover, the obese individuals of the study had higher numbers of Prevotellaceae and Methanobacteriales than the other two groups.