These values were lower than what was reported for 1975 as 14.6% and 13,8%, respectively, indicating a worldwide reduction in the extent of undernutrition. Causes
A person may be underweight due to genetics, improper metabolism of nutrients, lack of food (frequently due to poverty), drugs that affect appetite, illness (physical or mental) or the eating disorder anorexia nervosa.Being underweight is associated with certain medical conditions, including type 1 diabetes, hyperthyroidism, cancer, and tuberculosis. People with gastrointestinal or liver problems may be unable to absorb nutrients adequately. People with certain eating disorders can also be underweight due to one or more nutrient deficiencies or excessive exercise, which exacerbates nutrient deficiencies.A common belief is that healthy underweight individuals can ‘eat what they want’ and then burn it off either by high levels of activity or elevated metabolism. It has been shown however that individuals with BMI < 18.5 eat about 12% less calories than individuals with normal BMI (21.5 to 25) and they are 23% less physically active (by accelerometry). Problems
Being underweight can be a symptom of an underlying condition, in which case it is secondary. Unexplained weight loss may require a professional medical diagnosis.Being underweight can also cause other conditions, in which case it is primary. Severely underweight individuals may have poor physical stamina and a weak immune system, leaving them open to infection.
It can be distinguished by its pale color when stained with haematoxylin and eosin. The presence of odontoblastic processes here allows the secretion of matrix components. Predentin can be 10-40μm in width, depending on its rate of deposition. : 134–137

Types
There are three types of dentin, primary, secondary and tertiary. Secondary dentin is a layer of dentin produced after the root of the tooth is completely formed. Tertiary dentin is created in response to a stimulus, such as a carious attack or wear. Primary dentin
Primary dentin, the most prominent dentin in the tooth, lies between the enamel and the pulp chamber (near dentinoenamel junction). The outer layer closest to enamel is known as mantle dentin. This layer is unique to the rest of primary dentin. Mantle dentin is formed by newly differentiated odontoblasts and forms a layer consistently 15-20 micrometers (µm) wide. Unlike primary dentin, mantle dentin lacks phosphorylation, has loosely packed collagen fibrils and is less mineralized. Below it lies the circumpulpal dentin, more mineralized dentin which makes up most of the dentin layer and is secreted after the mantle dentin by the odontoblasts. Circumpulpal dentin is formed before the root formation is completed. Newly secreted dentin is unmineralized and is called predentin. It is easily identified in hematoxylin and eosin stained sections since it stains less intensely than dentin. It is usually 10-47μm and lines the innermost region of the dentin. It is unmineralized and consists of collagen, glycoproteins, and proteoglycans. It is similar to osteoid in bone and is thickest when dentinogenesis is occurring.