This study aimed to evaluate the association between PDs and undernutrition in Yemeni population and the correlation between undernutrition and other risk factors including age, gender, level of education and habits as smoking and khat chewing.
The results showed that there were non-statistically significant association between PDs and undernutrition participants. Mild undernutrition was found in (60.7%) of study participants and gingivitis was diagnosed in (81.2%) of study participants. This association may reveal that the reason of gingivitis presence in most of the study sample was the presence of mild undernutrition that may cause a little effect on the immune response.
BMI is the most common non-invasive tests to assess malnutrition . Alb is also a well-known marker of nutritional status . Low BMI and Alb in both genders were not significantly associated with PDs. The literature has reported that the severity of undernutrition has massive effects on the immune system which plays a role in progression of immunodeficiency . There are many studies which clarified that the pathogenesis of periodontitis is significantly related to the host response with microbial factors [30, 36-38] . The Cytokines, which are chemical mediators of inflammatory response, can be influenced by nutrition as a related factor . People with severe undernutrition are more susceptible to many microbial opportunistic infections . Therefore, this may clarify why low BMI and Alb is not significantly associated with PDs as most of the participants were in a mild undernutrition conditions.
Rodrigues et al., (2014)  published a positive correlation between hypoalbuminemia and periodontitis. A meta-analysis of 63 studies (2125 patients) discussed the effect of starvation on Alb and notified that the level of Alb remains normal before the patients reach the ultimate stage of starvation where undernutrition became physically evident . In other words, hypoalbuminemia requires a more destructive inflammation and severe undernutrition to occur. Therefore, this may explain the non-significant relation between low BMI and Alb with PDs.
The present study showed that most of participants are between late adolescence and early adult and the majority of the samples were males (58.5%) similar to Degarage et al., 2015 . This can be explained by the neglect behavior and the inability of males to take care of themselves specially those who are studying away from their families and cannot cook food. Regarding the occupation, most of participants were students (60.3%) and most of their education level was bachelor degree (49.8%). Among males group, age, education level and smoking were significantly associated with PDs (P=0.001, P=0.004, P=0.002; respectively). There is a high prevalence of khat chewing in Yemen 43.27% . Khat chewing habits promote the development of other habits like cigarette smoking [45-46]. smoking is considered as one of the most or significant life style factors that is associated or linked to PDs. It is considered as a detrimental factor that influence the occurrence and progression of periodontitis [47-48]. This can be explained by the Yemeni society lifestyle of chewing khat which is a common practice among high schools, colleges and university students, since it is considered as a mild stimulant that promote energy during working or studying . Most chewers get a good degree of concentration and raise energy levels and awareness, promote imaginative ability and capacity to incorporate ideas, and enhance the ability to communicate especially among Yemeni males . khat chewing caused obvious CNS symptoms such as loss of appetite (anorexia) and may be associated with the mixed effect of the central amphetamine-like with delaying of gastric empty and insomnia that leads to late waking up in the morning and a reduced activity performance caused by the central release of noradrenergic neurotransmitters [51-52]. Moreover, cathinone promotes or elevates the sympathomimetic activity that results in a late discharge of food from stomach . This may explain why khat chewing can lead to malnutrition.
Poor oral hygiene among males is another risk factor of gingivitis. This can be due to the masculinity behavior of thinking that oral hygiene is not connected to men strength . In contrast, chronic periodontitis presented in the older age  and persons of age 40 and above were four times more probably to have periodontitis than younger ages . This may explain why the majority of the sample was diagnosed with gingivitis.
Among females group; age, level of education, smoking and khat chewing were significantly associated with the PDs (P=0.014, P=0.000, P=0.002, P=0.000; respectively) due to the previous reasons that explained in males group.
Limitation was the lack of knowledge of patient about the importance of BMI and Alb in diagnosing undernutrition which was an obstacle during conducting sample.