Mixtures of hot water, lemon and honey are often used to alleviate symptoms of the cold or flu. Honey and lemon are said to soothe sore throats and speed recovery from these respiratory ailments. In one demonstration on the BBC, a honey and lemon mixture eased coughing brought on by chili powder. One study also found that honey with lemon improved children's coughs (2).
Honey is known to have antimicrobial effects due to its low water content, high phenol content and enzyme content. Phenols are secondary metabolic products of plants, many of which have been found to be beneficial to human health. Lemon juice is a rich source of alkaloids and flavonoids, which are plant compounds also known to have numerous health benefits, including antibacterial and antiviral effects.
One study tested the effects of honey and lemon mixtures on bacteria isolated from respiratory tract infections (streptococcus pyogenes and streptococcus pneumoniae) (1). These bacteria are responsible for sore throat and ear infection among other types of disease.
A mixture of honey and lemon (50:50) had a greater antibacterial effect than either honey only or lemon only or mixtures of honey and lemon diluted with water. Antibacterial activity in this case was measured by the diameter of the area where bacterial growth was inhibited. The amount of honey and lemon required to stop bacterial growth (Minimum Inhibitory Concentration, (MIC)) was also less than the MIC of either honey or lemon alone.
The rate of kill was also examined as a more accurate measure of antimicrobial activity. This measured the decrease in bacterial cell populations and complete killing effect. The honey and lemon mixture and lemon juice only killed all bacterial cells (streptococcus pneumoniae) in the shortest period of time (120 minutes). Some antibiotics (Ceftriaxone, Levofloxacin) and honey alone took up to 1440 minutes.
The greater effectiveness of the honey and lemon mixture suggests that the compounds in both acted synergistically. The mixture was administered at room temperature which may have preserved the antibacterial activity of the honey and lemon compounds. However diluting the mixture with water weakened its effectiveness, possibly because the highly acidic pH of the undiluted honey and lemon juice mixture increased antibacterial activity. This suggests that honey and lemon can be used for respiratory infections, and the mixture should be undiluted and unheated.
References
(1.) Mshelia, B. M., G. O. Adeshina, and J. A. Onaolapo. "The antibacterial activity of honey and lemon juice against Streptococcus pneumoniae and Streptococcus pyogenes isolates from respiratory tract infections." Advances in Biotechnology and Microbiology4, no. 5 (2017): 001-008 (PDF Link).
(2). Khalil, Amira Mohammed Saed Mohammed, and Rasha Mohamed Gamal. "Honey with lemon Improves Children’s Nocturnal Cough and their Sleep Quality as well as Their Parents." International Journal 3, no. 6 (2015): 143-152. (PDF Link)