According to several studies ,  and expert opinions  high blood glucose level affects the optimal functioning of the immune system. This might lead to deadly complications in the Flu and Possibly COVID-19 .
The lowest normal blood sugar level is 3.9 mmol/l (70 mg/dL) .
Gotosurvive.com suggests the target blood glucose limit throughout the day at a range of 4.4 ± 0.5 mmol/L (79 ± 9 mg/dL).
1. Type 2 Diabetes and its Impact on the Immune System.
Retrieved April 28, 2020 from https://www.ncbi.nlm.nih.gov/pubmed/31657690
2. Case Western Reserve University. (2015, August 6).
High blood sugar of diabetes can cause immune system malfunction, triggering infection: Scientists show how sugar-derived molecules can weaken infection-fighting antimicrobial beta-defensin peptides. ScienceDaily.
Retrieved April 28, 2020 from www.sciencedaily.com/releases/2015/08/150806151354.htm
3. American Diabetes Association
People with diabetes do face a higher chance of experiencing serious complications from COVID-19.
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This is also caused by above-target blood sugars, and both could contribute to more severe complications.
Retrieved March 30, 2020 from www.diabetes.org/covid-19-faq
4. Scientific American.
How Blood Sugar Can Trigger a Deadly Immune Response in the Flu and Possibly COVID-19.
Glucose metabolism plays a key role in the cytokine storm seen in influenza, and the link could have potential implications for novel coronavirus infections.