Certain unripe fruit -- such as lychees -- can be dangerously toxic, and cause low blood sugar levels by metabolic poisoning

From the news by Adam Withnall @adamwithnall 15 June 2019 at The Independent.

India: 43 children dead in brain disease outbreak 'linked to lychee fruit toxin'

State officials express alarm at outbreak, which only appears to affect malnourished children
At least 43 children have died in less than two weeks in the Indian state of Bihar from a brain disease that scientists believe could be linked to consumption of lychee fruit.
Health officials said the children died at two hospitals in Muzaffarpur, a region of the state that is well known for its many lychee orchards harvested throughout May and June.
The state government is yet to confirm the cause of the outbreak but is attributing most of the deaths to hypoglycaemia - low blood sugar level...

From the medical literature: 

In Vivo Studies on the Mechanism of Methylenecyclopropylacetic acid and Methylenecyclopropylglycine-Induced Hypoglycemia

Ingestion of toxin-containing fruit such as the unripe ackee and litchi fruits causes potentially fatal hypoglycemia. Outbreaks of hypoglycemia, diffuse encephalopathy, seizures, and cerebral edema in children in litchi-growing regions of Asia and India have been reported during the litchi harvest, resulting in numerous deaths annually and occurrences are increasing due to the expansion of litchi fruit production.
Similarly, ingestion of unripe ackee fruit can lead to protracted vomiting followed by hypoglycemia and CNS depression, a syndrome known as the Jamaican Vomiting Sickness. While numerous factors may potentiate these syndromes, such as exposure to banned pesticides, both hypoglycemic syndromes require exposure to toxins.
Structurally similar toxins that have been implicated in causing these syndromes have been isolated from both ackee and litchi fruits. Hypoglycin A (L-2-amino-3-methylenecyclopropylpropionic acid) is the hypoglycemia-inducing toxin of unripe ackee fruit, and methylene cyclopropyl acetic acid (MCPA) is the hypoglycemia-inducing metabolite of hypoglycin A . In addition to hypoglycin A, methylene cyclopropyl glycine (MCPG) has been isolated from litchi fruit, and administration of MCPG to rodents also causes hypoglycemia.
Previous in vitro and in vivo studies have found that MCPA and MCPG can inhibit β-oxidation , suggesting that these toxins may inhibit hepatic gluconeogenesis by several mechanisms: 1) depletion of the hepatocellular energy charge (ATP/ADP), 2) depletion of acetyl-CoA, which is an allosteric activator of pyruvate carboxylase, leading to reduced activity of pyruvate carboxylase activity and/or 3) increases in butyryl-CoA, which is an allosteric inhibitor of pyruvate carboxylase . However, the molecular mechanism by which MCPA and MCPG cause hypoglycemia in vivo has not been established due to technical limitations assessing in vivo rates of hepatic pyruvate carboxylase flux...

Qiu Y, Perry RJ, Camporez JG, Zhang XM, Kahn M, Cline GW, Shulman GI, Vatner DF. In vivo studies on the mechanism of methylene cyclopropyl acetic acid and methylene cyclopropyl glycine-induced hypoglycemia. Biochem J. 2018 Mar 20;475(6):1063-1074. doi: 10.1042/BCJ20180063. PubMed PMID: 29483297; PubMed Central PMCID: PMC5884121.