A recently published randomized controlled clinical trial has found that flavonoid-rich apples can improve endothelial function in individuals at risk for cardiovascular disease.
The age old saying that “an apple a day, keeps the doctor away” may not be that far away from the truth.
Apples are one of my favorite fruits for supporting health and they are packed with many nutrients including Vitamin C, fiber(pectin) to support healthy digestive function and phyto-chemicals such as polyphenols, flavonoids and anti-viral and anti-tumor chlorogenic acid.
Endothelial dysfunction is a systemic pathological state of the endothelium (the inner lining of blood vessels) and can broadly be defined as an imbalance between vasodilating and vasoconstricting substances produced by (or acting on) the endothelium.
Endothelial dysfunction is characterised by reduced NO(nitric oxide) bioavailability, is now recognised by many experts to be an early reversible precursor of atherosclerosis.
One of the main mechanisms by which dietary flavonoids are thought to influence cardiovascular disease is via protection of the bioactivity of the endothelium-derived nitric oxide (NO). 
The cardioprotective effects of apples are primarily attributed to flavonoids, found predominantly in the skin. Many of the phyto-nutrients are 3-4 times more concentrated in the skin, so it’s wise to eat the full apple and not to discard the skin as many often do.
Flavonoid-Rich Apple Improves Endothelial Function in Individuals at Risk for Cardiovascular Disease: A Randomized Controlled Clinical Trial
The randomized controlled clinical trial published 2018 in the Journal of Molecular nutrition and food research aimed to determine if acute and/or chronic (4 weeks) ingestion of flavonoid-rich apples improves endothelial function, blood pressure (BP), and arterial stiffness in individuals at risk for cardiovascular diseases (CVD).
In this randomized, controlled cross-over trial, acute and 4 week intake of apple with skin (high flavonoid apple, HFA) is compared to intake of apple flesh only (low flavonoid apple, LFA) in 30 participants.
The primary outcome is endothelial function assessed using flow-mediated dilation (FMD) of the brachial artery, while main secondary outcomes were 24 h ambulatory BP and arterial stiffness.
Other outcomes include fasting serum glucose and lipoprotein profile, plasma heme oxygenase-1 (Hmox-1), F2 -isoprostanes, flavonoid metabolites, and plasma and salivary nitrate (NO3– ) and nitrite (NO2– ) concentrations.
Compared to LFA control, the HFA results in a significant increase in FMD acutely (0.8%, p < 0.001) and after 4 weeks chronic intake (0.5%, p < 0.001), and in plasma flavonoid metabolites (p < 0.0001). Other outcomes are not altered significantly.
The study concluded:
A lower risk of CVD with higher apple consumption could be mediated by the beneficial effect of apple skin on endothelial function, both acutely and chronically. 
 Modulation of nitric oxide by flavonoids.
 Flavonoid-Rich Apple Improves Endothelial Function in Individuals at Risk for Cardiovascular Disease: A Randomized Controlled Clinical Trial.
 Flavonoid-rich apples and nitrate-rich spinach augment nitric oxide status and improve endothelial function in healthy men and women: a randomized controlled trial.
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.