Oxidative stress has been associated with disease and ageing since the 1950’s when Denham Harman proposed the free radical theory of cellular damage. Since that time evidence has accumulated to suggest that a high level of oxidative stress is linked with cardiovascular disease and its associated disorders such as diabetes, metabolic syndrome and obesity. Much research has suggested that the oxidation of the low density lipoprotein (LDL) particle is necessary for the progression of the disease, and in response research has been keen to find dietary behaviours that can limit this process. Foods high in compounds with antioxidant properties are therefore of great interest to this branch of nutritional research. Strawberries are a good source of antioxidant chemicals and contain phenolic compounds such as quercetin, ellgic acid, anthocyanins, catechins and kaempferol, which may have beneficial in vivo effects.
When researchers1 fed 24 hyperlipidaemic subjects a strawberry beverage containing 10g of freeze dried strawberries or a placebo in a 12 week cross-over trail, subjects consuming the strawberry drink had a significantly lower concentration of oxidised LDL compared to the placebo, in response to a high fat meal. In addition, triglyceride levels were lower following the high fat meal in those subjects consuming the strawberry drink. The authors concluded that strawberry phenolics attenuate the postprandial lipaemia and LDL oxidation following a high fat meal. This generally supports other research in this field that shows beneficial effects on oxidation levels for foods with high levels of polyphenolic compounds such as wine, tea, fruit and chocolate. Although the in vitro evidence of their antioxidant ability is well reported, care should still be taken when interpreting in vivo studies, due to the extensive polyphenol metabolism that occurs during absorption.
Strawberries contain high levels of anthocyanidins, the glycosylated forms of anthocyanins. The most common anthocyanidin in strawberries is pelargonidin 3-glucoside, which can represent around 87% of all anthocyanidins. Following absorption, anthocyanidins are found in the urine mainly in the monoglucuronide forms of the parent anthocyanidin, suggesting extensive metabolism has occurred. Anthocyanidins are unusual in that they are absorbed intact, rather than being deglycosylated in the gut lumen or the enterocytes. Strawberries, black raspberries, black currents and boysenberries have all been shown to be absorbed in their glycoside forms, possibly through the stomach lining. The metabolism of polyphenols in humans is important nutritionally, because conjugation may significantly alter their in vivo effects. Because the benefits of individual compounds are not well understood, diets rich in a range of polyphenolic compounds would appear to be the most sensible health consideration at present.
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