The importance of vegetables and fruit in the diet

The importance of vegetables and fruit in the diet

The importance of vegetables and fruit in the diet

Mieke Faber and Paul van Jaarsveld, Medical Research Council

Vegetable and fruit intake is a significant determinant of disease burden in both developed and developing countries, and a low intake is among the top 10 risk factors contributing to attributable mortality. In the year 2000, about 11.1 million males and 12.5 million females over 15 years of age in South Africa did not eat enough vegetables and fruit.

1 Worldwide low intakes accounted for an estimated 2.6 million deaths.2 Eating a variety of different vegetables and fruit as part of a balanced diet will help prevent major non-communicable diseases such as, for example, cardiovascular disease and certain cancers.

Vegetables and fruit are rich in vitamins, minerals, dietary fibre, phytochemicals and antioxidants. Phytochemicals are bioactive compounds found in plants. Although phytochemicals are not essential in the diet, it is believed that they contribute significantly towards the health benefits of diets high in vegetables and fruit. Some of the best known phytochemicals are carotenoids, such as α- and β-carotene, β-cryptoxanthin, lutein, lycopene and zeaxanthin. Other phytochemicals include limonene (found in e.g. citrus fruit such as lemon and orange), ellagic acid (found in e.g. pomegranate, strawberry, grape and peach), and sulphoraphane (found in e.g. cruciferous vegetables such as broccoli, Brussels sprouts or cabbage). Flavonoids, another classification of phytochemical, include compounds such as quercetin (found in e.g. apple, red onion, red grape, citrus fruit, tomato, broccoli and other leafy green vegetables), anthocyanins (found in e.g. blueberry, cranberry, black raspberry, red raspberry, cherry and red cabbage) and hesperidin (found in e.g. citrus fruit such as lemon and orange).

Dark-green vegetables and colourful fruits are particularly rich sources of antioxidants, which are substances that may protect the body’s cells against the oxidative damaging effects of free radicals. Free radicals are molecules produced during normal metabolism, or by environmental exposures like tobacco smoke, certain chemicals, pollution and radiation. Free radicals can damage cells in the body, and may play a role in heart disease, cancer and other diseases. Antioxidants can neutralize free radicals and may reduce or even help prevent some of the damage they cause. Examples of antioxidants include β-carotene, lutein, lycopene, selenium, vitamin A, vitamin C and vitamin E.

Non-communicable diseases

There is convincing scientific evidence showing that a high intake of vegetables and fruit has a protective effect against cardiovascular disease,2 stroke,3 certain cancers,2,4,5, type-2 diabetes mellitus6 and the metabolic syndrome.7

– Vegetable and fruit consumption is inversely associated with the risk for developing coronary heart disease.8 Increasing consumption of vegetables and fruit up to 600 g per day could reduce ischemic heart disease by about 31% worldwide.2

– Increased consumption of vegetables and fruit is also associated with a reduced risk of stroke. Vegetables and fruit are rich sources of potassium, which has a blood-pressure-lowering effect that contributes towards a reduced risk of stroke.3 Increasing consumption of vegetables and fruit up to 600 g per day could reduce ischaemic stroke by about 19% worldwide.2

– Increasing the intake of green leafy vegetables could significantly reduce the risk of type-2 diabetes mellitus.6 Consumption thereof has been shown to be associated with a lower risk for type-2 diabetes mellitus, probably because the low energy density, low glycemic load and high fibre and micronutrient content, particularly magnesium.9 Consumption of fruit juices, however, is associated with a higher risk for type-2 diabetes mellitus, probably because of the high sugar and high glycemic load.9

Vegetables and fruit contain varying amounts of several micronutrients and, on average, are low in energy content, resulting in a high nutrient density. With other words, they are nutrient rich in relation to their low energy content.10 It is therefore important to eat vegetables and fruit as part of a healthy balanced diet to reduce the risk of obesity and to maintain a healthy body weight. People with a healthy body weight generally have a lower risk for non-communicable diseases.

Micronutrients

Low intake of vegetables and fruit contributes significantly towards micronutrient deficiencies in developing countries, including South Africa. A large majority of the South African population consumes a cereal based diet, with vitamin A-rich vegetables and fruit being the least consumed foods.11 This is a concern as vitamin A deficiency is one of the major nutritional deficiencies in South African children.12 Yellow and orange-fleshed fruit (e.g. ripe mango, pawpaw, yellow peach) and vegetables (e.g. pumpkin, butternut, carrot) and dark-green leafy vegetables contain high amounts of β-carotene, which is converted to vitamin A in the body. An adequate amount of vitamin A is needed to maintain eye health and boost the body’s immune system to fight infectious diseases.

Citrus fruit (e.g. orange, grapefruit, naartjie), guava and tomato contain large amounts of vitamin C, which is a powerful antioxidant that can protect cells from cancer-causing agents, increases absorption of calcium (an essential mineral for strong bones and teeth) and iron from other foods. (Low iron levels can lead to anaemia, one of the most severe nutrition-related disorders, affecting about 2 billion people worldwide.) Vegetables and fruit are rich in potassium which is needed for the nerves and muscles to function properly. B-vitamins are necessary for converting food into energy. Folate, commonly found in green leafy vegetables significantly reduces the risk of neural tube birth defects in newborns and contributes towards the prevention of heart disease.

How much vegetables and fruit should we eat?

The World Health Organization (WHO), Food and Agricultural Organization (FAO) of the United Nations and the World Cancer Research Fund recommend a minimum daily intake of 400 g of a variety of vegetables (excluding potatoes and other starchy tubers) and fruit for the prevention of chronic diseases such as heart disease, cancer, diabetes mellitus and obesity, as well as for the prevention and alleviation of several micronutrient deficiencies, especially in less developed countries. This translates to 5 servings per day, where one serving equals approximately ½ cup of vegetables or one small fruit. The South African food-based dietary guidelines suggest at least 5 servings per day, with at least one vitamin C-rich fruit/vegetable and one dark-green or dark-yellow vegetable per day. A daily consumption of 400 g vegetables and fruit will provide, on average, approximately 14 g fructose.13

Whole fruit (the skin and the pulp) provides more nutrition than fruit juice. The edible skins of many fruits, such as apple, apricot, blueberry, grape, pear, plum, prunes, raisins, raspberry and strawberry are sites of important biological activity, but are removed when fruits are juiced. The skin of the fruit interacts with sunlight, and forms a variety of coloured pigments such as, for example, carotenoids and flavonoids. The skin is also an important source of dietary fibre.

References

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2. Lock K, Pomerleau J, Causer L, Altman DR, McKee M. The global burden of disease attributable to low consumption of fruit and vegetables: implications for the global strategy on diet. Bulletin of the World Health Organization 2005; 83(2): 100-108.

3. He FJ, Nowson CA, MacGregor GA. Fruit and vegetable consumption and stroke: meta-analysis of cohort studies. Lancet 2006; 367: 320-326.

4. Gandini S, Merzenich H, Robertson C, Boyle P. Meta-analysis of studies on breast cancer risk and diet: the role of fruit and vegetable consumption and the intake of associated micronutrients. European Journal of Cancer 2000; 36: 636-646.

5. World Cancer Research Fund / American Institute for Cancer Research. Food, Nutrition, Physical Activity, and the prevention of cancer: a global perspective. AIR, Washington DC, 2007; Chapter 4.

6. Carter P, Gray LJ, Troughton J, Khunti K, Davies MJ. Fruit and vegetable intake and incidence of type-2 diabetes mellitus: systematic review and meta-analysis. British Medical Journal 2010; 341: c4229 doi:10.1136/bmj.c4229

7. Ford ES, Mokdad AH, Giles WH, Brown DW. The metabolic syndrome and antioxidant concentrations. Findings from the third national health and nutrition examination survey. Diabetes 2003; 52: 2346-2352.

8. Dauchet L, Amouyel P, Hercberg S, Dallongeville J. Fruit and vegetable consumption and risk of coronary heart disease: a meta-analysis of cohort studies. Journal of Nutrition 2006; 136: 2588-2593.

9 Bazzano LA, Li TY, Joshipura KJ, Hu FB. Intake of fruit, vegetables, and fruit juices and risk of diabetes in women. Diabetes Care 2008; 31: 1311-1317.

10. Darmon N, Darmon M, Mallot M, Drewnowski A. A nutrient density standard for vegetables and fruits: nutrients per calorie and nutrients per cost. Journal of the American Dietetic Association 2005; 105: 1881-1887.

11. Labadarios D, Steyn NP, Nel J. How diverse is the diet of adult South Africans? Nutrition Journal 2011; 10: 3 doi:10.1186/1475-2891-10-33.

12. Labadarios D. (ed) (2007). National Food Consumption Survey – Fortification Baseline (NFCS-FB): South Africa, 2005. South African Department of Health, Pretoria.

13. Li BW, Andrews KW, Pehrsson PR. Individual sugars, soluble, and insoluble dietary fiber contents of 70 high consumption foods. Journal of Food Composition and Analysis 2002; 15: 715-723.