Human fat consumption has certainly changed drastically from the hunter-gatherer conditions, through the beginning of agriculture to modern times.
Changes concerned both the amounts and the quality (Simopoulos, 1999), from the low amounts of fats, especially of vegetable origin, with relative abundance of long-chain polyunsaturated fatty acids (LC-PUFA), components of structural lipids in lean meat of wild animals and fish, in prehistoric conditions, to the progressive increment in the consumption of fats from farmed animals and cultivated vegetables.
The introduction and development of agriculture have changed fat intake markedly, although for a long time changes concerned mainly the continuity of fat supply after agriculture development as opposed to the sporadic intake in hunter-gatherers. Following the progressive depletion of food obtained from small mammals, fish, fowls and gathered plants, associated with the increase in human population numbers, cereal grains became the dominant caloric and protein source of most early cultures.
Drastic changes in fat intakes have occurred however, especially in recent times, i.e. in the period after the Second World War, for a number of reasons: fats represented in the past the most expensive part of the diet, since fat/oil productions in developing countries were limited by climatic and economic reasons, and importation from fat-producing countries was expensive. Fat consumption was therefore strictly correlated with national per capita incomes (FAO, 1977). With the introduction of extensive cereal grains and seed oil-raising crops, the availability of fats for human consumption and animal feeding increased dramatically. Fats became recently rather inexpensive, even used as fuel, and available on a global scale to most populations, which in several situations appear to be exposed to hypercaloric and yet deficient (in several essential micronutrients) dietary conditions. In addition, increments in seed oil consumption brought about marked increments in the consumption of PUFA, especially of the omega-6 series (i.e. linoleic acid, 18:2 omega-6). However, differences in fat intakes among populations are still present, with generally lower intakes (7±15 energy per cent) in countries from the East and Far East, e.g. Bangladesh, Korea, China, India, Philippines, and from Africa, e.g. Tanzania, Nigeria, Ethiopia (FAO, 1994), vs. around 32±38 energy per cent (en per cent) in several countries on Western diets. A relatively recent study carried out in Tanzania dealt with populations on diets with 8±13 en per cent from fats (Pauletto et al., 1996), i.e. still much lower than the levels in Western countries. High fat intakes are generally associated with high saturated fatty acids (SFA), and also relatively high intakes of PUFA, especially of the omega-6 series.
There are also still appreciable differences in fat intakes among Western populations as indicated by a cross-evaluation in 14 European countries (Hulshof et al., 1999). Variations concerned both the absolute intakes with values ranging from around 31 en per cent, in Finland, Italy, Norway and Portugal, up to over 40 en per cent in Germany, Iceland, Spain and Belgium. As to the qualitative differences, SFA range between around 10 en per cent, in most Mediterranean Countries to about 19 per cent, monounsaturated fatty acids (MUFA) contribute to about 9±12 en per cent, with higher values in Greece and the southern parts of Italy and Spain (high olive oil intake), and PUFA ranging between 3 and 7 en per cent. Trans FA range between 0.5 en per cent in Greece up to round 2 en per cent in Iceland, and are therefore not considered to be a major problem. As to the trends in nutrient intakes over time, it is of interest that a study carried out in 10-year-old children over two decades (1973±94) in Louisiana, revealed that total energy intake remained unchanged during that time period (although it declined as Kcal/body weight), but there was a trend toward weight gain. There was a significant increase in percentage energy from proteins and carbohydrates and a decrease in percentage energy from fat (mainly SFA and MUFA). In general, although more children met the recommendations for total fat, SFA and dietary cholesterol, the vast majority continued to exceed prudent diet recommendations.
Recently, it has been proposed that the role of the diet, and particularly of dietary fats in vascular disease and in its protection, have been vastly underestimated, owing to failure to understand the importance of postprandial events (Spencer, 2002). The compounds that repeatedly enter the circulation every day during our lifespan certainly result in the exposure of vessel walls to a large variety of nutrients, and also of potentially stressful factors. These include postprandial oxidative stress, consequent to the consumption of meals containing oxidized and oxidizable lipids. This results in the postprandial elevation of plasma lipid peroxides (Ursini and Sevanian, 2002), while on the other hand, AO in meals may minimize postprandial oxidative stress.
Based on the above considerations, it appears that the consumption of heated/ fried fats may be a contributing factor in the impact of dietary fat on health. Alterations of fats and oils and of the lipid components of meals are induced by various factors (heat, light, irradiation, pH, oxygen, moisture, pro-oxidizing agents, storage at room temperature) through several processes. Refining of vegetable fats and oils has no deleterious effects upon their composition as far as deliming, neutralization or bleaching are concerned, but during deodorization or physical refining, small amounts of dimeric triglycerides and of trans fatty acids are formed depending upon temperature and duration (Billek, 1992). In general, boiling and baking have no effects, and short-term shallow frying shows only minor changes in quality. The situation is different for deep-fat frying, which can cause serious alterations, especially if the oil is used for too long. The chemical reactions involved are predominantly isomerizations, polymerization and oxidation processes. There are certainly differences related not only to the cooking/frying conditions, e.g. conventional cooking methods vs. microwave cooking (Regulska-Ilow and Ilow, 2002), but also to the type of fat, the oils containing more unsaturated fatty acids, e.g. several seed oils rather than olive oil, being more susceptible to oxidative changes. In addition to alterations in the chemistry of fats, changes can also occur in antioxidant levels and antioxidant activity of the oils (Warner, 1999).
The addition of antioxidants to the oils may protect the fatty acids from oxidation, and during heating/frying the loss of lipid-soluble vitamins (e.g. the tocopherols) from the oil precedes that of more polar compounds, e.g. phenolics in the case of olive oil (Gomez-Alonso et al., 2003), suggesting that they may act as protecting agents against AO vitamins. Fried foods are generally considered detrimental to our health especially in relation to lipid oxidation, but not all food components are equally affected, since there is, for example, little or no effect on the protein or mineral content of fried food. In addition, it should be considered that when fat/oils are used in frying food, e.g. potatoes, the temperature reached at the surfaces between food and oils is markedly lower than the temperature of the boiling oily phase, owing to the extensive evaporation of the water in the food, and that the formation of a `crusty’ surface prevents a significant penetration of the oxidized products into the food.
Among the oxidative products generated from lipids in foods, attention has been expressly paid to cholesterol oxides. Several cholesterol oxides are commonly found in foods with high cholesterol contents, such as meat, egg yolk and egg-based products (cakes, sweet biscuits, mayonnaise) if fresh materials are not used in their manufacture, and in full-fat dairy products (Savage et al., 2002). Fresh foods generally contain very low levels of cholesterol oxides, while their levels are increased by storage, cooking and processing. Dietary cholesterol oxides appear to be well absorbed, and to influence postprandial lipoprotein particle size and composition. These changes may have effects on the clearance of chylomicrons from plasma, arterial delivery of oxysterols and possible deposition in arterial lesions (Vine et al., 1997). In general, lipid peroxides from the diet may contribute significantly to the whole process of lipid peroxidation, especially during the postprandial phase, in addition to the peroxides produced through endogenous processes.
A number of studies have been devoted to investigate the health effects of thermoxidized oils and fats (Billek, 2000). While early studies using extremely overheated fats showed toxic effects in animals, the administration of fats and oils heated in equipment for deep-fat frying under the conditions of good commercial practice did not show detrimental effects on classical parameters (e.g. growth, toxicity tests) even when fed in high amounts for long time periods. However, human studies specifically related to the postprandial effects of unheated or heated oils showed increments of markers of lipid oxidation in serum related to the type of oil (e.g. safflower had greater effects than olive oil, both when uncooked and especially when cooked) (Sutherland et al., 2002). The effects of the administration of both cooked oils on major functional parameters, e.g. endothelium-dependent dilatation were, however, minimal (Williams et al., 2001). Other parameters not directly related to lipid peroxidation and to changes in plasma antioxidants have also been shown in animal studies after the administration of thermally oxidized fats: an increase in plasma thyroxine concentrations irrespective of the vitamin E and selenium status (Eder et al., 2002). In general, this type of study needs to be substantially extended and applied to more practically relevant conditions. The experimental design is crucial in this respect since the type of oxidized fats to be administered and their actual chemical composition, the context of the other components (macro- and micro-nutrients) of the diet, the doses and duration of the experiments and selection of subjects are major determinants in the outcomes.
Several health organizations over the last few years have provided recommendations on fat intake with the aim to improve our health status especially with respect of CV disease and cancer. The Scientific Conference on Dietary Fatty Acids and Cardiovascular Health (AHA, 2001) and the Executive Summary of the NCEP Expert Panel (NCEP, 2001) have provided the following recommendations: total fat 25±35 en per cent, SFA < 7 en per cent, MUFA up to 20 en per cent and PUFA up to 10 en per cent. On the other side, since evidence has been accumulating on the differential and somewhat contrasting biological roles of the omega-6 and omega-3 fatty acids, it has also been proposed by the board of the International Society for the Study of Fatty Acids and Lipids (ISSFAL) (NIH Workshop 7±9 April 1999) that individual PUFA should be considered separately and that, in addition to a value not exceeding 7 en per cent for total PUFA, LA, the major omega-6, should not exceed 4-5 en per cent, while the omega-3 ALA (alpha linolenic acid) should be at least 1 en per cent and EPA + DHA in a range of at least 0.3 g up to 1 g/day. The omega-6/omega-3 ratio is also considered an important parameter and a ratio of about 4 or 5/1 has been recommended, i.e. a ratio in the range of that apparently present in the diet before the explosion of modern agriculture, and lower than the ratio greater than 10/1 in our diets. Practical approaches to the definition of a diet with an adequate FA composition and can be based on the use of food composition data, such as those in the web site of the USDA (http://www.usda .gov). Although information from databases may not be totally adequate with reference to some minor FA components, e.g. some omega-3 FA, their use is valuable.
As to the recommendations concerning the consumption/intake of AO, although randomized controlled trials of AO vitamins as supplements have shown that they have no beneficial effect on risk for myocardial infarction or stroke, increments in the consumption of vegetables and fruits should be highly recommended. As an example, the list of 10 foods recommended as very healthy by Time magazine (2002), on the basis of generally accepted scientific evidence, and selected also for the content in AO in addition to other bioactive compounds, include the following vegetables and fruits: tomatoes (rich in the carotenoid lycopene and vitamin C), spinach (rich in the AO phytochemicals lutein and zeaxanthine, in addition to providing iron and folate), broccoli (rich in beta-carotene and vitamin C, in addition to some phytochemicals, e.g. indole-3 carbinol with detoxifying activity), nuts (rich in vitamin E, as well as in the omega-3 FA alpha-linolenic acid, and in ellagic acid, with potential anticancer activities), red wine (polyphenolic AO derived from the skin of the grapes), oats (rich in tocotrienols, AO with vitamin E-like activities, and in fibres, e.g. betaglucan), (green) tea (rich in the AO phenols, the catechins), blueberries, very rich in several types of AO (especially the antocyanins). Dietary AO, in addition to providing precious protective and health-promoting agents, may play a special role by acting at the gastrointestinal tract, possibly a major site of production of toxic oxidized products (Halliwell et al., 2000).
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