Kestin et al. (1990) compared three different cereal brans (wheat, rice, oats) in mildly hypercholesterolaemic men. The bran was incorporated in bread and muffins and was given to the subjects for four weeks in a crossover design. In comparison with wheat and rice bran, oat bran significantly reduced the plasma cholesterol concentration with 5.6 and 3.8 per cent, respectively. The main difference between the test products was that oat bran contained twice as much water-soluble fibres as rice and wheat bran.
The dose±response effect of oat bran and oatmeal was studied in hypercholesterolaemic subjects by Davidson et al. (1991). The oatmeal or oat bran were given in doses of 28, 56 and 84 g/day for six weeks. Oat bran in doses of 56 and 84 g and oat meal in a dose of 84 g significantly reduced the total and LDL-cholesterol concentration compared with a control group given 28 g farina. The higher efficiency of the oat bran is probably due to its higher betaglucan content. The conclusion that beta-glucans is the active component was further confirmed in a study by Braaten et al. (1994a). They gave a purified preparation containing 80 per cent beta-glucans mixed in a beverage to hypercholesterolaemic participants for 4 weeks. The preparation significantly reduced the total and LDL-cholesterol levels without changing HDLcholesterol in comparison with a maltodextrin placebo drink. In this study blood samples were taken each week and it was thus possible to follow the hypocholesterolaemic effects developed in more detail.
When the participants took the beverage containing beta-glucan, the LDL-cholesterol level was reduced almost linearly during 4 weeks, and when the intake was stopped, the LDL-cholesterol level went back to the baseline value in 2 weeks. Other, longer, studies have indicated that the cholesterol-lowering effects of an intake of beta-glucans can diminish with time (Uusitupa et al. 1992). In this study there were significant cholesterol-lowering effects after 4 weeks but not after 8 weeks. Oat bran can also alter the postprandial effects after a meal in normolipidaemic humans (Dubois et al. 1995). The oat bran was added to a test meal when the subjects have been on a low-fibre diet or a diet supplemented with oat bran (40 g/day) for 2 weeks. No change in fasting blood lipid values or plasma insulin was observed after the 2-week oat bran period compared with the low-fibre period. Adding oat bran to the test meal markedly reduced the postprandial insulin rise. The postprandial effects were enhanced after 14 days of oat bran feeding and increased plasma phospholipids, increased plasma and HDL-free cholesterol, decreased plasma and HDL-cholesterol esters, were observed.
Some studies did not show significant effects on the blood cholesterol levels when oats was consumed. An oat bran concentrate was baked into a bread, one roll containing 11.2 g beta-glucan (ToÈrroÈnen et al. 1992). The bread was consumed for 8 weeks by 13 men with mild to moderate hypercholesterolaemia. Another group of 15 men instead took a control product (wheat bread). No significant blood cholesterol-lowering effect was observed even if the cholesterol level for the oat group was reduced from 6.30 to 6.05 mmol/L after 4 weeks of consumption. The authors conclude that the insignificant effect could be due to a poor solubility of the beta-glucan preparation, enzymatic hydrolysis after ingestion and thus a low viscosity in the intestine. The number of subjects in the study was also rather low which could have contributed to the insignificant result. Another study on healthy young men for 2 weeks did not detect any cholesterol-lowering effects of an oat beta-glucan concentrate (9 g beta-glucan/day) with a peak molecular weight of 1000 000 (Beer et al. 1995).
The authors point out that to be able to estimate the physiological effects of the beta-glucans not only the content but also the solubility and the viscosity should be measured. Fourteen days may also be too short a time to detect any significant effects of the beta-glucan diet. In another study the cholesterol lowering effect of 3 g oat beta-glucan/ day, the level prescribed as the minimum in the FDA health claim, was investigated (Lovegrove et al. 2000). The study had a parallel design and as a control wheat bran was used. The subjects were asked to eat the supplement together with low-fat yoghurt or low fat milk each day for 8 weeks. The betaglucan diet did not reduce total or LDL-cholesterol, despite having a high viscosity.
There are thus conflicting results concerning the hypolipidaemic effects of beta-glucans but most of the studies with a good design indicate that oats have a cholesterol-lowering effect. This was confirmed by Ripsin et al. (1992) who made a meta-analysis to estimate the effect of soluble fibre from oats on the blood cholesterol levels. They included studies that had different designs (parallel, crossover), intervention times (2.5±12 weeks), background diets (usual, American Heart Association step 1 diet (AHA-1), low fibre, low fat), study subjects (sex, age, cholesterol levels), selection of control product (low fibre, wheat bran) and soluble fibre dose (1.1±7.6g/day). When 12 well-designed studies were included in the meta-analysis, the soluble oat fibre reduced the blood cholesterol with 0.13 mmol/L, a modest reduction. The reduction was larger if the subjects had higher blood cholesterol values (>5.9 mmol/L) and if the dose was over 3 g/day.
A later meta-analysis (Brown et al. 1999) concluded that soluble fibre is associated with a small but significant decrease in total cholesterol (-0.045 mmol/L/g) and LDL-cholesterol (-0.057 mmol/L/g). The effects of soluble fibre from oats, psyllium or pectin were not significantly different.
To be able to document the relatively small cholesterol-reducing effects of beta-glucan containing functional foods it is important to use an appropriate study design. It is also important to check that the beta-glucans are soluble. In a recent study (Lia Amundsen et al. 2003), an oat bran concentrate were added to food products such as bread, teacake, muesli, muffins, macaroni, pasta and apple drink. The authors also checked how much of the beta-glucans that were soluble with the method of Aman and Graham (1987). It was found that the solubility of the beta-glucans in the products was surprisingly low (50 per cent) but the daily dose of soluble beta-glucans consumed by hypercholesterolaemic subjects (2.7 g) was still high enough to decrease the blood cholesterol levels significantly compared with a control diet.
Tables below summarise clinical studies on hypercholesterolaemic subjects given oats. The reduction in blood lipids are compared with a control group given no oat supplement, wheat bran, wheat, rice, maltodextrins, farina or corn flakes. For some of the studies the oat supplement was consumed for 8 weeks but to make a more similar comparison a study length of 4 weeks was selected. There are large variations in the type of oats (oat bran concentrate, oat bran, oats), the preparation (hot, cold, cereal, beverage, bread, cookies) and in the daily doses (2.2±13.4 g soluble fibre or beta-glucan) in the studies. However, all studies except one seems to reduce the total and LDL-cholesterol level in comparison with the control group. The reduction in total cholesterol varied between 0 and 12.6 per cent and for LDL-cholesterol between 0 and 16.5 per cent. The effect on the HDLcholesterol and triglyceride levels was more variable. No clear correlation between the dose of beta-glucan/soluble fibre and the reduction in blood lipid values was found. Also it is difficult to conclude which preparation that is the most efficient one. In a study by Kerckhoffs et al. (2003) a larger reduction of the cholesterol values was observed when the oat bran was mixed in a juice compared with when it was incorporated in bread and cookies.
Only few studies so far have investigated the effects of an intake of barley on the lipid metabolism in humans. There are somewhat more animal studies in this area. In a study in hamsters, barley were given in doses of 0, 25, 50 and 75 per cent of the diet and it was shown that barley lowered the total cholesterol concentration but no dose±response was found (Ranhotra et al. 1998). The cholesterol-lowering effects of beta-glucan fractions from barley and oats were also compared in hamsters (Delaney et al. 2003). The cereals were given in three doses; 2, 4 and 8 g/100 g diet. The diets gave a clear dose-dependent decrease in the total cholesterol level but no differences in the cholesterol-lowering effects between barley and oats were observed. Chicken has also been fed beta-glucans from barley and the effect on the blood cholesterol concentration was followed (Fadel et al. 1987). A non-waxy (Franubet) and a waxy (Washonupana) starch genotype was compared and it was shown that only the waxy genotype had an effect on the blood cholesterol. This was probably due to the waxy genotype having a higher viscosity when mixed with water, a greater average degree of polymerisation and a lower endogenous beta-glucanase activity (Bengtsson et al. 1990).
Blood lipid values in the end of the diet period indifferent clinical studies on hypercholesterolaemic subjects given oats. The study with the largest total blood cholesterol-lowering effect in comparison with the control group is listed first1
In a human study, McIntosh et al. (1991) investigated the cholesterol-lowering effects of barley bran and barley flakes using a 4-week crossover design. The bran and the flakes were incorporated in different foods: bread, muesli, spaghetti and biscuits. Wholewheat flour was substituted for barley in the control products. The intake of beta-glucan was about 8 g/day in the barley period and 1.5 g/day in the wheat period. Consumption of barley led to a significant fall in total (6 per cent) and LDL-cholesterol (7 per cent) compared to the wheat diet. It was also shown by Lupton et al. (1994) that barley bran flour enhanced the cholesterol-lowering effect of the National Cholesterol Education Program (NCEP) step 1 diet in individuals with hypercholesterolaemia. In another study barley was cooked with rice (50/50) and consumed by people with normal or increased lipid levels (Ikegami et al. 1996). In the participants with normal lipid levels no effects on the cholesterol level was seen when barley was included in the diet, while for those with hypercholesterolaemia the total and LDL-cholesterol levels were decreased significantly. In a recent study no significant effects on the total and LDL-cholesterol levels were observed in a group of mildly hypercholesterolaemic men that were given 8.1±11.9 g barley beta-glucans/day (Keogh et al. 2003). More human studies are needed to confirm if beta-glucans from barley have similar cholesterol-lowering properties to oat beta-glucans.
The main biomarkers for the efficiency of a food to control diabetes are measurement of blood glucose and insulin after a standardised meal (glycaemic index, GI, postprandial effects) or the fasting glucose, insulin or HbA1c levels (long-term effects). Both metabolic and epidemiological evidence suggests that replacing high-GI forms of carbohydrates with low-GI forms of carbohydrates will reduce the risk of acquiring type 2 diabetes (Willett et al. 2002).
Granfeldt et al. (1995) investigated the postprandial effect of two oat products, flaked oats (muesli) and boiled oat flakes (oat porridge) in healthy subjects. Both these products had a similar GI as white bread, while intact boiled oat kernels tested at the same time gave lower glucose and insulin responses. The same is also valid for barley that consumed as porridge gave a similar postprandial response as white bread in healthy subjects (Liljeberg et al. 1996). A porridge with a high fibre barley genotype gave however a lower glycaemic and insulin response. Other studies (van der Sluijs et al. 1999) also showed that if a more concentrated oat extract (Oatrim) is consumed in a cooked, boiled or baked form, it lowers the glucose and insulin responses. This effect of more concentrated forms of oats (oat bran and oat gum) seems to be valid both for people with type 2 diabetes and for healthy people (Braaten et al. 1994b). Tappy et al. (1996) gave diabetic subjects a cooked extruded oat bran concentrate for breakfast in different doses (4.0, 6.0, 8.4 g beta-glucan). The maximum increase in plasma glucose for the oat bran meals were 67, 42 and 38 per cent compared with a continental breakfast (35 g available carbohydrates).
Battilana et al. (2001) have studied the mechanism of action of beta-glucans on postprandial glucose metabolism. Healthy men were given a diet with (8.9 g/ day) or without beta-glucans for 3 days. On the third day the diet was administered as fractionated meals ingested every hour for 9 hours. In this way it was possible to study effects on the metabolism that were unrelated to a delayed carbohydrate absorption, for example fermentation effects. However, the glucose metabolism (glucose and insulin concentrations) was similar for the diet with or without beta-glucans. Thus, the main effect of the beta-glucans seems to be a delayed intestinal absorption of carbohydrates.
Wood et al. (1994, 2000) suggested that the reductions in glucose and insulin responses after a meal are mainly due to the viscosity of oats. They studied mixtures of oat beta-glucans with different viscosity and there was a highly significant linear relationship between the viscosity and the glucose and insulin responses. This can be explained by a delay in the carbohydrate absorption due to the high viscosity. Wuzrsch and Pi-Sunyer (1997) have in a review concluded that a concentration of 10 per cent beta-glucan in a cereal food gives a 50 per cent reduction in the postprandial glucose peak.
The effect on the glucose metabolism of a long-term intake of oat betaglucans has also been investigated. An intake of oat beta-glucans (3 g in muesli) taken for breakfast for 4 weeks in men with type 2 diabetes led to a decreased cholesterol level and lower postprandial glucose peaks but no effects on the fasting plasma glucose, insulin and HbA1c were observed (Kabir et al. 2002). The intervention period of 4 weeks may be too short to detect a change in the glucose metabolism in the fasted state. In a longer 12±week pilot study, a bread containing an oat bran concentrate (9 g soluble fibre/day) improved the postprandial glucose metabolism in partcipants via type 2 diabetes (Pick et al. 1996).
Cereals rich in beta-glucans may be a useful nutritional tool to control the metabolic disorders hyperlipidaemia and type 2 diabetes mellitus. One problem, however, is that such products may be unacceptable to many consumers. This can be improved by offering a wider range of foods enriched in beta-glucans. This is the goal for an ongoing EU-funded project coordinated by the author Gunilla Onning `Design of foods with improved functionality and superior health effects using cereal beta-glucans (QLR1±2000±00535)’. In the project beta-glucans are isolated and incorporated in different foods that normally do not contain cereals, such as ready-meals. The sensory and functional properties are followed carefully in the designing of the new, food enriched with beta-glucan. This and other projects will lead to enhanced possibilities for the consumer to select from a wider range of beta-glucan containing foods in the future.
The main part of the studies on the health effects of beta-glucans has been devoted to their effects on glucose and lipid metabolism. In the future other physiological aspects will certainly attract increased attention. An interesting area is, for example, the importance for gut health of the beta-glucans. To reach a deeper understanding of the health effects of beta-glucans, newly developed nutrigenomic techniques can be used. From a nutrigenomic perspective, nutrients are dietary signals that are detected by the cellular sensor systems that influence gene and protein expression and, subsequently, the production of metabolites. It is known that nutrients can be potent dietary signals that influence the metabolic programming of cells and thus have an important role in the control of homeostasis (MuÈller & Kersten 2003). Transcription factors are probably the main agents through which nutrients influence the gene expression. For example, dietary polyunsaturated fatty acids potently repress the hepatic expression of several genes involved in fatty acid synthesis by binding to the receptor family of PPARs (Kersten et al. 2000). By using nutrigenomics it is possible to measure the response of thousands of actively transcribing genes in a cell.
So far, only a few human trials in the nutrigenomic area have been done. In a study by Vidon et al. (2001), a high-carbohydrate diet and a high-fat diet gave the same mRNA concentration for the LDL receptor in blood lymphocytes. Other studies have analysed the gene expression profile and investigated the effect of diabetes (Sreekumar et al. 2002; Maier & Olek 2002) and the function of short-chain fatty acids in the colon (Mariadason et al. 2000).
1986) and Oat Bran (Wood 1993). The first book is very comprehensive and gives a good review of oats chemistry, usage, and nutritional value including health aspects and one post on oat beta-glucans. Oat Bran focuses on the dietary fibre components, including the beta-glucans. Later, another book on oats was published: The Oat Crop: Product and utilization (Welsch 1995), including chapters presenting the botany, production, processing and food uses of oats. The AACC has also published one corresponding book on barley entitled Barley: Chemistry and Technology (MacGregor and Bhatty 1993). The book covers aspects such as the use of barley in malting, feed and as a human food.
There are also several recent review articles in this area. Mazlkki has written a chapter in the Handbook of Dietary Fibre (2001) with the title `Oat fibers: production, composition, physico-chemical properties, physiological effects, safety and food applications’. Wood and Beer have written a chapter about `Functional oat products’ included in Functional Foods. Biochemical & processing (1998). A review of the role of viscous soluble fibre in the metabolic control of diabetes with special emphasis on cereals rich in betaglucans has been published by Wuzrsch and Pi-Sunyer (1997). A more recent review on the influcence of beta-glucans on the human serum lipoproteins has been made by Kerckhoffs et al. (2002). This article also reviews the effect of other dietary components such as soy protein, plant sterols and isoflavones.
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