Other Dietary Approaches for the Prevention and Management of CVD

Very Low-Fat/High-Carbohydrate Diet and High-Protein/Low-Carbohydrate Diet

When considering diets very low in fat and high in carbohydrates ('very low-fat' diets), it is important to separate the effects of the composition of the diet from confounding factors associated with intentional weight loss. For the purposes of this discussion, a very low-fat diet will be defined as less than 15% of energy as fat. Consumption of a very low-fat diet without a decrease in energy intake frequently decreases blood total, LDL, and HDL cholesterol levels and increases the total cholesterol:HDL cholesterol ratio (less favorable) and triglyceride levels. A mitigating factor may be the type of carbohydrate providing the bulk of the dietary energy: complex (whole grains, fruits, and vegetables) or simple (fat-free cookies and ice cream). The reason for this later observation has yet to be investigated. Notwithstanding these considerations, for this reason moderate fat intakes, ranging from <30% to 25 to 35% of energy as fat, are currently recommended to optimize lipo-protein profiles with respect to decreasing CVD risk.

Current interest in the area of weight loss is centered on high-protein/low-carbohydrate (high protein) diets. Recently, high-protein diets were shown to result in significantly more weight loss than standard reduced energy diets and were accompanied by more favorable blood lipid profiles (lower triglyceride, higher HDL cholesterol levels). However, by 1 year the advantage in terms of weight loss attributed to the high-protein diet did not persist (Table 2). The major concern with high-protein diets is that in the absence of steady weight loss the higher intakes of saturated fat and cholesterol can ultimately have an adverse effect on LDL cholesterol levels. Ongoing work will most likely resolve some of these issues.

Fiber

Dietary soluble fiber, primarily ^-glucan, has been reported to have a modest independent effect on decreasing blood total and LDL cholesterol levels. A meta-analysis concluded that 3g of soluble fiber (equivalent of three servings of oatmeal) reduced both total and LDL cholesterol levels approximately 0.13mmoll—1 (Figure 5). Most evidence suggests that soluble fiber exerts its hypo-cholesterolemic effect by binding bile acids and cholesterol in the intestine, resulting in an increased fecal loss and altered colonic metabolism of bile acids. The fermentation of fiber polysaccharides in the colon yields short-chain fatty acids. Some evidence suggests that these compounds may have hypocholesterolemic effects via alterations in hepatic metabolism. At this time there is no evidence to suggest that insoluble fiber has an effect on blood lipid levels.

Soy Protein

The potential relationship between soy protein and the risk of developing CVD has a long history dating back to the 1940s. Despite this relatively protracted lead-time attempts at more precisely defining this relationship have been slow in coming and somewhat inconsistent. Renewed interest developed in the relationship between soy protein and blood lipid levels after a meta-analysis was published in the mid-1990s suggesting that soy protein resulted in significant reductions in total and LDL cholesterol levels, with the most pronounced effect in hyper-cholesterolemic individuals. Changes in HDL cholesterol levels were not significant. Whether the effect on total and LDL cholesterol levels was attributable to the soy protein per se or other soybean derived factor(s), the most likely being the constitutive isoflavones, had yet to be determined. Since then a number of well-controlled studies have re-examined the effect of soy protein and/or isofla-vones on blood lipid levels in humans. The results of more recent studies are variable. Declines in LDL cholesterol levels attributable to the substitution of 25-50 g of soy protein for animal protein range from null to small (3-6%) in normocholesterolemic and hypercholesterolemic individuals. Changes in HDL cholesterol levels were highly variable, ranging from —15% to +7%. Soy-derived isoflavones do not appear to have an independent effect on blood lipid levels. On the basis of the most recent data it can be concluded that, although helpful when used to displace products containing animal (saturated) fat from the diet, despite the current claims, individuals should be cautioned against an overreli-ance on the casual use of soy protein containing foods or the use of isolated isoflavones to control serum lipid levels.

Plant Sterols

Sterols compare for a group of compounds that are essential constituents of cell membranes in animals and plants. Cholesterol is the major sterol of mammalian cells. Phytosterols, such as beta-sitosterol, campesterol, and stigmasterol, are the major sterols of plant cells. In humans, plant sterols are not synthesized, are poorly absorbed, and appear to interfere with cholesterol absorption. It is this later property that has been exploited in the use of these compounds as blood cholesterol-lowering agents. Maximal LDL cholesterol lowering attributable to plant sterols occurs at a dose of about 2gday—1 (Figure 6). Although a relatively wide range of responses has been reported, the majority of work suggests an expected LDL cholesterol lowering of about 10% in hypercholesterolemic subjects. Plant sterol-enriched margarines and other foods are currently available in some countries. Few side effects of plant sterols have been reported with the exception of decreased levels of circulating carotenoids; the long-term effect of this is unclear at this time but should continue to be monitored carefully.

Antioxidant Nutrients

Considerable interest had been generated in the potential benefit of dietary supplementation with vitamin E and other antioxidant nutrients in reducing CVD risk. Support for this hypothesis came from two sources. First from the epidemiological observations suggesting that vitamin E supplement use was associated with decreased risk of CVD.

Table 2 Summary mean outcomes

Carbohydrates in diet (g day 1)

No. of diets

No. of participants

Summary mean change (SD)

No. of diets

No. of participants

Summary mean change (SD)

Weight change (kg) All studies, all participants 34 RCT and R-Cross only 7

Caloric content of the diet (kcal day-1)

<1500 18

>1500 16

Diet duration (days) <15 14

16-60 9

>60 10 Participant age (years) <40 22

Baseline weight (kg) <70 3

70-100 13

BMI (kg/rrr2) in all studies, 1

all participants Body fat (%) in all studies, all 5

participants

614 53

72 142 447

426 242

22 365 138 113

130 75

45 84

25 52 45

59 62

19 77 18 27

2092 1122

870 1222

198 827 968

642 1231

230 1357 301 739

Adapted from Bravata DM, Sanders L, Huang J, Krumholz HM, Olkin I, and Gardner CD (2003) Efficacy and safety of low-carbohydrate diets: a systematic review. JAMA 289: 1837-1850.

Figure 5

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0

Relationship between fiber intake and change in total cholesterol levels. (Reproduced from Brown L, Rosner B, and Sacks FM (1999) Cholesterol-lowering effects of dietary fiber: a meta-analysis. American Journal of Clinical Nutrition

□ Age 50-59 years

o Age 40-49 years

t

o Age 30-39 years

1 1

1

Dose of plant sterol or stanol (g day 1)

Figure 6 Effect of plant sterols or stanols on LDL cholesterol levels. (Reproduced from Law M (2000) Plant sterol and stanol margarines and health. British Medical Journal 320: 861-864.)

Dose of plant sterol or stanol (g day 1)

Figure 6 Effect of plant sterols or stanols on LDL cholesterol levels. (Reproduced from Law M (2000) Plant sterol and stanol margarines and health. British Medical Journal 320: 861-864.)

Second from the in vitro work demonstrating that vitamin E in LDL was correlated with decreased susceptibility of the lipoprotein particle to oxidation and that in cell culture oxidized LDL resulted in foam cell formation. A number of recent intervention studies have failed to demonstrate a benefit of vitamin E or other antioxidant vitamins. At this time the data do not support a recommendation to use antioxidant vitamins for the prevention or management of CVD.

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