The judicious replacing of animal protein with soy protein reduces saturated fat and cholesterol intakes, indirectly resulting in a more favourable blood cholesterol level and potentially reducing the risk of coronary heart disease 10. The cholesterol-lowering effects of soy protein, compared to animal protein, have been recognised in animals for more than 90 years 37. A number of human studies over the past 20 years have shown that the daily consumption of 30-60g of soy protein contributes to a decrease in total and low-density lipoprotein cholesterol (LDLC) of between 10% and 20% in individuals with elevated serum cholesterol 37. High-density lipoprotein cholesterol (HDLC) either remains unchanged or is increased under these circumstances. In addition, a significant 10% reduction in triglycerides has been reported in several studies, as noted in a meta-analysis by Anderson et al. 37. These authors reviewed controlled clinical trials examining the effect of soy protein, either textured or isolated, on serum lipid concentrations. Soy protein intake averaged 47g/day in these studies. Of the 38 studies, 34 (89%) reported improved serum lipid and lipoprotein profiles (TC: -9,3%, LDLC: -12,7%, triglyceride: -10,5%, HDLC: +2,4%). According to these studies changes in lipid concentrations were independent of changes in body weight and dietary intake of total fat, saturated fat and cholesterol.
Although Type 2 diabetes (noninsulin-dependent diabetes mellitus, NIDDM) is regarded as a multifactorial disease with a strong genetic basis, available evidence suggests that carbohydrate containing foods rich in dietary fibre and with a low glycaemic index (GI) protect against the development of Type 2 diabetes mellitus, the effect being independent of body mass index 3,22. Tsai et al. 40 studied obese patients with Type 2 diabetes mellitus. These subjects were given a standard meal with and without 10g of soy fibre. The soy fibre supplement significantly enhanced the return of serum glucose levels to fasting levels during the latter half of the test meal. Verster 41 studied the long-term effect of either an energy-restricted high-carbohydrate, high-fibre, low-fat (HCHFLF) diet with a daily addition of 150g (cooked weight) of dry beans compared to the influence of the same diet with the addition of 50g (raw weight) of soy protein isolate on the metabolic Type 2 diabetes control of patients for a twelve-week period. Both diets improved glycaemic control as indicated by decreased glycated haemoglobin concentration. Lo et al. 42 conducted glucose tolerance tests on patients with hyperlipidaemia. Adding 25g of soy fibre in a cookie to the diets of subjects significantly reduced their fasting glucose levels by 8,5%. Although some studies found a positive effect on control of diabetes, other studies did not support this conclusion. More research is needed in this area. Substituting soy protein for animal protein may further protect diabetic individuals from diabetic nephropathy. Anderson et al.43 recently postulated that the increased glomerular filtration rate after the ingestion of animal protein, is absent or mild with soy protein and that the effects are related to the unique amino acid composition of soy affecting renal blood vessel physiology and the favourable effects of the isoflavones on renal glomerular biology and haemodynamic function. The renal protection hypothesis has been tested in animal models and patients with Type 2 diabetes. Although beneficial results from feeding rats soy protein, rather than casein, have been reported 52, (provided soy protein was half of the daily protein intake), had no distinct effects on renal function or proteinuria43.
Evidence from epidemiological studies suggest, although not entirely consistently, that soybean-based diets protect against cancer of the breast, prostate and colon 26,44. An epidemiological study carried out in Singapore found an inverse relation between the consumption of soybean products and the risk of breast cancer in premenopausal women 45, but a subsequent study of Chinese women failed to find a similar association 46. Further evidence that soy may protect against breast cancer development was provided by studies of rodent cancer models in which dietary soy supplements inhibited chemical and radiation-induced breast tumours 47, prostatic dysplasia 48 and colon cancer 30. Cell culture experiments have also shown that soybean constituents completely prevent or suppress the induction of tumours in those systems 49. Epidemiological studies as well as animal and cell culture experiments therefore provide evidence that suggests that the intake of soybeans might play a role in lowering the risk of cancer.
Soy isoflavones are proposed to preserve bone mineral density 12. Animal studies support the potential benefits of soy isoflavones on bone mineral density and preliminary human studies also support the potential role of soy isoflavones in increasing bone mineral density in post-menopausal women 8. Potter et al. 50 recently reported a significant increase of 2% in both bone mineral content and density in the lumbar spine of post-menopausal women after six months on a diet including 40g protein per day from isolated soy protein containing 2,25mg isoflavones/g protein. Isoflavones may to some degree inhibit osteoporosis, but as a single prevention strategy may be insufficient for complete protection 51.
Traditional hormone replacement therapy (HRT) offers protection from cardiovascular disease, reduces the extent of osteoporosis, improves cognitive function and relieves menopausal symptoms associated with acute ovarian oestrogen loss. Despite these beneficial effects, questions remain about the risk–benefit ratio of conventional HRT because of the possibility of increased risk from oestrogen-dependent reproductive cancers (breast and endometrium). Thus, alternative sources of exogenous oestrogen are constantly being sought 31. Diet has been claimed to offer potential relief of the symptoms of the menopause, with vegetarians reporting fewer symptoms, although much of the evidence is anecdotal. Hypothetically, soy isoflavones have the potential to provide an exogenous source of phyto-oestrogen-like effects. The lower incidence of menopausal symptoms in women in countries consuming soy as a staple has been attributed in part to the high intake of isoflavones 31. A number of clinical trials of soyfoods have been conducted in postmenopausal women aimed at evaluating the effects on hot flushes and vaginal dryness. Results and conclusions have been variable but promising with regard to an oestrogenic effect. However, a strong placebo effect has been observed 31,53. Further studies must continue to explore a soy-based HRT as well as to investigate the existence of a dose-response effect.
The prevalence of soy allergy has often been debated 56. In most studies the diagnosis of soy allergy was not substantiated by scientific diagnostic criteria 56. Some researchers reported that 4% 58, 5% 59 and 7% 60 of children had symptoms after double blind, placebo-controlled food challenge with soy. Others 61 estimated that gastrointestinal symptoms occur in 30% of infants fed soymilk in the treatment of gastrointestinal cow’s milk hypersensitivity. Therefore, they do not recommend the use of SBFs in infants younger than 6 months of age with cow’s milk hypersensitivity manifested by gastrointestinal symptoms 61. They do use SBFs in infants older than 6 months and have not observed any adverse effects 61. Hypoallergenic formulas derived from the hydrolysis of cow’s milk proteins may be useful in young babies when there is scientific evidence that the child is allergic to cow’s milk and soy. However, these formulas are quite expensive 56. Hosking, et al 62 reported a 2,0% prevalence of allergy to cow’s milk in Australian children aged 0–2 years (The Melbourne Food Allergy Study) compared to 0,1% to soy.
Using their influence as experts in the area of nutrition and cardiovascular disease (CVD), cancer, osteoporosis and menopausal syndrome, dieticians can lead the way in educating both their patients and the public about the many benefits of healthy soy foods – a superior way to improve both nutrition and health. Although soy and soy products have a large number of potential health benefits, there are as yet no large long-term studies in humans actually demonstrating improved morbidity and mortality. While results from such studies may be many years away, considerable progress has been made since the First International Symposium on the Role of Soy in Preventing Chronic Disease was held in 1995. The foundation has now been laid for research to determine the effects of soy on the incidence of cardiovascular disease, cancer and osteoporotic factors. Although adverse effects of soyfoods appear to be few and minor these nevertheless also need to be shown in long-term studies.
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Despite recent decreases in prevalence, coronary heart disease (CHD) remains the major killer in South Africa’s White and Indian populations, while stroke is a major cause of disability and death in the Black and Coloured populations36. CHD and stroke are both multifactorial diseases in which complex interaction between genetic predisposition and environmental factors, including diet, influence the atherosclerotic and thrombotic processes underlying the clinical manifestations of these diseases. 
Cancer is a disease associated with well-recognised genetic abnormalities, but diet is regarded as an important risk factor for colorectal, breast and uterus cancers. Meat and fat intakes are associated with increased risk of colorectal cancer and total energy with increased risk (leading to obesity) of breast and uterus cancer 22,. 
Soy-based infant formulas (SBF) were first described as cow’s milk substitutes in 1909 54 but were not used for feeding babies with cow’s milk allergy (CMA) until 1929 55. Since then, SBFs have been widely used for feeding babies with CMA. Current commercial SBFs are lactose free and enriched with methionine, taurine, carnitine and iron 56. The distribution of nutrients in SBFs is quite similar to that in cow’s milk formulas. SBFs and cow’s milk formulas contain the same amount of proteins, lipids derived from vegetable oils, and carbohydrates in the form of maltodextrins, cornstarch or sucrose. Several clinical studies showed that feeding SBFs to full term infants is associated with normal growth, protein nutritional status and bone mineralisation 56. Because the nutrient requirements of small preterm infants differ from those of full-term infants, the American Academy of Pediatrics does not recommend that soy formulas be fed to small preterm infants 57.