A casein variant in cow's milk is atherogenic
Introduction
Cow milk constitutes a major part of the diet of developed nations and it contains an average 32 g of protein per litre [1], mostly casein (82%) and whey (18%). The caseins have been divided into four subclasses, αs1, αs2, β and κ [2], [3], whose major function is to transport calcium phosphate [4]. β-Casein is the second most abundant casein behind αs1 [1] and is essential for curd formation and important in determining the surface properties of micelles [4]. To date 10 genetic variants of β-casein have been characterised with regard to sequence [5], [6], and of these only A1, A2, A3 and B are found in nearly all Bos taurus populations [7], [8]. The allelic distribution of variants is such that the majority of cows in Western nations produce milk rich in the genetic variant β-casein A1. For example, Holstein and Ayrshire breeds of cow produce 63 and 67% A1, respectively, and 35 and 33% A2 [9]. In contrast, the Guernsey breed produces 1% A1 and 98% A2. Alignment of β-casein A1 and β-casein A2 protein sequences reveals near-perfect homology with the only difference at codon 67 where β-casein A2 has a proline residue and β-casein A1 has a histidine residue [4], [10].
While some epidemiological studies show no association between reported milk consumption and cardiovascular mortality [11], comparative data from several populations have suggested a strong correlation between the consumption of β-casein A1 and deaths from ischaemic heart disease. McLachlan [12] examined the calculated consumption of β-casein A1 (excluding cheese) against IHD mortality in males aged 30–69 in 16 developed nations and found a correlation value (r2) of 0.86. Upon comparing IHD and β-casein A1 consumption in the states of the former West Germany, the same researcher found an r2=0.66, three times greater than the correlation between smoking and IHD in the same population. This implies that β-casein A1 consumption may cause more vascular damage than a well-known risk factor for atherosclerosis.
There have been almost no cases of IHD-related mortality reported in either the Masai (East Africa) [13] or Samburu (Northern Kenya) [14] communities. Furthermore, these tribes showed normal to low cholesterol levels despite having a diet rich in animal milk and meat protein [13], [14]. Interestingly, almost all of the African Zebu (B. indicus) cattle populations produce milk containing β-casein A2 [15].
Ingestion of total casein, the protein fraction isolated by acid precipitation of all casein subclasses from milk, has been shown to promote atherosclerosis in several animal models including rabbits [16], monkeys [17] and mice [18]. Casein consumption is also associated with hypercholesterolemia [19] and reduced LDL catabolism [20]. However, to date no controlled diet studies have been carried out to definitively determine whether a specific casein subclass or subclass variant is more atherogenic than others. The aim of the present study was to determine whether dietary administration of β-casein A1 in a rabbit model of atherosclerosis promotes the disease state compared with rabbits fed β-casein A2.
Section snippets
Study groups
Sixty New Zealand white/Lop cross rabbits of both sexes (16–24 weeks old) had their right carotid artery balloon de-endothelialised at t=0 [21], then randomly divided into 10 groups (n=6 per group). Group 1 was fed pellets containing 20% whey protein for 6 weeks while Groups 2 and 3 had 10% β-casein A1 and A2, respectively, plus 10% whey, all with no added dietary cholesterol. The diets for the remaining seven groups contained 0.5% dietary cholesterol and 20% milk protein, which differed only
Rabbit diets and weights
While the amount of the study pellets eaten each day varied between rabbits, on group averages the amounts consumed varied by only 12 g per day over the 6 weeks experimental period, and there was no significant difference (P<0.05) between any of the groups (mean=30.05±0.9 g per day). The addition of 0.5% cholesterol to the diets of Groups 4–10 did not influence the amount eaten, or did the type or amount of casein. Almost all rabbits lost weight over the 6 weeks of study (mean −5.62%) with no
Discussion
When rabbits are fed a cholesterol-enriched diet, subendothelial accumulations of fat-filled macrophages (“foam cells”) are developed in most arteries [22]. These lesions are termed fatty streaks and closely resemble human juvenile fatty streaks that are present in early childhood and are considered the precursors of advanced atherosclerotic plaques [23]. Since juvenile fatty streaks are first seen in infants under 3 years of age [23], the question was raised: is casein, in addition to milk
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