Milk, plasma cholesterol and controls in nutritional experiments

Influence of dairy product and milk fat consumption on cardiovascular disease risk: a review of the evidence

Although evidence has linked the consumption of saturated fat (SF) to increased LDL levels and an increased risk of the development of cardiovascular disease (CVD), recent findings have indicated that the link between CVD and SF may be less straightforward than originally thought. This may be due to the fact that some food sources high in SF contain an array of saturated and unsaturated fatty acids, each of which may differentially affect lipoprotein metabolism, as well as contribute significant amounts of other nutrients, which may alter CVD risk. The purpose of this review is to examine the published research on the relationship between milk fat containing dairy foods and cardiovascular health. The findings indicate that the majority of observational studies have failed to find an association between the intake of dairy products and increased risk of CVD, coronary heart disease, and stroke, regardless of milk fat levels. Results from short-term intervention studies on CVD biomarkers have indicated that a diet higher in SF from whole milk and butter increases LDL cholesterol when substituted for carbohydrates or unsaturated fatty acids; however, they may also increase HDL and therefore might not affect or even lower the total cholesterol:HDL cholesterol ratio. The results from the review also indicate that cheese intake lowers LDL cholesterol compared with butter of equal milk fat content. In addition, the review highlights some significant gaps in the research surrounding the effects of full-fat dairy on CVD outcomes, pointing to the need for long-term intervention studies.

Dairy fat in cheese raises LDL cholesterol less than that in butter in mildly hypercholesterolaemic subjects

OBJECTIVE

To determine whether dairy fat in cheese raises low-density lipoprotein (LDL) cholesterol as much as in butter, since epidemiology suggests a different impact on cardiovascular disease.

DESIGN

A randomised crossover trial testing the daily consumption of 40 g dairy fat as butter or as matured cheddar cheese, each of 4 weeks duration, was preceded by and separated by 2-week periods when dietary fat was less saturated.

SETTING

Free-living volunteers.

SUBJECTS

A total of 14 men and five women of mean age 56+/-8 y, with mean total cholesterol of 5.6+/-0.8 mmol/l.

MAIN OUTCOME MEASURES

Plasma cholesterol, LDL cholesterol (LDL-C), HDL cholesterol (HDL-C), triacylglycerol and glucose.

RESULTS

Saturated fat intake was significantly lower during the run-in than during the cheese and butter periods. Mean lipid values did not differ significantly between the cheese and run-in periods, but total cholesterol and LDL-C were significantly higher with butter: total cholesterol (mmol/l): butter 6.1+/-0.7; run-in 5.6+/-0.8 (P < 0.05; ANOVA with Bonferroni adjustment); vs cheese 5.8+/-0.6 (P > 0.05); median LDL-C (mmol/l): butter 3.9 (3.5-4.1) vs run-in 3.4 (3.0-4.1) (P < 0.05; Tukey test); vs cheese 3.7 (3.3-3.9) (P > 0.05). Among 13 subjects whose initial LDL-C was >4 mmol/l, the difference between butter (4.4+/-0.3 mmol/l) and cheese (3.9+/-0.3 mmol/l) was significant (P = 0.014). HDL-C was highest with butter and triacylglycerol with cheese (neither was significant).

CONCLUSION

A total of 40 g dairy fat eaten daily for 4 weeks as butter, but not as cheese, raised total and LDL cholesterol significantly compared with a diet containing significantly less saturated fat. Dietary advice regarding cheese consumption may require modification.

Milk consumption, stroke, and heart attack risk: evidence from the Caerphilly cohort of older men

OBJECTIVE

To examine associations between milk consumption and incident heart disease and stroke.

DESIGN

A representative population sample of men was asked to weigh and record their food intake for seven days. The total consumption of milk was obtained from these records. Details of all deaths and vascular events were collected during the following 20 years. Incident ischaemic strokes and heart disease events were diagnosed by standard criteria.

SETTING

The Caerphilly cohort, a representative population sample of men in South Wales, aged 45-59 when first seen in 1979-83.

PARTICIPANTS

A representative 3:10 subsample of the men in the cohort.

MAIN RESULTS

665 men (87% of those approached) returned satisfactory seven day diet diaries. After adjustment, the relative odds of an event in the men whose milk consumption was the median or higher, relative to those with lower intakes of milk, were 0.52 (0.27 to 0.99) for an ischaemic stroke and 0.88 (0.56 to 1.40) for an ischaemic heart disease event. Deaths from all causes were similar in the two milk consumption groups (relative odds 1.08; 0.74 to 1.58).

CONCLUSIONS

These results give no convincing evidence of an increased risk of vascular disease from milk drinking. Rather, the subjects who drank more than the median amount of milk had a reduced risk of an ischaemic stroke, and possibly a reduced risk of an ischaemic heart disease event. These conclusions are in agreement with the results of a previously reported overview of 10 large, long term cohort studies based on food frequency intake records.

The A2 milk case: a critical review

This review outlines a hypothesis that A1 one of the common variants of beta-casein, a major protein in cows milk could facilitate the immunological processes that lead to type I diabetes (DM-I). It was subsequently suggested that A1 beta-casein may also be a risk factor for coronary heart disease (CHD), based on between-country correlations of CHD mortality with estimated national consumption of A1 beta-casein in a selected number of developed countries. A company, A2 Corporation was set up in New Zealand in the late 1990s to test cows and market milk in several countries with only the A2 variant of beta-casein, which appeared not to have the disadvantages of A1 beta-casein. The second part of this review is a critique of the A1/A2 hypothesis. For both DM-I and CHD, the between-country correlation method is shown to be unreliable and negated by recalculation with more countries and by prospective studies in individuals. The animal experiments with diabetes-prone rodents that supported the hypothesis about diabetes were not confirmed by larger, better standardised multicentre experiments. The single animal experiment supporting an A1 beta-casein and CHD link was small, short, in an unsuitable animal model and had other design weaknesses. The A1/A2 milk hypothesis was ingenious. If the scientific evidence had worked out it would have required huge adjustments in the world's dairy industries. This review concludes, however, that there is no convincing or even probable evidence that the A1 beta-casein of cow milk has any adverse effect in humans. This review has been independent of examination of evidence related to A1 and A2 milk by the Australian and New Zealand food standard and food safety authorities, which have not published the evidence they have examined and the analysis of it. They stated in 2003 that no relationship has been established between A1 or A2 milk and diabetes, CHD or other diseases.

Milk drinking, ischaemic heart disease and ischaemic stroke I. Evidence from the Caerphilly cohort

OBJECTIVE

To test the hypothesis that milk drinking increases the risk of ischaemic heart disease (IHD) and ischaemic stroke in a prospective study.

DESIGN

In the Caerphilly Cohort Study dietary data, including milk consumption, were collected by a semiquantitative food frequency questionnaire in 1979-1983. The cohort has been followed for 20-24 y and incident IHD and stroke events identified.

SUBJECTS

A representative population sample in South Wales, of 2512 men, aged 45-59 y at recruitment.

MAIN OUTCOME MEASURES

In total, 493 men had an IHD event and 185 an ischaemic stroke during follow-up.

RESULTS

After adjustment, the hazard ratio in men with a milk consumption of one pint (0.57 l) or more per day, relative to men who stated that they consumed no milk, is 0.71 (0.40-1.26) for IHD and 0.66 (0.24-1.81) for ischaemic stroke. At baseline, 606 men had had clinical or ECG evidence of vascular disease, and in these the vascular risk was even lower (0.37; 0.15-0.90). The hazard ratio for IHD and ischaemic stroke combined is 0.64 (0.39-1.06) in all men and 0.37 (0.15-0.90) in those who had had a prior vascular event.

CONCLUSION

The data provide no convincing evidence that milk consumption is associated with an increase in vascular disease risk. Evidence from an overview of all published cohort studies on this topic should be informative.

SPONSORSHIP

: The Medical Research Council, the University of Wales College of Medicine and Bristol University. Current support is from the Food Standards Agency.

Does Fat in Milk, Butter and Cheese Affect Blood Lipids and Cholesterol Differently?

OBJECTIVE

To compare the effects of isoenergetic amounts of milk, cheese and butter (adjusted to the same content of lactose and casein) on fasting and postprandial blood lipids and lipoproteins, and on postprandial glucose and insulin response.

DESIGN

The experiments were designed to provide 20% of total energy from dairy fat, as either whole milk, mean (+/-SD) 2164 (+/-97) g, butter 93 (+/-4) g, and hard cheese 305 (+/-45) g, which were served to 14 healthy young men for three periods of three weeks each, separated by washout periods, in a randomized, cross-over study with strictly controlled dietary intake. Fasting blood samples were taken at the end of the study periods. Measurements of the postprandial effect of the three different dairy test products (0.7 g of milk fat/kg body weight) were carried out on day 4 of each intervention period. Blood samples were taken before and at 2, 4, 6 and 8 hours following intake of the meals.

RESULTS

Fasting LDL cholesterol concentration was significantly higher after butter than cheese diet (p = 0.037), with a borderline significant difference in total cholesterol (p = 0.054) after the experimental periods of three weeks. Postprandial glucose showed a higher response after cheese diet than after milk diet (p = 0.010, diet x time interaction).

CONCLUSIONS

A different effect of fat in milk and butter could not be confirmed in this study. The moderately lower LDL cholesterol after cheese diet compared to butter diet should be investigated further.

Milk, coronary heart disease and mortality

STUDY OBJECTIVE

To study the association between reported milk consumption and cardiovascular and all cause mortality.

DESIGN

A prospective study of 5765 men aged 35-64 at the time of examination.

SETTING

Workplaces in the west of Scotland between 1970 and 1973.

PARTICIPANTS

Men who completed a health and lifestyle questionnaire, which asked about daily milk consumption, and who attended for a medical examination.

MAIN RESULTS

150 (2.6%) men reported drinking more than one and a third pints a day, Some 2977 (51.6%) reported drinking between a third and one and a third pints a day and 2638 (45.8%) reported drinking less than a third of a pint a day. There were a total of 2350 deaths over the 25 year follow up period, of which 892 deaths were attributed to coronary heart disease. The relative risk, adjusted for socioeconomic position, health behaviours and health status for deaths from all causes for men who drank one third to one and a third pints a day versus those who drank less than a third of a pint was 0.90 (95% CI 0.83, 0.97). The adjusted relative risk for deaths attributed to coronary heart disease for men who drank one third to one and a third pints a day versus those who drank less than one third of a pint was 0.92 (95% CI 0.81, 1.06).

CONCLUSIONS

No evidence was found that men who consumed milk each day, at a time when most milk consumed was full fat milk, were at increased risk of death from all causes or death from coronary heart disease.

Effect of skim milk and whey diets on plasma lipid levels of rabbits in a cross-over study

The influence of a skim milk and a whey-based diet on plasma lipids and lipoproteins in rabbits was investigated in a crossover design, in which the diets were switched after 12 weeks. The whey diet increased plasma lipid levels regardless of when it was consumed in the feeding protocol. The milk diet, on the other hand, modestly elevated the lipid levels if provided in the first half of the feeding protocol, but reduced lipid levels if given in the second half of the feeding regime. Both diets also induced similar changes in lipoprotein patterns when given in the first half of the feeding protocol, but opposite effects during the second half. Atherosclerotic changes were also observed in the aorta of animals receiving the whey diet during the second half of the experimental protocol.