Thursday, February 15, 2018

Whole vs. Skim Milk -- Increased HDL and no Effect on LDL, Glucose and Insulin in 3-Wk Crossover Study... But WAIT!

"Whole" or "skim" - eventually it may make much less of a difference than either low-fat or high-fat proponents may still believe.... that is if you're young and healthy and not doing the "1 gallon o' milk"-routine, bro ;-)
Now that more and more people begin to get that eggs are not toxic cholesterol bombs, it's about time that scientists re-address the myth that putting the "whole" in the "milk" would have serious health consequences... oh, wait: Eventually, Sara Engel and colleagues from the University of Copenhagen found statistically sign. health consequences - allegedly heart-healty ones!

More specifically, the authors observed that only 3-weeks on whole vs. skim milk will significantly elevate the HDL levels of 18 healthy adults who were randomly assigned to a sequence of treatments consisting of 0.5 L/d of whole milk and skimmed milk as part of their habitual diet.
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While only 17/18 subjects completed the intervention, the statistical analysis of the data doesn't give us reason to believe that the significant increase in HDL cholesterol (compared to skimmed milk | P < 0.05), was just a statistical artifact.

Why would you even care about HDL? 

Admittedly, the role of HDL as a marker of heart health has been vastly overrated in the past decade. Nevertheless: higher HDL levels at stable LDL cholesterol, triacylglycerol, insulin, and glucose concentrations, as they were observed in the study at hand have been found to be consistently associated with a reduced risk of heart disease.
Table 1: Whole milk increased the subjects' total energy intake, but that doesn't seem to be a problem in the short run (Engel 2018)
No, whole milk won't make you fat! Remember: calories count and in the study at hand, switching the subjects to whole milk did result in "a significantly higher energy intake with whole milk compared to skimmed milk" (Engel 2018), but "it seems that the study subjects compensated for the extra energy with whole milk by lowering their intake of carbohydrate which was significantly lower compared to skimmed milk" (ibid). In conjunction with the potentially beneficial health effects of the milk fat globule membranes (MFGM), whose concentration is obviously much higher in whole milk, this may explain the overall positive effects on the subjects' lipid profile.
The question the study at hand cannot answer, though, is whether the 3.7% increase in the study at hand is enough of an improvement to have physiologically relevant effects. I have my doubts about that. Hell, you could reasonably doubt that this is more than a statistical artefact.
Figure 1: Markers of metabolic health after skimmed milk and whole milk periods (Engel 2018)
In view of the fact that the previously mentioned auxiliary markers of metabolic disease, i.e. the subjects' LDL cholesterol, triacylglycerol, insulin, and glucose concentrations worsened non-significantly, the study at hand clearly isn't going to change the opinion of the initially referenced dietitian... I mean, Steinmetz' 1994 study, which observed significant increases in HDL and concomitant decreases in Apolipoprotein B (the primary apolipoprotein of chylomicrons, VLDL, IDL, and LDL particles | more about its role in heart disease in Brunzell 2008) didn't affect the public opinion, either (Steinmetz 1994)... and let's be honest: the 3.8% increase looks rather random - statistically significant or not, it's probably not physiologically relevant, anyway.

Which reminds me. Unlike you may have read on a couple of blogs, there's no convincing evidence that high-fat dairy would be significantly heart (and metabolically) healthy than low-fat airy. In fact, the totality of (albeit mostly observational studies) suggests either the opposite or (just as most experimental trials) no difference, at all... ah, and NO butter, grass-fed or not is not a suitable alternative for eating fish / taking omega-3 capsules (in case you hate fish), simply because the n-3 concentration is still low (same for cows fed n-3 enriched diets, <4% of EPA and DHA will actually make it into the milk | Lock 2004).
Cheesy, but heart healthy: Cheese come in all forms and colors | more
Saturated fat has always been good for HDL: When all is said and done, the increase in HDL Engel et al. observed in the study at hand didn't come as the surprise as which I announced it. Research has, after all, shown significant increases in HDL cholesterol in response to increasing amounts of allegedly "bad" saturated fatty acids (SFAs) in the diet - even more importantly, a meta-analysis showed that this increase was pronounced enough to improve the physiologically much more relevant HDL/LDL ratio (Mensing 2013) - especially if SFAs replace plant-based transfats.

Does this mean that you should ignore the Danish and American Dietary Guidelines advice "that SFA should be limited to less than 10 E%, due to the predicted effect on LDL cholesterol" (Engel 2018), because the more saturated the fats in your diet, the healthier your heart is going to be? Not exactly. Not just, but also because "it is necessary to be cautious when interpreting low concentration of HDL cholesterol as a CVD risk factor" (Engel 2018) - if you're genetically wired to have low HDL levels, that's not associated w/ increased risk of myocardial infarction. That's a result which is relevant not just for the selected few, but does, in fact, as Engel et al. point out, question "the causality of an association between low HDL concentration and CVD" (Engel 2018) -- or as Holmes et al. have it: "genetic findings support a causal effect of triglycerides on CHD risk, but a causal role for HDL-C, though possible, remains less certain" (Holmes 2014) | Comment!
  • Brunzell, John D., et al. "Lipoprotein management in patients with cardiometabolic risk: consensus statement from the American Diabetes Association and the American College of Cardiology Foundation." Diabetes care 31.4 (2008): 811-822.
  • Engel, Sara, Mie Elhauge, and Tine Tholstrup. "Effect of whole milk compared with skimmed milk on fasting blood lipids in healthy adults: a 3-week randomized crossover study." European journal of clinical nutrition (2017): 1.
  • Holmes, Michael V., et al. "Mendelian randomization of blood lipids for coronary heart disease." European heart journal 36.9 (2014): 539-550.
  • Lock, Adam L., and Dale E. Bauman. "Modifying milk fat composition of dairy cows to enhance fatty acids beneficial to human health." Lipids 39.12 (2004): 1197-1206.
  • Steinmetz, Kristi A., et al. "Effect of consumption of whole milk and skim milk on blood lipid profiles in healthy men." The American journal of clinical nutrition 59.3 (1994): 612-618.