A few notes on “Dietary carbohydrate restriction improves metabolic syndrome independent of weight loss“, since I just got to looking at this.
Small LDL changes aside, the findings are not really a reflection of carbohydrate restriction.
Here is a spreadsheet of the fatty acid composition of the diets in Excel. While the low-carbohydrate diet does have a higher saturated fat intake, its unsaturated fat intake is proportionally even higher.
We know that saturated and unsaturated fats have competing effects on blood lipids. We also know that unsaturated fats lower blood lipids compared to carbohydrates. (link)
So if you increase saturated fats but unsaturated fats to an even greater degree, guess what? According to the published research, blood lipids improve.
That’s exactly what we saw here. The low-carbohydrate diet had 4X more MUFAs and nearly 6X more PUFAs than the high-carbohydrate diet, yet only 2.5x more SFAs. Ta-da! Improved lipids. That means better LDL, better trigs, better LDL, etc.
Importantly, this was the primary outcome on the trial registration page. (link)
There is also some hoopla about a reduction in saturated fat in the blood. In fact, I hated myself enough to be reading Shawn Baker’s instagram and found that reported there.
In fact, the paper authors seemed to want to communicate the same hype: “Despite being higher in saturated fat, carbohydrate-restricted diets decrease plasma total saturated fatty acids”. Here is the figure. Can anyone else see the problem?
It is reported as %.
Yes, that is right. In a diet where the ratio of SFA to UFA goes down relative to a high-carbohydrate diet, the % plasma SFA (as a fraction of the total pool of fatty acids) goes down.
Well, duh? Who the heck would expect it to go up or stay the same?
Blood glucose was also reported. It was improved on the low-carbohydrate diet. Expected and a benefit of carbohydrate restriction, to be sure.
Blood pressure was also reported to be lowered. This was confusing to me, so I looked more closely, and yep, there was no difference in blood pressure between diets.
There was no significant difference in fatty liver.
This does not mean that low-carbohydrate diets might not exert a modest beneficial effect on fatty liver. It just means that if there is one, this effect is small. That said, if the high-carbohydrate diet contained lots of added sugars (glucose or sucrose does not matter)… we would expect a modest improvement in fatty liver. This has already been demonstrated and might account for the fatty liver improvements in this trial. I cover that here:
There was also no difference in resting energy expenditure in this trial. Subjects did not turn into fat-burning furnaces.
What does this study tell us? On an isocaloric basis:
1. Replacing carbs with fat lowers blood sugar;
2. Replacing carbs with PUFAs and MUFAs improves lipids;
3. Replacing carbs with fats does not improve blood pressure, fatty liver, visceral fat, or waist circumference.
What this paper should have been called is:
“Diet with much higher PUFAs and MUFAs improves blood lipids compared to higher carbohydrate diet”
But then that wouldn’t be too interesting because we’ve known that since the 1950s…