There have been some recent rumblings about the ingestion of saturated fat, particularly butter, having a negative effect on insulin sensitivity. The study demonstrating this affect is discussed in the Lopez et al paper.

I think there are some aspects of this study and its conclusions that should be kept in mind.

There are a number of things about this study that can reduce its external validity. First, this was a very short-term study, 5 separate test meals. Therefore, the results may not be the same when this type of eating style is followed long-term. There does seem to be 1-4 week time lag/adaptation phase to a high fat/low carb diet and most likely vice versa (Phinney). Related to this aspect is the fact that low carb and keto type diets, compared to low fat/high carb diets, have far better results on indices of lipids and blood sugar control (Westman et al; Reaven; Sharman et al). Also related to this aspect is the evidence of extraordinary health from a number of groups, such as the Masai and the Pukapuka/Tokelau (high fat diet mostly from coconuts, therefore high saturated diet) that eat a high fat diet. Therefore, over the long-term, a high fat diet, including a high saturated fat diet, is not likely to be pathological. Another shortcoming of this study is that there were only male subjects; therefore this may not apply to females. The macro ratios were not what are typically recommended for many low carb or keto diets, particularly the protein. Typically the fat intake is closer to 60% (72% in this study), protein is 20-30% (6% here, big difference) and carbs are 10-20% (22% here) and usually the carbs are derived from non-starchy vegetables, nuts and seeds, not pasta, bread and sugary yogurt (Westman et al; Volek et al). Therefore, this diet does not represent what is typically recommended. Interestingly, ALL of the fat-enriched challenge meals produced negative results. The authors (Lopez et al) state; “All the indexes [which include the insulin sensitivity ones] were significantly higher after the high-fat meals [high poly, MUFA & SAT] than after the control meal” (p.640). The saturated fat (butter) did do the worst, but they all did poorly. This conclusion seems to be in opposition to the evidence of high fat diets not causing insulin resistance and actually improving glycemic control (Westman et al). For instance, Cordain et al stated “…fat alone and under isocaloric conditions, unlike refined sugars, does not cause insulin resistance in humans…a range of isocaloric diets containing up to 83% fat did not directly cause insulin resistance, and the 83% fat diet actually improved certain aspects of glucose homeostasis” (p.100). Is this the difference between short-term verses long-term metabolic processes?

Another aspect related to this study is the flawed view that saturated fat should not be thought of as a single type of fat. There are a number of different types, i.e., carbon chain lengths, of saturated fats each with potentially different biological functions (Enig). This is the same as acknowledging that polyunsaturated fats are not all the same; the omega 6 and omega 3 fats, although they are both poly’s, have very different biological effects. Drilling down a bit further, the different types of omega 3’s, EPA and DHA, can have different effects. Therefore, it would seem important to not lump all types of saturated fats affects into one category. Until research is done on other types of saturated fats, the effects of butter should not represent the affects of all types of saturated fats.

This is not the first study to look at what type of effects different types of fats would produce. In fact, a 2008 review paper looked at this specific question. The authors of the paper concluded “Most studies (twelve of fifteen) found no effect relating to fat quality [relative amounts of saturated, mono, or polyunsaturated types] on insulin sensitivity” (Galgani, p.471). It would seem that the weight-of-the-evidence does not support a deleterious effect of saturated fat on insulin function.

One final thing before concluding; I am assuming that the butter the authors used was not from grass-fed cows. I wonder if the effects would have been different, due to the different fatty acid profile of grass-fed vs. grain-fed, if this type of butter was used?

Based on the recent Lopez et al study and the many other papers on this specific topic (high saturated fat diet and insulin function), and related topics (high fat, low carb diets and long-term health) I would conclude that a person transitioning (metabolically, likely 2 to 4 weeks) to a low-carb, high fat diet should not get a majority of their fats from saturated fats, particularly the long-chain type (palmitic, myristic and stearic acid). From that point forward a high saturated fat intake may be okay, but it would seem wise to get a good amount of monounsaturated fats as well as the proper amount of omega 3’s and the proper ratio of omega 3 to omega 6. This last scenario would typically be the natural result of following a high-quality low-carb diet, i.e., one based on quality meats, seafood, eggs, nuts, seeds, olives, avocados, some butter and cream (preferably from grass-fed cows), lots of non-starchy veggies and a bit of fruit. Therefore, from a real world perspective, any concern of using butter or other high-saturated fat foods should not be overblown.


Enig, M. (2000). Know your fats: The complete primer for understanding the nutrition of fats, oils, and cholesterol. Bethesda Press. Silver Spring, MD.

Galgain, J. et al (2008). Effect of the dietary fat quality on insulin sensitivity. Br J Nutr; 100: 471-479.

Lopez, S. et al (2008). Distinctive postprandial modulation of b cell function and insulin sensitivity by dietary fats: monounsaturated compared with saturated fatty acids. Am J Clin Nutr; 88: 638-644.

Phinney, S. (2004). Ketogenic diets and physical performance. Nutrition & Metabolism; 1(2).

Reaven, G. (2005). The insulin resistance syndrome: definition and dietary approaches to treatment. Annu Rev Nutr; 25: 17.1-17.6.

Sharman, M. et al (2002). A ketogenic diet favorably affects serum biomarkers for cardiovascular disease in normal-weight men. J Nutr; 132: 1879-1885.

Volek, J. & Westman, E. (2002). Very-low-carbohydrate weight-loss diets revisited. Cleveland Clinic J Med; 69(11): 849-862.

Westman, E. et al (2007). Low-carbohydrate nutrition and metabolism. Am J Clin Nutr; 86: 276-284.

Saturated fat and insulin function; should low-carbers be concerned?
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