I have defended dietary fat as healthy in many blog posts, including

• Jason Fung: Dietary Fat is Innocent of the Charges Leveled Against It
• Which Is Worse for You: Sugar or Fat?
• Faye Flam: The Taboo on Dietary Fat is Grounded More in Puritanism than Science.

But there is one circumstance in which dietary fat might not be so great: if you are still eating sugar. In last week's post, "Heidi Turner, Michael Schwartz and Kristen Domonell on How Bad Sugar Is" I quote this:

Schwartz agrees that sugar can cause major health problems, but says it isn’t acting alone. The most potent way to activate the brain’s reward system is actually by combining sugar with fat, he says. And much of the American diet contains both of these components.

There is a claim here about complementarity in badness: that is, in the presence of sugar, dietary fat is worse than in the absence of sugar. I take the view that in the absence of sugar, dietary fat—other than the big mistake of transfat—is quite healthy. But it is logically possible that dietary fat combined too close in time to sugar is unhealthy. Let me spin out a possible theory. I should say first that I am not really persuaded by the "overwhelmingly rewarding" theory that Michael Schwartz is putting forward. Sugar is extremely rewarding. Fat is extremely rewarding. Is the combination of sugar and fat that much more rewarding than sugar in combination with nonfat foods or fat in combination with nonsugar foods?

Instead, let discuss things from the standpoint of the satiation to calorie ratio I talk about in "Letting Go of Sugar." Dietary fat by itself, or in combination with other foods low on the insulin index (see "Forget Calorie Counting; It's the Insulin Index, Stupid") is quite satiating: it will make you feel full quite fast. But sugar has a negative satiation to calorie ratio: it makes you feel less full. So add enough sugar to your dietary fat, and the dietary fat's normal tendency to make you feel full will be neutralized. 

The idea that sugar neutralizes the tendency of dietary fat to make you feel full still doesn't seem to make dietary fat any worse than anything else combined with sugar. But what if, in addition to the mechanisms that normally make dietary fat satiation, there is a volumetric mechanism that makes you full if there is a high volume of food in your stomach. It would make sense that sugar can neutralize some mechanisms that make you feel full, but it can't neutralize the volumetric mechanism. But dietary fat doesn't have a lot of volume per calorie, so if the normal mechanism that makes dietary fat so very satiating is neutralized, there isn't a volumetric backup mechanism for fat. Sugar gets past the main safeguard that makes you not want to overeat dietary fat and that's it. 

Solution? Don't eat sugar. See "Letting Go of Sugar" for how to get there. Sugar is bad whether or not it is combined with dietary fat. Even on the theory above, dietary fat is only bad when combined with sugar.

Don't miss these other posts on diet and health and on fighting obesity:

• Stop Counting Calories; It's the Clock that Counts
• 4 Propositions on Weight Loss
• Forget Calorie Counting; It's the Insulin Index, Stupid
• Obesity Is Always and Everywhere an Insulin Phenomenon
• The Problem with Processed Food
• Anthony Komaroff: The Microbiome and Risk for Obesity and Diabetes
• Letting Go of Sugar
• Heidi Turner, Michael Schwartz and Kristen Domonell on How Bad Sugar Is
• Prevention is Much Easier Than Cure of Obesity
• Which Is Worse for You: Sugar or Fat?
• Our Delusions about 'Healthy' Snacks—Nuts to That!
• My Giant Salad
• Using the Glycemic Index as a Supplement to the Insulin Index
• How Fasting Can Starve Cancer Cells, While Leaving Normal Cells Unharmed
• Why You Should Worry about Cancer Promotion by Diet as Much as You Worry about Cancer Initiation by Carcinogens
• Good News! Cancer Cells are Metabolically Handicapped
• How Sugar, Too Much Protein, Inflammation and Injury Could Drive Epigenetic Cellular Evolution Toward Cancer
• Meat Is Amazingly Nutritious—But Is It Amazingly Nutritious for Cancer Cells, Too?
• The Keto Food Pyramid
• Sugar as a Slow Poison
• How Sugar Makes People Hangry
• Why a Low-Insulin-Index Diet Isn't Exactly a 'Lowcarb' Diet
• Hints for Healthy Eating from the Nurse's Health Study
• The Case Against Sugar: Stephan Guyenet vs. Gary Taubes
• The Case Against the Case Against Sugar: Seth Yoder vs. Gary Taubes
• Gary Taubes Makes His Case to Nick Gillespie: How Big Sugar and a Misguided Government Wrecked the American Diet
• Against Sugar: The Messenger and the Message
• A Conversation with David Brazel on Obesity Research
• Magic Bullets vs. Multifaceted Interventions for Economic Stimulus, Economic Development and Weight Loss
• Nina Teicholz on the Bankruptcy of Counting Calories
• Mass In/Mass Out: A Satire of Calories In/Calories Out
• Carola Binder: The Obesity Code and Economists as General Practitioners
• Carola Binder—Why You Should Get More Vitamin D: The Recommended Daily Allowance for Vitamin D Was Underestimated Due to Statistical Illiteracy
• Jason Fung: Dietary Fat is Innocent of the Charges Leveled Against It
• Faye Flam: The Taboo on Dietary Fat is Grounded More in Puritanism than Science
• Diseases of Civilization
• Katherine Ellen Foley—Candy Bar Lows: Scientists Just Found Another Worrying Link Between Sugar and Depression
• Ken Rogoff Against Sugar and Processed Food
• Kearns, Schmidt and Glantz—Sugar Industry and Coronary Heart Disease Research: A Historical Analysis of Internal Industry Documents
• Eating on the Road
• Intense Dark Chocolate: A Review
• In Praise of Avocados
• Salt Is Not the Nutritional Evil It Is Made Out to Be
• Confirmation Bias in the Interpretation of New Evidence on Salt
• Whole Milk Is Healthy; Skim Milk Less So
• Is Milk OK?
• How the Calories In/Calories Out Theory Obscures the Endogeneity of Calories In and Out to Subjective Hunger and Energy
• Putting the Perspective from Jason Fung's "The Obesity Code" into Practice
• 'Forget Calorie Counting. It's the Insulin Index, Stupid' in a Few Tweets
• Julia Belluz and Javier Zarracina: Why You'll Be Disappointed If You Are Exercising to Lose Weight, Explained with 60+ Studies (my retitling of the article this links to)
• Diana Kimball: Listening Creates Possibilities
• On Fighting Obesity
• The Heavy Non-Health Consequences of Heaviness
• Analogies Between Economic Models and the Biology of Obesity
• Debating 'Forget Calorie Counting; It's the Insulin Index, Stupid'
• Podcast: Miles Kimball Explains to Tracy Alloway and Joe Weisenthal Why Losing Weight Is Like Defeating Inflation

Also see the last section of "Five Books That Have Changed My Life" and the podcast "Miles Kimball Explains to Tracy Alloway and Joe Weisenthal Why Losing Weight Is Like Defeating Inflation." If you want to know how I got interested in diet and health and fighting obesity and a little more about my own experience with weight gain and weight loss, see my post "A Barycentric Autobiography."