You may have heard the news: Fat is back.

It's well documented that our decades-long demonization of fat (and saturated fat in particular) has been based on biased, industry-funded, and often erroneous data. This paved the way for sugars, refined grains, and unhealthy seed oils to creep their way into nearly every crevice of the modern food supply. Saturated fat, on the other hand, has been largely exonerated, at least for the general population. The most recent large scale meta-analysis found no association between saturated fat consumption and death by any cause, and a recent small trial even found it to be protective to cardiovascular and metabolic health (RR).

That said, there are still some instances where eating excessive saturated fat might be a bad idea. Here are 5 of them.

1. You haven't cut the sugar and other sources of concentrated carbohydrates from your diet.

Insulin, the body's fat storage hormone, becomes elevated when we eat sources of concentrated carbohydrates, like a baked potato or bowl of brown rice. When you consume saturated fat and carbs in one meal (a slice of pizza or cheesy burrito, for example), more insulin is secreted for the same amount of carbs. (R) Hyperinsulinemia (or chronically elevated insulin) is so problematic to our health—and our waistlines—that it's now being considered a "unifying theory" of chronic disease! One recent estimate stated that nearly 50% of Alzheimer's cases alone may be owed to hyperinsulinemia. (R) On the other hand, saturated fat by itself or with protein and leafy vegetables is likely a-ok.

2. You have genetically very high cholesterol.

About 1 in 250 people have gene variations that make them less efficient at processing LDL cholesterol, while also causing their livers to create more of it. This is called Familial Hypercholesterolemia (FH). For this group, eating excessive saturated fat may not be a good idea. They also over-absorb dietary cholesterol, and so while for the VAST majority of people, eating cholesterol (from egg yolks, for example) has no bearing whatsoever on cholesterol in the blood, this small group may be an exception. Not a whole lot is known about this condition, but it proves that we actually are unique snowflakes—while there are dietary principles that may work for most, there is truly no such thing as a "one-size fits all" diet.

3. You're a fiber-phobe.

If you haven't heard of the microbiome by now, you are living under a rock. But if you truly have missed one of the most exciting health developments of our time, I'll spell it out for you: We have a commune of 30 trillion microorganisms that live in our gut. And their collective health is our health. Problem is, they have different tastes than us—they only like to eat one thing: fiber. Without fiber, various bacterial byproducts can enter our circulation, causing damaging and mood-zapping inflammation. This ability of bacterial products to be ferried into the blood seems to be enhanced by saturated fat, but is prevented when saturated fat is consumed with fiber (from dark leafy greens, for example). (R)

4. You carry the ApoE4 allele (which is actually pretty common).

The ApoE4 allele is the most well-defined Alzheimer's risk gene, putting carriers at higher risk for developing Alzheimer's and other neurological conditions. Some researchers suspect that carriers of either 1 or 2 copies of the gene, which make up ~25% of the population, are worse at processing cholesterol, causing it to become "backed up" in the blood. (R) While the science of ApoE4/diet interaction is SUPER complicated and VERY far from being settled, going out of the way to eat loads of saturated fat if you're a carrier may be unwise until science can clarify. (Dietary cholesterol, on the other hand, seems safe and possibly even protective.) (R, R)

Wanting to get some perspective from practicing clinicians, I interviewed two cardiologists specifically on this topic: William Davis, MD, and Steven Gundry, MD. Both have differing opinions, despite being relatively aligned in overall dietary approach. Dr. Davis has a more liberal view on saturated fat for ApoE4s (video), while Dr. Gundry believes that ApoE4s should avoid concentrated saturated fat sources and opt for foods high in monounsaturated fat (olive oil, avocado, macadamia nuts) instead.

To find out your ApoE status, sign up for a gene testing service like 23andMe, which tells you which variants of the gene you carry under your Alzheimer's risk status.

5. If ketogenic/paleo/low carb/Atkins/Bulletproof/LCHF diets drive your cholesterol way up.

Most people will see their numbers improve on low carb diets such as these, which IMO are generally healthful. Some, however, can see their numbers begin look worse (unusually elevated LDL for example). Peter Attia, a low-carb-friendly physician, has shared a case study on his blog of a patient following a ketogenic diet (very low carb, high fat diet) with a high percentage of calories from saturated fats only to see their LDL particle count (thought to be a predictor of heart disease risk) shoot through the roof. When they replaced sources of saturated fat (butter, coconut oil) with monounsaturated fat (olive oil, avocado oil), their lipid profile improved dramatically.

If eating lots of saturated fat causes your lipids to become high, this does not mean that ketogenic diets "don't work for you" or that your Paleo diet is wrong, or that you've got bad genes. It just means that your version of the diet needs a little bit of tinkering to make it work for your unique biology. For 30 days, stop putting coconut oil and butter in everything (including your coffee) and replace commonly recommended saturated cooking fats with avocado oil, which is very heat stable. Draw new labs. See what happens. Just remember: There's no such thing as a singular diet that's perfect for everybody.

In the words of Stephen Hawking: "Without imperfection, neither you nor I would exist."