When it comes to diet and disease, who isn’t wondering: What the fat?

Should we eat it with abandon, as the meaty keto people advise? Should we try to keep it mostly low and “good” — some olive oil, some fish and the occasional avocado?

Fat, protein and carbohydrates are the three major players that fuel the human machine, supplying nearly 100% of our body’s energy. For decades, there’s been a push-pull regarding two of them — fat and carbs — with everyone from scientists to celebrities trying to come up with the optimal mix for good health and long life. The public is also looking for big fat answers to coronary heart disease, diabetes, cancer and all the rest, trying to identify that nutritional silver bullet.

Disease detectives (aka epidemiologists) at Fred Hutchinson Cancer Research Center have been studying dietary fat and health for more than 30 years. Clinical researchers here are also examining how diet and nutrition can impact cancer treatment and recurrence.

What do they say when it comes to its benefits and harms, particularly in the realm of cancer? Here’s your big fat update. 

Low fat for long life

New results just came out on a long-term dietary modification trial with the Women’s Health Initiative that specifically looked at fat and women’s health. Participants were healthy and disease-free, aged 50 to 79 at the outset of the study in 1993; data was gathered via biological samples as well as self-reporting.

Led by the Hutch’s Dr. Ross Prentice and a pack of WHI researchers around the country and published last month in the Journal of Nutrition, the study followed nearly 50,000 women for almost 20 years to see if cutting back on dietary fat reduced the risk of breast and colorectal cancers and heart disease.

Nearly half of the participants cut their fat intake to 20% of total calories, eating more fruits, veggies and whole grains than meat, cheese, nuts and other fat sources. The other participants ate a “usual diet” with about 35% of their calorie intake coming from fat. 

Their findings: The women who kept their fat low and bumped up their vegetable, fruit and grain intake lived longer, healthier lives — or at least they reduced the likelihood of death following breast cancer, slowed diabetes progression and prevented coronary heart disease as compared to the women who ate the usual, higher-fat diet.

Dietary fat: friend or foe?

Fat first started getting the side eye about 60 years ago, when Americans began gaining weight and getting sick; experts concluded dietary fat must be driving obesity and diseases like diabetes, cardiovascular disease and maybe even cancer. Suddenly, fat was bad and carbs were the better option to break off our big fat love affair. Sausage and eggs gave way to breakfast cereals — many loaded with sugar. Hamburgers and steaks were hipchecked by pizza, pasta and packaged foods — many highly processed.

Hutch nutritional expert Dr. Marian Neuhouser and colleagues explained the genesis of this fat-carb smackdown in “Dietary fat: From foe to friend?” a review of nutritional science published late last year. The focus on fat, they wrote, was “driven by a prevailing belief that carbohydrates — all carbohydrates, including highly processed grains and sugar — were innocuous and possibly protective against weight gain, cancer, and cardiovascular disease through multiple mechanisms.”

As the diet pendulum swung from high fat to high carb, though, the rate of obesity and diabetes continued to climb while life expectancy dropped. So, the macronutrient skirmishes and studies raged on. Now fat’s back and bigger than ever, boosted by the wildly popular keto diet (short for ketogenic).

Fat’s a major component of keto — making up about 70% to 80% of total calorie intake, with 10% to 20% from protein and a measly 5% to 10% from carbs. Drastically cutting back on carbs forces your body to bring energy via a different (and potentially harmful) chemical process known as ketosis. Glucose (sugar) from carbohydrates is the body’s primary fuel; without carbs, it turns to its secondary source, fat. Particularly, the body’s fat stores (often located in the rear).

As a result, you lose weight. And as a result of that, there are now thousands of keto diet experts with books, blogs, YouTube channels, Twitter testimonials and all the hoopla, hype, confusion and questionable advice that comes with a new diet craze. Case in point, there’s actually a bacon and butter keto cookbook. (Read about the link between processed meat and cancer.) 

Keto-curious cancer patients

Cancer patients are very keto-curious, although it’s usually more about fending off recurrence or progression than fitting into skinny jeans. And there are plenty of cancer keto resources online. What there isn’t, is much clinical data.

There are many keto diet cancer trials in the works, in research centers from Florida to Frankfurt. But some are still recruiting, others have shut down, and many more haven’t yet published results. So, some patients are DIYing it, going keto during chemo or other protocols without solid evidence as to whether it will make their cancer shrink or grow.

Others are turning to keto in lieu of standard treatment — and advising peers to do the same.  

Carol Oxford Tatom, a 55-year-old research scientist and breast cancer patient from Vacaville, California, worries that patients will end up hurting themselves by taking things too far, too soon.

“The papers I read on keto and cancer were all in mice,” she said. “They’re still all in mice. And we’re not mice. There’s no clinical evidence that it’s healthy.”

Tatom gets the rationale for trying it — “the idea is the Warburg effect, that cancer feeds on sugar” — but she thinks people often look at dietary issues too simplistically.  

“That whole idea that sugar feeds cancer? Well, sugar also feeds us, it feeds all cells. If you you eat a reasonably balanced diet, that’s really been shown to be the healthiest,” she said.

Randomized clinical trials — on people, not mice — are what’s needed to suss out whether there’s any there there, Tatom said. Until then, crusading for keto as a cure can “promote false hope and … cause harm.”

Pairing keto with cancer treatment

Some answers are trickling in. Columbia University oncologist Dr. Siddhartha Mukherjee, author of  “The Emperor of All Maladies,” has been researching the ketogenic diet as a potential tool in cancer treatment for a few years. Among other things, he found that a keto diet can actually accelerate certain leukemias.