For those unfamiliar with the “Penn & Teller” magic act, Penn Jillette is a long-time Las Vegas entertainer, TV personality, and author of several books. We originally published a review in 2016 and have had a lot of searches on this diet, so we updated the article for 2023.

His latest book discusses his rather atypical approach to weight loss. [https://www.amazon.com/Presto-Pounds-Disappear-Other-Magical/dp/1501140183].

If one is familiar with Penn Jillette, then it should come as no surprise that Penn’s approach to weight loss was a radical departure from the conventional. Approaching his 60th birthday at 330 lbs and with a systolic blood pressure topping 200 mmHg, Penn underwent heart surgery for blocked heart arteries. After the heart surgery, Penn’s doctor recommended he undergo bariatric surgery to help him lose the excess weight.

While bariatric surgeries have become an increasingly conventional medical treatment for severely obese people like Penn Jillette with serious co-morbidities, Penn’s skyrocketing blood pressure, despite consuming numerous prescription drugs, made it apparent that he might not live long enough to see his children reach adulthood. Neither conventional dieting nor increasingly conventional bariatric surgery appealed to the unconventional Penn. However, after the heart surgery, he realized that his children were more important to him than another piece of chocolate cake and decided to do something to improve his health.

After one of his shows in Vegas, Penn met Ray Cronise, who asked Penn to “interrupt his current relationship with food” via a major lifestyle transformation. Ray Cronise, a NASA scientist turned nutrition and weight loss expert recommended that Penn starts by picking just one food to consume for the next two weeks.

Penn chose the potato. He says Cronise told him he could have picked several other starchy plant-based foods like corn or beans, but Penn settled on potatoes. He was told he could have as many potatoes as he wanted and would not have to go hungry or count calories to lose weight. Penn says he ate about five potatoes a day. Penn ate nothing but potatoes, either baked or boiled. No butter, cheese, sour cream, or bacon bits… just plain old potatoes.

After 14 days of eating nothing but potatoes, Penn had lost 14 pounds. After these initial two weeks, Penn's weight loss continued as he added bean stew, salad, and other mostly minimally-processed plant-based meals, while seldom consuming animal products and almost no red or processed meats. His weight loss continued even though he ate whenever he got hungry and never counted calories. He also did not start exercising until after he had lost 100 lbs. Penn admits that he had become addicted to foods loaded with salt, sugar, and/or fat. The extreme “mono diet” Cronise had recommended designed to break Penn of his unhealthy food cravings for typical modern foods loaded with fat, sugar, refined grains, and added salt.

One might say that Penn’s diet shifted from "living to eat" to an "eating to live" priority so that he could hopefully live a longer and healthier life.

Penn has kept off the 100 lbs he lost in the first 3-4 months. His drug-resistant hypertension is now well controlled but with far less need for BP meds, and no doubt his other CVD risk factors have also improved. Penn insists that he is eating whatever he wants today but notes that what he wants to eat now has profoundly changed. He reports that his cravings for salty, fatty, and sugar-rich foods are largely gone. He does report that he still eats the occasional (once every few weeks) typical modern foods but claims that his desire for such foods is now largely gone. For more on Penn’s take on his largely plant-based, low-salt diet, here’s a link.

I would describe Penn’s current eating plan as a low-calorie-dense diet similar to a DASH-style or Pritikin-style diet.

What Can We Learn From Penn's Experience?

Well, the first thing is that potatoes and other minimally-processed high-carbohydrate foods do not promote obesity, despite numerous reports that claim that eating potatoes is associated with obesity or somehow promotes overeating and obesity.

Here’s one example," Walter C. Willett, who chairs the Department of Nutrition at the Harvard School of Public Health, has warned us about potatoes for years. Here’s what he says in his book, Eat, Drink, and Be Healthy: The Harvard Medical School Guide to Healthy Eating. (Full disclosure: I [Patrick J. Skerrett] co-authored this book.) ‘Nutritionists and diet books alike often call potatoes a ‘perfect food.’’ But while eating potatoes daily may be fine for lean people who exercise a lot or who do regular manual labor, for everyone else, potatoes should be an occasional food consumed in modest amounts, not a daily vegetable. The venerable baked potato increases blood sugar and insulin levels more quickly and to higher levels than an equal amount of calories from pure table sugar”.

How well does Dr. Willett's claim match reality? Not at all, as demonstrated in the recent Omnicarb trial. This trial fed people four healthy DASH-style diets that varied in the amount and type of carbohydrate foods. It was a randomized crossover-controlled feeding trial where 163 overweight adults (systolic blood pressure, 120-159 mm Hg) were given four complete diets containing all their meals, snacks, and calorie-containing beverages.

Each diet was fed for five weeks, and all subjects completed at least 2 of the 4 study diets. This study primarily focused on minimally-processed carbohydrate-rich foods designed to have either a high glycemic index (GI) or a low GI. Dr. Sacks and colleagues found a 20% reduction in fasting insulin levels and 6% lower LDL-C on the high-GI diet compared to the low-GI diet.

So it appears that when the diet is high in carbohydrates, the higher GI foods modestly improved insulin sensitivity and LDL-C levels compared to the lower GI group.

Dr. Sacks et. al. concludes: "In this 5-week controlled feeding study, diets with a low glycemic index of dietary carbohydrate, compared with a high glycemic index of dietary carbohydrate, did not result in improvements in insulin sensitivity, lipid levels, or systolic blood pressure.

In the context of an overall DASH-type diet, using the glycemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance." [https://www.ncbi.nlm.nih.gov/pubmed/25514303].

Of course, in this Omnicarb Trial, calorie intake was controlled, so even though insulin sensitivity and fasting insulin levels actually improved modestly, its results do not prove that eating potatoes without the butter, sour cream, melted cheese, etc., won't promote weight gain, although according to Dr. Willett, potatoes (like other high-GI foods) promote overeating by increasing insulin levels.

And how about history? In the 1800s, Irish peasants ate a diet with about 90% of their total calories coming from potatoes, and yet finding an obese Irish peasant back then was like finding a needle in a haystack.

Of course, Dr. Willett will point to data from about 120,000 health professionals that do find a correlation between eating more potatoes and becoming overweight or obese. Then again, those were American health professionals, and when they ate potatoes, they were usually French fries or chips. The "mashed potatoes" in restaurants are typically loaded with butter, salt, and cream.

Usually, Americans eat their baked potatoes with melted cheese, butter, bacon bits, sour cream, etc... so the correlation between eating more potatoes and being more likely to gain weight is likely confounded by the fact that few Americans ever eat a plain baked or boiled potato with nothing on it, as Penn Jillette did.

Correlation, as they say, does not necessarily prove causation. While Dr. Willett's questionable ideas about potatoes and high GI foods have been cited by numerous low-carbohydrate diet advocates who maintain that potatoes promote overeating because they increase blood sugar and insulin levels, this makes little sense as higher levels of insulin and sugar in the blood result in people feeling more satiated, not less. Indeed, when Dr. Holt examined how much satiety people felt consuming a set number of calories from various food items, she found that a plain potato had the highest satiety index (SI) score of all the individual foods she tested. She reported the SI produced by "boiled potatoes … was seven-fold higher than the lowest SI score of the croissant." [https://www.ncbi.nlm.nih.gov/pubmed/7498104]. The humble potato produced far more satiety per calorie than did fish, chicken, lean meat, or even beans (which have a low GI despite their high carbohydrate content.

Bottom Line:

Penn's unconventional approach to weight loss and control does not mean that eating only potatoes will make excess pounds magically disappear. However, Penn's experience does confirm Dr. Holt's research showing that the humble potato does provide more satiety per calorie than most other foods, including lean meat, fish, yogurt, or eggs. Indeed, while some health professionals still cling to the questionable theory that high-GI foods promote weight gain and disease, this theory seems to be without credible scientific merit [http://ajcn.nutrition.org/content/76/1/290S.full.pdf+html].

While potatoes may not be the perfect food, they can be a healthful addition to any diet designed to aid weight loss, lower blood pressure, and reduce other CVD risk factors. [http://jamanetwork.com/journals/jama/article-abstract/2040224].

Hopefully, Penn Jillette's experience will help make the naïve notion that potatoes somehow promote obesity and type 2 diabetes because of their high glycemic index disappear.

James J. Kenney, PhD, RD, FACN

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