Tom Kerridge has a captivating story to tell. The popular chef and presenter on BBC’s Proper Pub Food and Best Ever Dishes lost 70 kilograms (down from 190 kg) and many viewers witnessed that he slimmed down not knowing what his secret recipe to success was. Motivated by the growing interest, Tom Kerridge wrote a book that recently entered the top ten book sales list at amazon.co.uk. It could have been another simplistic take on a low-carb diet, but the publisher decided to go a different route. They dubbed it “Tom Kerridge’s dopamine diet”.
If you are wondering what a dopamine diet is, it is simple to shed some light in these dark ages of nutrition science (see my last post on the Wansink pizzagate):
“It’s about homely great cooking,” he says. “It’s protein led … a lot of it is about big flavours, spices and strong tastes.”
Oh, sorry. That was the new wine in old wineskins part of the book, which is acknowledged by the author by the way:
The answers contained in his new cookbook, Tom Kerridge’s Dopamine Diet, aren’t earth-shatteringly original, as the Michelin-starred chef from Marlow gastropub The Hand and Flowers, is the first to concede.
The recipes are essentially revved-up versions of low-carb cooking, ditching anything sugary or starchy in favour of meat, fish, eggs, nuts, dairy and vegies, with starring roles given over to omelets, mince and broth.
The “novelty” in his diet is the reference to neuroscience.
What then, of the book’s sciency-sounding title? It refers to what Kerridge calls his “dopamine heroes” – beef, chicken, chocolate, fruit, nuts, spices and dairy. Feel-good foods he says are high in a chemical called tyrosine, which helps boost the release of dopamine, a neurotransmitter associated with pleasure and emotional rewards.
Certainly, dopamine is involved in many important processes in the brain such as the control of movement or reinforcement learning. However, “pleasure” is what is often being confused in the media with the motivational aspects of a reward. Decades of neuroscience research have established a dissociation between pleasure circuits in the brain encoding “liking” and motivational circuits encoding “wanting” of a reward. Importantly, only the latter is consistently linked to dopamine function. At this point, it is already obvious that the book has very little to do with state-of-the-art science.
Another (scientific) problem is the simple-minded idea of “dopamine heroes”. I eat feel-good food, some of it is supposedly high in tyrosine (the food examples are far from convincing in that regard), so it must upregulate dopamine in my brain! This is a dangerous mix of circular and reverse inference errors. Likewise, I could argue: Chefs cook well, I like my freshly cooked food, so I must be a professional chef. I get that one needs to provide good analogies to sell a book, but if they are blatantly wrong, this is doing damage to the empirical studies of the subject.
If it’s not about dopamine, why did Tom Kerridge lose so much weight?
Okay, maybe I am just nitpicking some sciency buzzwords while the big picture is still intact and convincing. In the end, if he was all wrong, how could he lose that staggering amount of weight? There might be some truth in the advice he is giving in his book, for example:
“There are restrictions, of course – no alcohol, a 90-gram limit on carbohydrates per day, and mince meals without the usually obligatory serve of rice or pasta.”
Basically sounds like cutting calories to me. There is good evidence that heavy alcohol intake (≥ 30 g/d or >2 glasses of wine) is a risk factor for weight gain and obesity in middle-aged men like Tom Kerridge (e.g., Wannamethee & Shaper, 2003; http://ajcn.nutrition.org/content/77/5/1312.short). Another grain of truth is that all “natural” foods are either high in carbohydrates or fat, but not high in both at the same time. To illustrate this, I have created a scatter plot from a spreadsheet containing the nutritional value of many common foods that I downloaded from the California State University of Northridge (https://www.csun.edu/science/ref/spreadsheets/xls/nutrition.xls).
We can see that most foods cluster along the axes of carbohydrates or fat, but only few dots (caloric density is mapped onto dot size) are scattered along the diagonal. These dots come primarily from processed foods such as baked goods. (The high carb + high fat vegetable dot comes from potato chips, reminding me of the inflated rates of vegetable consumption in the US due to French fries.) In other words, processed food might be offering something that we like considerably more than carbs or fat alone and it might be easier to stay clear of the one or the other in trying to lose weight. My guess is that this mechanism might have evolved because it promoted balanced diets, but we are clearly facing different challenges in our environment today. However, there is only preliminary evidence to date supporting this idea (e.g., Hoch et al., 2015; http://www.nature.com/articles/srep10041) and I think it is still useful to treat a calorie as a calorie when it comes to body weight until we have better evidence.
Isn’t more tyrosine in food still going to boost dopamine in the brain?
Maybe yes, but we don’t know for certain. Tyrosine is a precursor of dopamine so a higher availability should increase dopamine levels all things being equal. But in neurobiology, all things being equal is a very poor model of the complexities involved in dopamine synthesis. For example, it has been shown that tyrosine availability becomes a rate-limiting factor only when it is combined with administration of haloperidol (a dopamine D2 receptor antagonist) in animals (Chance et al., 1990; http://dx.doi.org/10.1016/0091-3057(90)90226-8 ).
Most of the evidence for an effect of tyrosine on dopamine function actually comes from phenylalanine and tyrosine depletion studies, but these effects cannot be extrapolated to the effects of tyrosine loading. This is a fundamental problem in nutritional neuroscience and extends beyond the realm of dopamine to similar methods such as tryptophan, a precursor of the “feel-good” neurotransmitter serotonin, challenges. Unfortunately, the brain is a not a simple linear input-output system. Thus, even if lacking nutrient A is bad for your health, it does not indicate that eating more and more of A will increase your health proportionally. Just like the pleasure derived from eating one’s favorite meal does not increase linearly per bite or with the frequency of eating it.
At least people are talking about dopamine and food now and that’s a good thing, right?
I often read and hear excuses that this is simply business and any kind of attention is good for a subject so there is no real harm in promoting a book with neuroscience bullshit. I don’t agree (as you can see). Andrew Gelman recently wondered in one of his blog posts why Brian Wansink was not called out earlier for publishing questionable research. He speculated that Wansink was flying under the radar and did not face any opposition because his research was not important or challenging enough to really bother anyone and take care. Now the damage is done to a whole scientific endeavor. I have also cited some related social science apologies in my last post. If we silently accept these practices, tomorrow’s image of neuroscience might be dragged down by today’s neuro-marketing schemes. These topics need to be studied in detail to improve public health one day and the loose use of neuro buzzwords induces the wrong impression of profound knowledge where there is very little insight. It is worth to keep in mind that such books are read by many more than the perfectly controlled and balanced reports that should be the cornerstones of evidence-based policies. If we are not going to be active, someone else will be.