Review: The Hungry Brain by Stephan Guyenet

Last update: 15 August 2021

Disclosures

I happen to like Dr Guyenet (from our interactions on Twitter), and am biased in favor of his ideas on obesity. I will attempt to be harsher and more nit-picky than I would have been otherwise, to give his work a fair shake, despite the fact that he hasn’t annoyed me with his Twitter antics like his arch-rival, Gary Taubes.

General impressions

It’s readable – I should know, having read it three times. Some parts are easy to understand, while others are pretty taxing, especially the explanation of how the brain works. The chapter about sleep is boring, IMO, probably because I already sleep well.

Guyenet, surprisingly, paints a very similar picture to Taubes in The Case for Keto. Pretty much all of the unobjectionable parts of TCFK are present in THB, in different words. The eminent differences being that Taubes doesn’t really buy into the calories concept (or at least not using the same causality arrows), whereas Guyenet does, and that Taubes blames dietary carbohydrate for obesity, whereas Guyenet blames convenient junk food in general.

If I were to give an elevator speech about the core claims of the book, it would be something to the tune of:

Changes in adiposity reflect energy balance, which is managed primarily by the unconscious parts of the brain. These systems are not adapted to the deliciousness, abundance and convenience of modern food. This causes chronic overeating, and therefore obesity in genetically susceptible individuals.

The unobjectionable

  • Energy balance determines changes in adiposity1,5. If you are getting fatter, you must be in a positive caloric balance; if you are getting leaner, you must be in a negative caloric balance.
  • Americans are eating more now than they were eating in the 1960s2. Even adjusting for uncertainty due to faulty methods, it seems likely to be true.
  • Highly palatable, ready-to-eat, industrially produced human foods (“junk food”) are exceptionally fattening in animals and in humans3. This happens through increased, spontaneous consumption.
  • The brain regulates appetite and energy expenditure4,5, by collecting information emitted biochemically by other tissues. This functionality has been in there for a long time, at least since primitive fish8. Damaging the hypothalamus in specific ways can induce ravenousness or disinterest in food7.
  • Food preferences are learned and socially constructed to some extent, but the brain is mostly concerned with calories and salt, while avoiding toxins9. Micronutrients are unmanaged in general, while calorie returns for time spent are maximized10. Some foods (like meat10) are regarded more valuable than their calorie content would imply.
  • Processing and refining increases the reward value (as the brain perceives it) of foods5.
  • Satiety is sensory-specific, and variety increases energy intake5.
  • How hard you are willing to work for food predicts weight gain. How impulsive you are predicts weight gain11. Reinforcement pathology is the mechanism here12.
  • Hunter-gatherers have diets that are low density of reward factors, limited variety, low ability to refine foods, and are very lean13,14.
  • Market forces are behind the increase in food environment obesogenicity15.
  • Convenience and cost of food have changed in ways that make them better deals under Optimal Foraging Theory5,10,15.
  • Congenital leptin deficiency causes extreme obesity in animals and humans5. In normal obesity, leptin is elevated, and adding more has little effect5,16. There probably exists some unidentified factor that defends against overfeeding6.
  • People gain fat as they age6. This doesn’t happen in primitives14,17,18.
  • Exercise (inasmuch as it maintains fitness) is probably good for you19.
  • High protein intake is good for you20.
  • Endogenous high insulin doesn’t cause obesity21.
  • Many different dietary approaches work (poorly)22.
  • Feasts without famines, and overeating should be avoided6.
  • Simple whole foods have high satiety value23.
  • Susceptibility to obesity, given the right environment, is genetic5,6. Some few people are resistant, most are not6. This seems enormously polygenic aside from a few really bad monogenic mutations24.
  • Sleep deprivation causes overeating25.
  • Fighting one’s own brain regarding hunger is a losing battle, success is not expected26.

The objectionable

Palatability and food reward

Guyenet uses the following definitions:

Food that brings us pleasure when we eat it is described as palatable. Palatable food tastes good. It’s a sign that the brain values a food, either as a result of instinct or reinforcement learning.
The brain presumably values certain food properties above others because they would have increased the reproductive success of our ancestors. The most highly palatable foods tend to be dense in easily digested calories and combine multiple innately preferred food properties in highly concentrated form: ice cream, cookies, pizza, potato chips, french fries, chocolate, bacon, and many others. These are the foods that are most likely to cause cravings and a loss of control over eating, because their physical properties make them exceptionally reinforcing, motivating and palatable. Researchers have an umbrella term for this combination of effects on the brain: food reward. Highly rewarding foods are those that seduce us.

He considers these to be a dominant factor in the common human obesity. This is plausible, and there is experimental evidence that lowering the tastiness of food results in weight loss29,30, but there are some corner cases that cast some doubt on this.

For one, it is possible to engineer a non-fattening diet that tastes good to the point of being universally preferred over a fattening diet – at least in rodents28. It is plausible to me that many of the successful individuals in dietary trials (individual variation in body mass delta is extreme, while the mean result is uninspiring31) owe their success to finding a diet that for them happens to be non-obesogenic and highly preferred over alternatives.

For another, recent (THB was written years before, mind) highly-controlled inpatient trials32,33 show that calorie intake can vastly differ between diets even when self-reported palatability is the same. Given the magnitude of these effects, it’s not clear to me at all that palatability or food reward drives modern obesity. It may be as simple as the fact that the current environment is pretty much never calorie-restricted and the set point ratchets upwards after every incomplete recovery from an overfeeding session6.

Behavioural advice

Guyenet gives some advice, summaries in brackets mine:

1. Fix your food environment [remove food cues from your perception]

2. Manage your appetite [eat foods with high satiety per calorie]

3. Beware of food reward [avoid junk foods problematic for you]

4. Make sleep a priority [fix anything that prevents good amount and quality thereof]

5. Move your body [physical activity every day]

6. Manage stress [fix causes of stress and meditate, don’t cope with food]

Again, this is all plausible and common sense… but at the same time, trials don’t really back up any sort of behavioural treatment for obesity much as truly effective at resolving the condition 34,35. Even if Guyenet’s specific advice were twice as effective, that’s barely a dent in the problem.

It’s not that this advice is wrong per se, it’s just that I think it’ll primarily help the sort of people who are already capable of supreme personal discipline, and just needed information on what specific procedures to implement.

Exercise

In particular, I object to exercise or physical activity having much to do with body fatness. Trials consistently indicate that the difference that exercise makes for body weight is about 1 kg36,37. Furthermore, total energy expenditure doesn’t explain the differences between obesity across countries38 and even hunter-gatherers don’t have different expenditures compared to the civilized39, so any effect of exercise would have to be via intake regulation (as Guyenet hypothesizes) – and that looks highly individual-dependent in terms of response40.

Conclusions

Overall, I like the work, and I think it’s closer to concordance with reality than Taubes’ book, and it’s academically valuable, but the advice is really not all that useful unless you don’t know the first thing about weight management. If you’ve got an iron will and unflinching discipline to apply them, the instructions will likely help, but if not, I doubt Guyenet’s program will be much more effective than existing behavioural weight management strategies.

References

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