Gut Reactions: The Science of Weight Gain and Loss

by Simon Quellen Field

How much do you really know about how the human body works and how it reacts to food, exercise, nutrition, and the environment? While most people have read about at least one fad diet, they're left wondering about the greater biochemistry, psychology, sociology, and physiology of the obesity crisis in the United States.
            Gut Reactions by chemist Simon Quellen Field shows readers how their bodies react to food and the environment and how their brains affect what and how much they eat. It reveals why some diets work for some people but not for others, based on genetics, previous weight history, brain chemistry, environmental cues, and social pressures. It explores how dozens of hormones affect hunger and satiety and interact with the brain and the gut to regulate feeding behavior. And it explains the addictive nature of foods that interact with the same dopamine and opioid receptors in the brain as cocaine, heroin, amphetamines, and nicotine.
            Whether you’re looking to lose weight, put on muscle mass, or simply understand how your metabolism or gut microbiome impact your food cravings, Simon Quellen Field has the scientific answers for you.
 

Genres: Nonfiction, Science, Health, Audiobook, Nutrition, Food, Medicine

256 pages, Paperback

Published January 8, 2019

Reviews

[Morgan Blackledge · Rating 5 out of 5]

True to the title.

This is a brief and potent (no fluff) primer on the science of weight gain/loss.

The book begins where every discussion on consummatory (eating, drinking, addiction etc.) behavior should.

With a discussion on the process of homeostasis.

For those of us who slept through (or were baked during) biology class. Homeostasis simply refers to an organisms (or systems) process for returning to balance (set point) after dysregulation.

Homeostasis applies to weight gain/loss in that your body weight is one of many set points that your bodies homeostatic processes tries to maintain via hunger and satiety signals that (under normal conditions) inform eating and drinking behavior, and keep us alive and at as optimal a body weight as the environment will allow.

In a modern western food environment (probably the one you’re in if you’re reading this) processed foods overstimulate your reward centers, and under stimulate your satiety sensors, and (basically) hack your brain, to all but ensure that you over eat (by a lot) unless you’re very careful.

Until I read this book, I was at least a little skeptical of the idea of food addiction. Now my world has been inverted. In other words, I’m a little skeptical of the whole construct of addiction, at least as some kind of special case of behavior run wild.

Basically, the implication of this book is.

Just about everything we (modern, western, post-capitalist creatures) do is subject to this exact type of brain hack => out of control, desperately unhealthy and unsustainable behavior that we call addiction.

Addiction isn’t some special case of substance use run amok.

In reality it is all of us all the time.

Slam cut to montage of earth burning (🌍 🔥 🥵).

The end.

Great book.

With surprising, big think implications.

Highly recommended.

[Marzi Pecen · Rating 5 out of 5]

If you love the scientific geekery of which hormones do what to regulate something, this is the book for you. If you want a step by step fad diet, keep looking. Interesting from the perspective of understanding systems and processes of heat regulation, ph of the gut and what’s going on in your gut.

[Clare Kirwan · Rating 4 out of 5]

3.5* Sugar is REALLY bad for us. I read this having embarked on a low carb diet and this highly scientific book about the chemical processes involved in the human digestive system really opened my eyes. With crisp explanations of specific hormones, prebiotics, probiotics etc it presents many fascinating facts and insights into nutrition and the mind-boggling complexity of our internal human biology, although it does so with an uneasy mix of highly technical language, replete with acronyms, and fairly obvious 'If you're hungry, go for a walk' type of advice. I listened free on audio but it was a bit overwhelming so I subsequently bought the book as a reference guide to highlight the really important stuff.

[Divyanshu Maithani · Rating 2 out of 5]

Listened to on runs, gets a bit too technical at places without clearly explaining the topic in simpler language.

[Kate Hageman · Rating 1 out of 5]

How many times can you say fat in a book? This one must be going for a world record. I read the audiobook and the narrator was horrible. Also, the recommendations were all - "just don't eat", get better willpower, don't be around people that don't help. It was overall pretty harsh while at the same time being demoralizing because of all the talk about vicious cycles (being fat does x but more of x makes you fat). I guess I should have known better - the title does say "the science of" so everything is pretty matter of fact. I wouldn't suggest reading this if you are sensitive at all to your weight. In fact, only read this if it's your job to study weight loss. I can't think this would be helpful otherwise.

[Lucy Batson · Rating 3 out of 5]

This book manages to pack a lot of dense scientific information in a short page count, but you can basically just read the last chapter and get the TL;DR on what works and doesn't when it comes to weight loss.

[Monica · Rating 3 out of 5]

I thought this book focused mainly on the gut microbiome based on its title, but realised that it covers a wider span of topics in readable chunks including a long section on different hormones. What I did was just zoom in on sections that I want to read so it is useful in that sense. There are short sections on pre/pro/anti biotics.

There are many factors contributing to weight gain and loss, not just calorie deficit so I do understand the range of coverage. But those picking up this book mainly to find help and support in weight loss may be overwhelmed. Eg, the book touched on a long list of hormones and that can be a bit baffling for some.

Since I only read selected sections, I am putting this under my "gave up on" shelf, not because it is bad, but just to remind myself that I did not read it in its entirety as it does not meet my current needs (maybe I should just re-name my shelf to "did not finish"). But the sections I read were very readable and helped me understand my body a bit more.

[John Poulain · Rating 3 out of 5]

A lot of the book isn't new (or focused on Gut Health) covering history of weight gain. Macro nutrients etc. a lot of the research is newer, less substantiated and deals with smaller effect sizes.

Choice Notes
 Obesity rates stabilised in America in 2010, having grown since 1980. That they changed in the 70s and stabilised in the 10s, which means obesity rates cab be changed. What was done can be undone. 
 We eat for pleasure long after we are satisfied. The saity hormones are being overriden by the pleasure hormones. We may pass up broccoli because we're full but we have room for dessert. Sugar and wheat flour don't produce the same saiety signals. 
 Fructose has all the calories of glucose but doesn't give you saiety signals. It cannot be stored and must be processed immediately by the liver the same as alcohol. 
 High satiety of protein diet and keeping muscle mass allowed for better weight loss (more and kept off longer). Low GI diet effects were additive to high protein diet. Very low calories diet lost 61% fat and 39% lean body mass 
 Low protein and low calories diet insulin sensitivity improved by 45% 
 Too much protein removes beenift of insukin sensitivity. Hugh protien rdducrd loss of lbm by 45% but no imporvement in i sulin sensitivity. 1.5g per kg vs 0.8g per kg. 
 When adrenal glands removed from rats they stop exercising. Ben and jerry's is not s long term solution. Exercise and meditation can be done for longer with better outcomes 
 Glucocortisiods improve seeking behavious. When adrenal glands removed from rats they stop exercising. Ben and jerry's is not s long term solution for depression. Exercise and meditation can be done for longer with better outcomes 
 Waist to hip ratio a better indicator of heart disease than BMI or waist circumference. Could be due to cortisol effects. 
 HgH counters Th e effects of in, tells the body to increase glucose which raises blood sugar, only safe when done whilst awake and fasting. Can lead to increased hearts and prostate issues. High intensity workouts can increase HgH. 
 High fat high sugar diets decrease HgH and so does alcohol and lack of sleep. Fasting leads to higher Ghrelin and therefore more HgH. 
 Reducing blood triglyceride and cholestorel and fat mass by taking oxytocin 19lbs in 8 weeks lost with no adverse side effects. 
 Oxytocin weight reducing is due to the breakdown of fats in fat cells. Another reason that depression sucks. 
 If thyroid pills thyroxine can cure obesity, why aren't doctors handing them out like candy? Because thyroid problems are only a small part of the obesity problem. 
 Gut microbes can ferment fiber into sugar, investing calorie intake by 2% 
 To become obese mice they needed to eat the standard western diet, high sugar, high fat, high protein and high digestible starches. 
 To become obese mice they needed to eat the standard western diet, high sugar, high fat, high protein and high digestible starches. 
 More of our tendency to be fat is related to our microbe genes than our human genes 
 Low carb and high protein diet has damaging effects on microbiome 
 We know genetics isn't the main reason for obesity, our genes didn't all change in the 70s. 
 RS1421085 SNP (pronounced snip) Single Nucleotide Polymorphism is 1% of variance in BMI in population but is present in 22% of the population of obesity. It is also less present in African populations (5%)suggesting that there was an evolutionary advantage tp putting on weight as people moved to Europe (44%) 
 There are many carriers of these genes and no gene following is found in more than 5% of population with BMI over 40. There a was less obesity in 1970s and genes haven't changed. 
 There is a U or J shaped distribution between birthweight and later obesity in life. This is linked to parental weight and can be changed, therefore likely to be fetal programming. Fat mothers lead to fat babies in a cycle. 
 Fasting is easier for most people than dieting as fasts until goal weight is not a lifestyle change and it's easier to have a "short term won't power than long term will power" 
 A cheat meal (not day) can improve sticking to a diet, improve letting levels and increase metabolism. It also decreases stress of hosting. 
 Only carbohydrate overfeeding leads to a boost. Fat overfeeding doesn't have these leptin or metabolism boosts 
 Cholesterol in body isn't changed much dye to diet. The liver will produce less in you eat more and if you avoid it the liver will make more 
 Waist to hip ratio <0.9 for men <0.8 for women in normal. 
 Curcumin research issues as the compound is fluorescent by itself and can't be tracked in the usual way. 
 In Thailand people eat a diet of 0.014% Capsaisin and when rats were fed that amount they ate the same by reduced visceral fat by 29% 
 Garlic and onions effect brown fat and the storage of fat 2-5% garlic by weight has this effect but is not considered viable for dieting 
 Visceral fat suppresses HGH and leading to visceral fat. Fat cells causes insulin resistance, causes leptin resistance which leads to always feeling hungry and more fat. Stress leads to weight gain leads to stress High sugar high fat diet increases fermicuties in the gut which increases food breakdown in gut leading to weight gain. Overweight mother's give birth to high birthweight children 
 Visceral fat suppresses HGH and leading to visceral fat. Fat cells causes insulin resistance, causes leptin resistance which leads to always feeling hungry and more fat. Stress leads to weight gain leads to stress High sugar high fat diet increases fermicuties in the gut which increases energy extraction in gut leading to weight gain. Overweight mother's give birth to high birthweight children. Fat cells increase autoimmune attacks on thyroid and decrease thyroid function. High fat diet causes intestinal k cells to proliferate creates GIP for fat cells to take up more fatty acids Depression leads overeating to obesity leads to depression. Western diet increases toxic bacteria to produce dpa which kills other bacteria leading to more energy extraction from fat extracting bacteria. Inflammation increases AGRP 

[Doreen · Rating 4 out of 5]

Very dense. Great, detailed account of the biochemistry of weight management. Listened to the audiobook. Has some practical tips. My takeaways: fiber and veggies are good. Sugar, especially fructose is bad. Your genetics have a lot to do with your weight, but they are no excuse. Intermittent fasting is something to seriously consider.

[Emily Kirik · Rating 3 out of 5]

Some valuable information but wasn’t written eloquently. The writing was choppy and contradictory. I found the chapter on the microbiome most useful, however. He explained many hormones that didn’t provide a lot of insight into heath as a whole, making the depth of that chapter unnecessary.

[Galia Angelova · Rating 4 out of 5]

I listened to this the book in two days. It’s packed with scientific explanations on how the metabolism works. I definitely forgot most of it, but I know what is good for me and what I’d better avoid. I can recommend this book.

[Clifford · Rating 4 out of 5]

A good scientific look at how your body handles food and stress. I would rate it a five, but this book would quickly fly over the head of someone not conversant in chemistry or biology.

[Sammi · Rating 3 out of 5]

Gives the hard science of chemical reactions

[Catherine Knight · Rating 5 out of 5]

This book is AMAZING. I have no idea why the other ratings are so low. This a book that *everyone* should read.

[For The Novel Lovers · Rating 4 out of 5]

Book Review

Title: Gut Reactions: The Science of Weight Gain and Loss by Simon Quellen Field

Genre: Science, Food, Weight Loss

Rating: 4.5 Stars

In Gut Reactions: The Science of Weight Gain and Loss, Simon Quellen Field takes on one of the most emotionally charged and culturally saturated topics of modern life—body weight—and approaches it with refreshing scientific clarity. Rather than offering another diet book or prescribing a simple formula for weight loss, Field dismantles common myths using biochemistry, physiology, and evolutionary reasoning, resulting in an accessible yet intellectually rigorous exploration of how and why human bodies gain and lose weight.

What distinguishes Gut Reactions is Field’s commitment to explaining mechanisms, not merely outcomes. Weight change, he argues, is not a morality tale about discipline but a complex interplay of genetics, hormones, gut microbiota, metabolic pathways, and environmental inputs. While many books attempt to simplify this complexity into digestible “hacks,” Field leans into it—though never at the expense of readability. His talent lies in translating detailed scientific concepts into approachable explanations, often enlivened with analogies and clear illustrations.

A central strength of the book is its nuanced discussion of metabolism. Field rejects the popular narrative that metabolism is a static “fast” or “slow” trait and instead traces how it is shaped by evolution, individual physiology, and energy balance. He also explains why calorie counting, while theoretically sound, often fails in practice due to measurement errors, differences in energy absorption, and the body’s adaptive responses. His explanation of gut bacteria and their role in nutrient extraction is especially compelling, highlighting how two people can eat the same meal but absorb different amounts of energy.

Field also excels at debunking common myths without resorting to condescension. Whether discussing “fat-burning foods,” miracle diets, or the simplistic demonization of carbohydrates and fats, he maintains a tone of curiosity rather than superiority. This makes the book not only informative but humane—an important achievement in a field often marred by shame-driven discourse.

One of the book’s more thought-provoking arguments concerns the way modern environments override evolutionary adaptations. Field explains that bodies optimized for energy storage in times of scarcity were never designed for the calorie-dense, sedentary conditions of contemporary life. The result is a mismatch between ancient biology and modern surroundings—a mismatch that no amount of willpower alone can fully overcome. This evolutionary framing helps shift the weight conversation away from blame and toward understanding.

A minor limitation of Gut Reactions is that, in its attempt to remain even-handed, it sometimes hesitates to deliver clear prescriptive guidance. Field provides principles—such as focusing on dietary patterns rather than single nutrients—but readers seeking a structured plan may find the book more enlightening than actionable. Yet the absence of a prescriptive diet may be intentional: Field seems to emphasize that sustainable weight management requires individualized, informed strategies rather than one-size-fits-all solutions.

Overall, Gut Reactions is a valuable contribution to the popular science of nutrition and metabolism. It stands out for its clarity, scientific grounding, and compassionate approach to a topic that affects nearly everyone. For readers who want to understand the biological “why” behind weight gain and loss—and who prefer explanation over oversimplification—Field’s work offers a compelling, thoughtful, and empowering read.

[Luca Nicoletti · Rating 4 out of 5]

Summary

A good book about nutrition. A details explanation of how the food we eat can influence many more things than our weight. How each macronutrient is absorbed and digested by our bodies and our guts. The strong relationship between our gut, our brain and our weight is under the lens in this book, which details explanations, examples and practical advice on how to get better nutrition and feel better while losing weight.

My evaluation
What did I like the most?
Many of the explanations of how some food, behaviours and emotion can highly impact how our bodies react to the food we’re eating. The fact that some food can influence how our bodies react to the calories we take in, how the presence of sweeteners can trigger the same mechanism of sugar in our body (experienced it myself - at least for the mental sugar craving I get).

What didn’t I like?
The fact that author blames weight gain, not on calories. Thermodynamics is not bullshit. You can transform more of the food you eat into fat, as much as you want, but if your total caloric intake is lower than your total caloric consumption, you will lose weight.

What would I have done differently?
I’d completely have avoided saying that calories don’t matter when considering weight gain or loss. That’s all it matters!

[Ben · Rating 5 out of 5]

Fantastic. Really good information and good scientific background. I can’t help feel that a laypersons version might help people who aren’t science based or don’t have a healthcare background to get more from it. But for me, this is easily the best book on dietary process I’ve read (well, listened to - I went the audiobook route so I didn’t have to stop eating). I also thought it was read very well. Dry? Maybe, but seems to be a good academic reading with some commendable pronunciation in some passages. If the narrator didn’t have to do a load of retakes when discussing chemical process, he’s a legend!
For the people who are saying the book doesn’t help with dieting? Perhaps read it again. It tells you exactly what to do and it’s incredibly simple. Remove sucrose and FRUCTOSE by a massive amount and try and remove what flour. It’s largely what we knew but nice to have it explained why. Fructose comes out as the bad guy of this story!
The book also debunks some significant diet myths and also explains why some people just don’t get the same effects from a diet as others do.

[Maura · Rating 2 out of 5]

This book is freely available if you have an Audible subscription which is why I gave it a listen.

I'm intrigued by the complex interplay between hormones in our bodies so I found those chapters somewhat interesting but also very technical. Some of the advice in the book was familiar to me, such as the information regarding fasting, but the author seems to adopt a very extreme approach. The author also frequently uses the word 'fat' to describe people in the book, which I dislike.

The narrator also gives an entirely monotone delivery, which doesn't enhance the very technical scientific descriptions or marry well with the somewhat extreme advice, so I found the book decidely odd overall!

[Nuno · Rating 2 out of 5]

There are those science books that avoid technical jargon to make it easier on the casual reader - this is not one of them. At the same time I can't say it's for scientists only as I believe with some effort people who are not already acquainted with the subject matter will be able to follow.
The conclusion I reached at the end is that having some knowledge of how our body works in relation to food can truly help us think about what we put inside it - and what should stay out.
The narration sounds like it was done by an AI... I think if I'd read it instead of listened to, it would be 3 stars.

[Conny · Rating 4 out of 5]

Diets and nutrition are a densely discussed and disputed subject, with many people weighing in on it with moral, ethical, idealized lenses.
Rather than focusing on judging your diet choices, this book strictly gets into the complex body chemistry of weight, appetite, sleep, genetics, hormones, etc. It explains on the basis of current understanding of body chemistry what works and what doesn't in weight management and why.
Very detailed and granular, but also very useful when you look for basic information that's not trying to sell you a specific diet fad book or meal subscription.

[Kristijan Kosutic · Rating 3 out of 5]

This book is good if you are already familiar with many of the mechanisms, hormones, behavior and nutrition. It then provides a good summary of current scientific research and gives you nice overview of everything you need to understand.

If you are relatively new to this, this book is probably not a good starting point. it will be a bit hard to follow and you will end more confused than when you started reading. If you do decide do start with this book, use it to note down all of the areas you need to research and plan follow up reading or investigation afterwards.

[Taylor · Rating 4 out of 5]

It’s alright, more like 3.5/3.75 stars. Good information, and I did take away information that I wasn’t aware of (or have completely forgotten), but it reads a bit dry. I listened to the audiobook and that might not be the correct format for me to personally take this in. Usually I love food books in audio format, but this one was certainly more science forward rather than common vernacular, so probably would have been better to read on paper or ebook than listen to while working out or cooking dinner.

[Steven · Rating 4 out of 5]

Recientemente leí el libro de ‘Hábitos Atómicos’ de James Clear mientras trabajo específicamente con mi sobrealimentación. Haz un hábito fácil, sostenible. Este libro básicamente es biología nutritiva 101, comenzando con el proceso de homeostasis y yendo por encima por muchas otras sustancias, hormonas, etc. Pero lo que se quedó conmigo es que hay personas propensas a la sobrealimentación. Yo, por ejemplo, soy propenso a comer dulces. Si hay un dulce en la casa, inmediatamente necesito comérmelo como si fuera una crisis. Por lo tanto, si esta es la cosa, es mejor no llevar dulces a la casa XD. El libro también va por la genética y aparentemente, no se la veracidad, puedes verificar estas cosas por medio de los test esos como 23&me. Curioso estoy yo de ver donde caigo en la tablita. Básicamente, es un buen libro como para empezar. Busca luego en qué concentrarte y seguramente encontraras un libro en algún lado sobre eso.

[Amy · Rating 2 out of 5]

This has an interesting textbook read although not written by a scientist and you can tell because the helpful insights drive me nuts- similar to supermarket magazine tips. Keep food out of your house, give leftovers to skinny people. It would have been more useful to list foods that provided building blocks for a number of the items. I listened to this and will check a print version for references and studies.

[Lizzie · Rating 2 out of 5]

The psychology elements were interesting but the science wasn't great, exagerating data for the goal of persuasion and then brushing past other elements to allow the readers own interpretation based on the previous interpretation provided by the aithor. There was also a lot of information that has been disproved or has no real base to state that it is correct. A podcast with Alan aragon or layne Norton would provide more reliable information, far less tribalism and more emphasis on nuance.

[Mathew Benham · Rating 4 out of 5]

A 7hr long audio book. When I started this book I didnt fully realize that this was about weight gain or loss. I was hoping for a book more on our gut bacteria and the thousands of rolls it plays in us. I was disappointed in that this book barely covers gut reactions and more focused on the weight topic.

[Reader · Rating 3 out of 5]

The book is full of facts and figures. For someone looking for the technical details of Gut, this is a great book. The details talks about why we experience what we do and what are the chemicals or hormones behind the scenes. Though I wished it has more solutions rather than just the problem statements.

[Roger March · Rating 4 out of 5]

Like others have noticed the narrator is very monotone. But the material was good enough that I felt like it is worth the slog. It is pretty technical and I enjoyed that in contrast to similar books that dumbed the concepts down.

[Cherry · Rating 4 out of 5]

This book had alot of good alot of interesting information & helpful tips, but also alot has gone over my head with all the scientific terminology. This is definitely a book you have to read a few times to take in all the information.

[rebecca · Rating 4 out of 5]

I retained half of the information and now want to study biology, lol. I need to read this again, the science went way over my head, since most of it was new information. Good tips to lose weight though, I have to try some!