Introduction

Generally speaking, I am a healthy person in his 50s:

• Whlie I may indulge in a coffee with milk and a good croissant now and then, I don't overdo it. My diet could be categorized as a Mediterranean diet.
• I am a computer programmer, which is a sedentary profession. However, try to have physical activity. I do weight lifting, cardio, and Pilates. Additionally, Barcelona is a very walkable city and I love walking. Anything in a 5 km radius I prefer to do by foot.
• My medical tests are generally normal. In particular, I know I don't have insulin resistance.
• I sleep well.
• Drink water regularly.

So, I check a lot of boxes, but have always had a tendency to be overweight, and I don't know why or what exactly am I doing wrong.

I have read the books, I have watched the channels, and have gone to a couple of nutrionists. It just does not work for me, for some reason.

It does not help that there are myths, contradictory information, biases towards certain diets or fasting routines or whatever. It does not help that the body is a complex system, and that it is hard for a non-specialist like me to digest all the material and come with a well-informed plan.

Yes, sometimes I have been able to lose weight. Like, doing intermittent fasting and increasing exercise. But, I am doing it well? Why do I plateau? Is my metabolism slowing down? Is it just water? Am I losing muscle? Is it sustainable? Which are the consequences in my body?

And why do I end back gaining weight again?

I need to understand how this works.

The GPT

So, at the start of last December, I decided to try something new: I defined a GPT for this. This is the profile (translation):

Your name is Ana. You are a specialist in endocrinology, nutrition, and sports/physical activity—top-notch, highly skilled, and educated at Harvard. You are also trained in related fields, giving you a vast and solid understanding of how the human body works.

You are a professional with scientific rigor who recognizes the false myths out there and disregards them. At the same time, you are kind, positive, and empathetic. Your advice has scientific rigor but also a great deal of common sense, balance, and a holistic view of the person.

Setup

Once I defined the specialized GPT, we start a conversation that will last.

First thing in the chat is to introduce myself with as many relevant details as possible.

Then I describe our goal as helping me lose weight in a healthy and sustainable way, no quick wins, and building a mental model of how this works along the way.

I remember it was encouraging that at the beginning she said something like:

Your health is overall good, this is probably going to be easy, maybe we will only need to tweak this or that, we'll learn about it.

I'll methodically register food, physical activity, and water, and she will evaluate the data, identify patterns, and keep track of my macros and energy balance. She is expected to proactively advice me on anything she considers relevant.

I also define here 20 or so aliases that will ease data entry, more on that below.

Tools

Smart scale

I purchased a high-quality smart scale, an InBody Dial H30.

These scales are not exact measuring body fat etc. If you seacrh in YouTube, there is a bunch of videos comparing DEXA scans with InBody scans with dometics scales. Not even DEXA scans are exact.

And these tests depend on the moment of the day, water, and ... at the end of they day, a domestic scale is trying to extrapolate how a bunch of currents go through your body. So that is what it is. What can I do? Get one of the best in the market for domestic use, and stick with it.

The most important thing is consistency and looking for trends. Weight alone is insufficient, you need to see if more or less you seem to be preserving muscle.

Do your best with the information that you got. And, hopefully, the % of fat will fluctuate, but after 6 months, maybe the fluctuation is near a different set point.

Kitchen scale

I need to weight food, got this model from Mastrad.

I had one with a dynamo before, but I am using this scale all day, this model is fast, has a battery, and the light allows me to read the weight in different light conditions.

Steps counter

I don't wear a smartwatch, but all I need is a steps counter. Google Fit is simple and enough.

Data entry

Scale data

Every day, first thing in the morning after going to the bathroom, I use the scale and register the data:

Good morning Ana! Today is Sunday 26 of January of 2025:

Weight ... kg
SMM ... kg
PBF ... %
MBR ... kcal
SLM ... kg

She responds stuff like:

You gained here and there, but this is not fat, the data says you are doing well, this is most likely influenced by water.

or

You are going through a stable period, the program is working, good job! 💪🎉

(Yes, Ana uses emojis.)

I am making this up, real answers are more complete and nuanced and take intro account what I reported yesterday.

Food

This is one of the best things about the GPT, data entry is extremely easy. I can say stuff like

15:00 Lunch

75 g of fresh spinachs with 6 g of EVOO
225 g of grilled chicken
1 Greek yogurt La Fageda
10 g of chocolate
1 macchiato

and the GPT automatically computes all the macros and calories in that food. This is awesome and a key part of this approach. Then, Ana tells me how I am doing that day so far.

If I am at home, I systematically enter that data after every meal or snack. If I am eating out, I describe the food, upload a picture, and Ana does her best.

Physical activity

As per physical activity, I register my training sessions and steps count, which are needed to estimate the calories you have burned.

From food intake and physical activity plus MBR and some correcting factors, Ana is able to compute an approximate energy balance.

Estimations all over the place, deal with it

As we see, it is all estimations.

For now, we do not have devices that enter your body cells and your food and report your exact body composition, exact energy expenditure, exact food composition, and so on.

So, you have to accept the inherent inexactitude of the data that is the basis of your actions and, again, do your best with the information that you have.

Aliases

I define a couple dozen of aliases to ease data entry.

For example, in the example above I say "chocolate", but what chocolate? In my setup phase I define "chocolate" to be specifically "Lindt 90%", unless I say otherwise. Folks, Lindt 90% is delicious and couldn't go better followed by coffee.

I also define the weight of the plates I use, because sometimes you cannot weight the food. So, I say:

786 g of grilled salmon served in plate A

and the GPT automatically subtracts the weight of plate A.

Adjusting dinner

I eat every day what I fancy, in moderation, and also that depends on what we cook for the family.

And then, we normally adjust for dinner. Like,

Ana, could you help me plan the dinner?

She may reply something like

Today you are a bit short on proteins, the rest is OK. Here are three options, depending on how hungry you feel, all of them would help you stay in an ideal moderate caloric deficit.

and perhaps we have a conversation about tweaking those options.

Building the mental model

Having a 24/7 nutriotinst at the tip of your fingers is priceless.

"Ana, you insist so much in this range of proteins, why is that so important?" (The aminoacids proteins provide are essential for body procedures and in particular for preserving muscle. Research suggests these many in your case.)

At the beginning, then I went and double-checked the recommendations in what seemed to be reputable sources. And they matched. That confirmed the setup.

"Ana, are the 70 g of white bread I just ate in this lunch a bad thing?" (Absolutely not, taking into account how balanced your diet is, those carbs provide energy and fit in the overall plan.)

I love bread, and fortunately in my city there are great healthy options.

"Ana, occasionally I don't feel like eating enough food to supply the necessary protein intake, are there alternatives that may help?" (Ana recommended some lean protein yogurts and shakes, as well as the standards they had to meet if I wanted to look for other options.)

"Ana, you say that my numbers today are reflecting water retention, but you recommend that I drink 2–3 liters of water? To compensate for retention, I need to add more of it? How does that make sense?" (Explanation about fluids in the body, water generation in some reactions, etc.)

This is the other key element in the plan. No way I can do that with a nutritionist I see twice a month for ten minutes. I have flooded Ana with questions.

I have had several threads, actually

My chats last on average a couple of weeks of intense conversation. Then, they may degrade a bit, like Ana forgetting something. When that happens, I start a new chat with the same GPT.

I don't have nice charts and tables that display my progress since I started (though my InBody app has some of that). But I don't need it, because I am building a system, not pursuing a short-term goal.

The outcome

At the end of each day, my macros are balanced, my water is in good levels, and my energy balance is moderately negative. And I have learned that is the trick in my particular case, measure, keep doing that consistently, and give time for the body to adapt.

I have been following this for about two months:

1. Have lost 4 kg for now
2. Tightened my belt by one notch
3. Eaten plenty of food
4. Preserved muscle (in fact, I have more tone)
5. Have a better understanding of how this works

I am so happy about this system, that I don't have right now a plan to stop using the scale and these chats. It is moslty effortless and the outcome is like nothing I have had before on this topic.