Before you begin

A few quick notes will make these guides easier to use.

• Use a desktop or laptop when possible: These guides often link out to calculators and supporting references. A larger screen makes it much easier to move between tabs and compare information.
• Use the linked tools instead of doing manual math: Interactive calculators are linked wherever they can save time. The equations are sometimes shown for reference, but you usually do not need to calculate them yourself.
• Treat the guide as a framework, not a rulebook: Use the guidance as a strong starting point, then adjust based on your body, recovery, and training response instead of following it rigidly.

1. Understanding Underweight

AI Fitness Advice (ChatGPT, Gemini, Claude)

AI models are trained on large amounts of online content, including forums like Reddit, where misinformation, bro-science, and repeated myths are common. Because of this, AI-generated workout programs and diet advice can often sound convincing while still being flawed or overly generic.

What is interesting is that AI’s biggest successes so far have mostly come from domains that are highly verifiable. In software engineering and mathematics, outputs can usually be checked quickly and cheaply: code either compiles, passes tests, or fails; a math solution can be verified directly. The feedback loop is immediate.

Fitness and nutrition are very different. There is rarely a fast or objective validation system. If an AI gives poor training advice, the consequences may only become visible after weeks or months, and even then the cause is difficult to isolate because progress depends on many interacting variables such as genetics, adherence, recovery, sleep, stress, exercise execution, and nutrition.

As a result, AI currently tends to perform best in fitness when guided by someone experienced enough to critically evaluate and iteratively refine its output. Unlike programming, there is no equivalent of a compiler or automated test suite that instantly flags a bad hypertrophy program or ineffective dieting strategy.

Why trust these guides? Where is the proof that they work? Who can I contact?

Nobody has to "trust" these guides blindly. If you find any inaccuracies or issues, simply email [email protected] and we'll look into it.

Feedback from Reddit users: https://www.reddit.com/r/tirzepatidecompound/comments/1omfgxd/everyone_should_read_this_guide_on_losing_fat/

Results found in the wild (user report, see screenshot): https://www.reddit.com/r/AskFitnessIndia/comments/1tgjvvo/how_to_backup_or_move_macrodex_data_locally_or_on/

We have linked evidence wherever possible, and users are encouraged to independently research the concepts presented in the guide.

Why read this Guide?

An average beginner spends 1–3 years not knowing how to diet properly. Most newbies focus too much on “optimizing” their workout plan when they could simply follow any proven beginner program. Workouts are much simpler than diet, all a newbie has to do is perform 10 sets per muscle group per week with high enough intensity with progressive overload to see results.

Without proper diet, at first, they still make progress because of newbie gains, but eventually their results plateau. In many cases, they may even build a decent amount of muscle simply by intuitively eating within the rough calorie/macro range needed for growth. However, this often leaves that muscle hidden behind a layer of body fat. So, the mirror ends up disappointing them.

This guide can save you 1–5 years of wasted time. After reading it, you’ll understand how to diet like professionals — including IFBB pros, Hollywood actors, models, and athletes.

Let's make a rule.

Nobody gets to say “my diet is fine” until they can define it like this:

“My maintenance calories are X. My goal is Y, so I’m running a deficit/surplus of Z. My target intake is A calories with B grams of protein, C grams of carbs, and D grams of fat.”

If someone cannot define their diet like this, then in most cases their idea of an “okay diet” is simply eating homemade/whole food without any real understanding of calories or macros.

But if you want consistent, measurable results, you need to know these numbers. This guide will teach you how to calculate and understand all of them.

Ask a celebrity, athlete, or model about their diet, and they can usually tell you immediately how many calories they eat and what their maintenance calories are.

Whether you’re aiming for a lean, athletic physique or the sheer size of a pro bodybuilder, your success ultimately hinges on a few fundamental principles. Mastering them is the difference between feeling lost and making consistent, tangible progress. It ensures every ounce of effort you pour into your training actually yields the results you’re after.

It all boils down to deliberately managing three key factors: the fuel you consume (calories and macros), your overall mass (body weight), and what that mass is made of (body fat percentage).

What the Scale Doesn't Tell You

It’s easy to get fixated on the number on the scale, but that figure doesn't tell the whole story. Your body isn't one solid mass; it’s a composition of fat mass and lean mass. Fat mass is exactly what it sounds like, while lean mass is everything else—muscle, bones, water, and organs. Knowing both your body weight and your body fat percentage allows you to understand this ratio, giving you a crystal-clear picture of your starting point and the quality of the progress you're making.

What's the Goal?

Your objective will dictate your entire approach, so it’s crucial to know exactly what you’re aiming for.

• Cutting is all about lowering your body fat while performing minimum required lifting to maintain existing muscle mass
• Body Recomposition is the process of building muscle and losing fat at the same time.
• A Lean Bulk focuses on maximizing muscle gain while accepting a minimal, controlled increase in body fat.
• An Underweight Bulk is a more aggressive approach for those who need to gain weight to reach a healthy range.

Manage expecations

Many newbies fail because they aim for multiple things in the same phase, while pros prioritize specific goals in specific phases.

In a weight-loss phase (cut), you should not expect to build much muscle. Yes, it's possible to gain muscle in a deficit, but the process is slow and difficult. It's like swimming against the current — possible, but requiring much more effort.

The rate of muscle gain drastically drops when the deficit exceeds 400 kcal, so why focus heavily on building muscle during a cut?

Muscle gain is maximized in a surplus.

A much better approach during a cut is to perform 1–1.5 hours of Zone 2 cardio daily. Vary the activity — fast walking, cycling, swimming — to avoid overuse injuries. Lift only 1-2x a week to maintain muscles with ease.

Once you cut down to a lean stage, transition into a lean bulk. Increase workout frequency to 3–4x/week and reduce cardio to around 30 minutes. Now you are gaining muscle faster due to the calorie surplus from the lean bulk.

Muscle comes easier during a lean bulk because of the calorie surplus.

Fat loss is relatively easy through hours of low-intensity Zone 2 cardio, but muscle gain is difficult during a cut or weight-loss phase.

Do not fight againist Calorie Balance, you'll burnout. Surplus for muscle gain, deficit for fat loss. Very simple.

Why not just continue eating at maintenance and avoid lean bulk or cut?

A slight surplus maximizes muscle gain. If you try to eat at maintenance, you'll most likely undershoot or overshoot your calories.

A slight surplus ensures that you are in a guaranteed calorie surplus environment where muscles have the best chance to grow.

The weight gained is partly muscle and partly fat. A lean bulk aims to maximize muscle gain while minimizing fat gain.

A cut is needed to get rid of the excess fat gained unintentionally during the lean bulk.

The Problem with Most Body Fat Measurements

Since your body fat percentage is such a critical metric, you need an accurate way to measure it. The truth is, most methods are wildly inconsistent.

First, you need an accurate body fat measurement.

The easiest way is to find a place with a Multi Segment Body Composition Analysis machine, like an InBody 970, 700,580, 380s, 270 or 260. It's accessible, cheaper than a DEXA scan, and accurate enough for tracking.

Don't worry it doesn't cost much (few dollars), many gyms offer it for free. Make sure it's a professional grade inbody machine, not consumer grade BCA anaylzers which tend to be highly inaccurate. (unfortunately some gyms install them)

A word of warning: don't trust consumer grade smartwatches and scales. While they're convenient, their body fat estimations are often unreliable and can lead you to make the wrong decisions. The professional equipment costs thousands of dollars for a reason that's a level of technology you simply won't find in a sub $1000 gadget. Finding a place for a real scan is easier than you think; a quick Google search for "body composition analysis near me" will likely point you to a local gym or clinic that offers InBody scans for a small fee.

For body fat % measurement, in terms of accuracy: MRI > DEXA > Hydrostatic Weighing > Bod Pod > InBody Multi Freq Device (specially the ones which also use Mhz freq, eg, InBody 970, 700) > InBody Multi Freq Device (no Mhz freq, eg, InBody 260,270, 280) > Other Multi-Frequency BIA Devices> Calipers (Skinfold, highly depends on operator skill) > Ultrasound > US Navy Formula > Single-Frequency BIA Devices.

Visual estimates, even by professionals, are significantly less accurate than DEXA scans, no matter what some forum discussions might claim. That said, for contest or photoshoot preparation, visual assessments for conditioning are a practical method. In such cases, it's beneficial to seek guidance from a coach's trained eye for a more reliable evaluation.

Coaches use Caliper and DEXA, onstage bodybuilders are evaulated based conditioning, accurate bodyfat% is not a judging criteria. So, while you'll see coaches telling people "eyes are final judges", nobodys eyes are more accurate than DEXA or MRI.

Back in time when these devices were not widely available, people used a much simpler rule. Lean bulk untill abs disappear, cut till abs become sharp. So, not having access to any of the bodyfat% measuring devices is not a roadblocker.

The "Dieting Tips" Fallacy

Many times you'll hear people say, "Just reduce your portions." The problem is that if you don't track calories, you won't know which portion to reduce.

Imagine your diet would benefit most from reducing excess fat intake, but instead you cut carbohydrates or protein. You may end up hurting your gym performance, recovery, or muscle gain while making little progress toward your goal.

Calorie tracking helps you avoid this problem by showing exactly where your calories are coming from. It allows you to remove the "wrong part" of the diet less often and make more informed adjustments.

A common misconception is that people who track calories are following an overly restrictive diet. In reality, tracking often provides more flexibility because you can fit foods you enjoy into your calorie target while still moving toward your goal.

Another common piece of advice is "cut carbs." Each gram of stored carbohydrate (glycogen) is associated with several grams of water, notice the word "hydrate" in the suffix. When people drastically reduce carbohydrate intake, they often lose a significant amount of water weight within a few days. Beginners see the scale drop rapidly and assume they have lost a large amount of body fat, when much of the initial change is simply water loss.

Without proper measurement, it is easy to mistake water loss for fat loss or make dietary changes that are not actually solving the problem.

Measure → Adjust → Measure → Repeat

Automation

This guide teaches you how to manually calculate and track everything step by step so you fully understand how the system works.

If you want to automate most of the process, you can use MacroCodex — a completely free app built around the methods explained in this guide. It handles nearly all of the repetitive calculations and tracking automatically.

You can still study the guide to understand the reasoning behind everything, but if you:

• do not have time for manual calculations,
• prefer learning by doing,
• or already understand the concepts and want a faster daily workflow,

then MacroCodex can make the process significantly easier and more consistent.

The dashed line represents your maintenance calories (TDEE).

Eat below the green dashed line to lose weight (deficit).

Eat above the green dashed line to gain weight (surplus).

For body recomposition, eat around the green dashed line (maintenance).

MacroCodex app has 12,000+ users already! Get free MacroCodex app here

All you've to do is log your calorie intake (daily) and weight (weekly) in the app

You can setup a goal like weight loss, recomp, cut, lean bulk

As your maintenance calories (TDEE) change, MacroCodex automatically updates your calorie and macro targets making your life easy.

Fueling the Machine: Calories and Macros

Your body's transformation is fueled by what you eat. Calories are simply energy—eat more than you burn, and you’ll gain weight; eat less, and you’ll lose it. But the type of calories you eat determines the quality of that change. These are your macros:

• Protein is the brick and mortar for building muscle. Without enough of it, your body simply can't repair and grow, no matter how hard you train.
• Fats are non-negotiable. Healthy fats are essential for regulating the hormones that drive muscle growth and recovery, not to mention keeping your joints healthy. Skimping on them will sabotage your progress.
• Carbohydrates are your body's go-to energy source, especially for powering you through high-intensity workouts.

Who is Underweight?

You are classified as underweight if your BMI < 18.5. Calculate your BMI here

Long Term Risks of Being Underweight

Maintaining a BMI below 18.5 over the long term can carry several health risks, especially if nutritional intake is inadequate. These risks are well documented in scientific literature:

Bone Health Issues

• Higher risk of osteoporosis and fractures due to reduced bone mineral density.
• Low body fat can disrupt hormones like estrogen, especially in women.
• Source: Feskanich et al., "Calcium, vitamin D, milk consumption, and hip fractures", JAMA, 1997

Mental Health Concerns

• Greater risk of depression, anxiety, and in some cases, cognitive decline.
• Could be linked to nutritional deficits and psychological stress.
• Source: McCrea et al., "Underweight and mental health disorders in the general population", Psychological Medicine, 2012

Weakened Immunity

• Chronic undernutrition impairs immune function, increasing susceptibility to infections.
• Source: Chandra RK, "Nutrition and the immune system", American Journal of Clinical Nutrition, 1996

Muscle Wasting

• Loss of lean muscle mass and reduced strength, especially in aging adults.
• May lead to fatigue, physical weakness, and frailty.

Hormonal and Fertility Issues

• Women may experience amenorrhea (loss of periods), and men may have lower testosterone.
• Both sexes can face fertility challenges.
• Source: Loucks et al., "Energy availability and reproductive function in women", Medicine & Science in Sports & Exercise, 2003

Increased Mortality Risk

• Epidemiological studies show a U shaped curve: both low and high BMI are associated with higher death risk.
• Source: Prospective Studies Collaboration, "Body mass index and cause specific mortality in 900,000 adults", The Lancet, 2009

Rule Out Medical Causes First

Before proceeding, it's important to understand why you may be underweight. In many cases, addressing the root cause can significantly improve your progress.

Some common medical reasons for being underweight include:

• Hyperthyroidism
• Celiac disease or other malabsorption issues
• Chronic infections or inflammatory diseases
• Eating disorders
• Depression or anxiety

If any of these apply to you, consult a healthcare provider before starting this guide. Treating the underlying issue first often leads to faster and more sustainable results.

Who Should Follow This Guide?

This guide is meant only for individuals who are underweight.

Use this guide only if you are underweight. If you are not underweight, refer to this tool instead:

Bulk or Cut Decision tool: Fitness Strategy Planner

Important: Following the underweight guide when you are not actually underweight can lead to excessive fat gain, poor body composition, and slower long term progress. Make sure you’ve correctly identified your current status before proceeding.

2. Calorie Needs and Surplus Calculation

Your BMR (Basal Metabolic Rate) is the amount of energy your body burns just to stay alive at rest. Your TDEE (Total Daily Energy Expenditure) includes your BMR plus all the calories you burn from daily life and exercise. While most online calculators give you an estimate, the only way to find your true TDEE is by tracking your food intake against your weight changes over a few weeks. For a deeper dive, see this accurate TDEE calculator.

After step2, your TDEE will be dead on center, not adjustment needed

TDEE is the maintenance calories, if you eat at matenance your weight will not change if you average it over 3 weeks which should take out water weight fluctations.

If you eat more than maintenance, you gain weight

if you eat less than maintenance, you lose weight.

Weight gain or lost will be some part fat, some muscle, depending on training/diet

Just use this calculator (it's the most accurate): TDEE Calculator

It will give you your Total Daily Energy Expenditure (TDEE).

Select a Surplus

Use Energy Phase Selector to figure out what Surplus % you need.

Info given below is for reference only, you can skip to next step.

Recommended Calorie Surplus by BMI:

Your ideal calorie surplus depends on your BMI and activity level. The Energy Phase Selector will calculate the right surplus based on your height, weight, lean mass, BMI, and activity level.

⚠️ Increase your surplus gradually to avoid digestive or metabolic distress. Don’t jump to your target surplus on day 1. For example, if your target surplus is 30%, start at 10% for a few days, then move to 20%, and finally reach 30% over a couple of weeks.

Write it down: Example: If Alex has a BMI of 16, he should eat at a surplus of 35% or more based on the table above. Now write down your target surplus from the table.

We’ll use this in the next step in the Energy Macro Planner.

3. Macronutrient Setup

If you have a low appetite, try drinking your calories through nutrient-dense shakes. Focus on incorporating healthy fats, which provide 9 kcal per gram—more than double the energy from carbs and proteins, which offer just 4 kcal per gram.

There’s no need to do all this math manually. The Energy Macro Planner will do the heavy lifting for you—just select the right surplus based on the info above, plug it in, and it'll calculate your exact protein, fat, and carb targets. If you're curious about the numbers behind that calculation, the details are below for you to read. Otherwise, feel free to skip ahead to the next step.

Protein

• 1.6-2.2 grams of protein per kilogram of total body weight per day
• This is overidden if protein falls below 15% of total calories consumed. Why? underweight people can end up with very low absolute protein intake, so this floor helps prevent undershooting protein even when body weight is low.
• Most accurate estimate for underweight or recovering individuals
• This is the most direct expression of the broader muscle-gain literature for hypertrophy focused phases.
• Supported by: Morton et al., 2018 (Br J Sports Med); Schoenfeld & Aragon, 2018 (J Int Soc Sports Nutr) - the broader literature supports roughly 1.6-2.2 g/kg/day for maximizing hypertrophy, and this guide keeps that total body weight based range while also using a calorie based floor to avoid undershooting protein in very light individuals.

Fat

• 1.0-1.5 grams of fat per kilogram of total body weight per day
• Provides energy dense calories to support weight gain and hormone function
• Recommended for underweight or recovering individuals to promote hormonal balance and overall health
• Supported by: ESPEN guidelines (2017) and general nutritional recommendations for weight gain emphasize the importance of energy dense foods. Higher fat intake (e.g., 1.0-1.5 g/kg total body weight, falling within broad bulking recommendations like Helms et al., 2019 [0.5-1.5 g/kg]) effectively contributes to the significant caloric surplus needed for overall weight restoration, ensuring essential fatty acid intake and supporting hormonal balance in underweight individuals. This approach prioritizes total mass gain over lean body composition optimization.

Carbohydrates

• Fill the rest of your calories with carbs
• Helps fuel workouts and recovery

4. Meal Plan Generation

Once you’ve got your calories and macros from the Energy Macro Planner, you can use AI to help create a diet plan.

AI often makes mistakes when estimating calories and macros, so always double-check everything using an accurate calorie tracker like Cronometer

See this guide on How to create a meal plan

5. Training Guidelines

Before you start any program

Hypertrophy just means muscle growth. You'll find plenty of programs but first you must learn what really matters in a program. Read Hypertrophy Blueprint, it contains all the knowledge you need to grow muscles. After reading this, you'll be able to make changes to any program.

Are you a beginner?

When you're new to fitness, the best thing you can do is follow a program built by an experienced coach. It's tempting to try and design your own routine, but you're unlikely to come up with something more effective. A solid plan is much more than just a list of exercises; it’s a careful balance of volume, intensity, and smart progression that takes expertise to get right.

If you’re working out from home or have limited equipment, this free beginner program list is an excellent place to start. It’s flexible and offers options based on the gear you have, including a full bodyweight program if you have no equipment at all.

So, what kind of schedule works best for beginners? A full-body routine three times a week or an upper/lower split four times a week are your best bets. As a novice, you get better results by training each muscle group at least twice a week, which helps maximize your body's muscle-building response. This is supported by the study "Effects of resistance training frequency on measures of muscle hypertrophy, a systematic review and meta analysis" by Schoenfeld et al., published in Sports Medicine, 2019. This approach also lets you practice the main lifts more often to build a solid foundation. If you can train four days a week, an upper/lower split is a fantastic choice because it hits every muscle twice while giving you plenty of time to recover and grow.

Ready to hit a fully-equipped gym? Here are a few great starting points:

Beginner Optimal Hypetrophy Program

If you're a late beginner or intermediate, proceed to Intermediate Programs

Resistance Training (Essential)

Underweight individuals should train with a moderate level of effort to build muscle without overtaxing their limited energy reserves. Avoid training to absolute failure or high volume in the beginning, as recovery capacity and nutritional reserves are typically lower.

• Frequency: 3 sessions per week to allow for full-body recovery
• Intensity: Keep 2–3 reps in reserve (RIR) on most sets. That means you stop 2–3 reps before failure

Research shows training 2–3 RIR still results in significant hypertrophy while reducing recovery burden (Schoenfeld, 2016; Helms et al., 2018). This makes it especially suitable for underweight individuals who may have reduced energy availability.

If you do not have access to gym, lift heavy objects for 30-40 minutes, it will boost your appetite and desire to eat.

Zone 2 Cardio (Optional)

• 1-2x/week, 20-30 minutes per session
• Improves cardiovascular fitness, recovery, and metabolic health
• Does not interfere with strength or appetite when kept low in intensity and duration
• Avoid long duration cardio, which can increase energy needs and suppress appetite

You can also sprint for short durations 20-30 minutes which may boost appetite.

For a practical explanation of Zone 2 and how to implement it, see: Zone 2 Cardio

(Sources: Venables et al., 2005; Schumann et al., 2018; Wilson et al., 2012)

6. Troubleshooting

Not gaining weight

• Increase intake by another 100-200 kcal (or raise surplus to +30-35%)
• Add calorie dense, low volume foods like oils, nut butters, trail mix
• Use liquid meals/shakes to reduce appetite burden

Low appetite

• Avoid excess fiber and raw vegetables
• Split food into 4–6 smaller meals/day
• Add calories to meals with toppings (cheese, ghee, nuts)

Poor recovery

• Ensure 7-9 hrs of sleep per night
• Reduce training volume if excessively sore
• Verify protein and fat intake are sufficient

7. Progress Tracking

How do I know if it's working?

See: Progress Tracking

8. When to Stop Following This Guide

You should stop following this guide once your BMI reaches or exceeds 20, which means you are no longer classified as underweight.

What to Do Next:

• Once BMI ≥ 20, do not continue with the same calorie surplus and diet plan, as it may lead to unnecessary fat gain and poor body composition. You can use this calculator to calculate your BMI
• Instead, switch to this guide to decide your next step: Bulk or Cut or Body Recomposition Decision tool: Fitness Strategy Planner
• Most likely you'll switch to Body Recomposition

Warning: Continuing this guide after reaching a healthy BMI can lead to excessive fat gain, which will make future cutting phases longer and harder. Transition appropriately.

9. Where to go from where?

Once you are Normal BMI, follow Body Recomposition guide. This will lean you out slowly, while adding more muscules to your frame. So, you'll look lean and muscular. Once you hit 10% bodyfat, you can always go for Lean Bulk to add more muscles to your frame slowly or just switch to matenance if you are happy with the results.