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What Are the Most Common Dieting Mistakes? Ask a PT

Welcome to the 30th edition of Ask a PT! What an excellent milestone. I’ve thoroughly enjoyed writing these each week. Last week, we talked about preparing for old age. This week, we’re looking at five common dieting mistakes.

Table of Contents

What Are the Most Common Dieting Mistakes?

Question: I’m just about to start a new diet, and wanted to know if there were any common mistakes I should be avoiding?

Answer: There are many mistakes people make with dieting. That’s why long-term diet success is so low. Of those who lose a significant amount of weight through a diet, 80% regain it within 12 months [1].

As a personal trainer, online coach, and fitness writer, I’ve found these five are the most common dieting mistakes. And the key point? Most of them are not the fault of the person dieting. They come from unrealistic expectations, lack of knowledge, and trying to follow a way of eating that isn’t enjoyable or sustainable.

Common Mistake #1: Lack of Measurements

Before starting a diet, take “before” measurements. Record your weight, waist, arms, chest, and leg circumference. Take photos too. These allow you to gauge progress. Many people either overestimate or underestimate how well they are doing. Re-measure each week to track trends over 12–24 weeks. Don’t stress over week-to-week fluctuations. Look at the long-term picture.

Common Mistake #2: Short-Term Thinking

Diets are often framed as 4–8 week programs. But what happens after? Without a plan, you will likely regain weight because you haven’t built long-term habits. Stop chasing quick fixes. Focus on changes you can sustain forever. Instead of cutting all junk food, try limiting it. Swap high-calorie foods for better alternatives. Build a diet that includes the occasional treat while keeping calories under control.

Common Mistake #3: Ignoring Other Areas of Health

You can’t out-diet poor sleep, high stress, and inactivity. If your step count is below 3k, your sleep is bad, and stress is high, you’ll need an extreme calorie deficit to see results. A better approach is to make small changes in several areas: improve sleep quality, exercise regularly, reduce stress, manage calories, and use supplements wisely. Addressing multiple factors will deliver better long-term health and physique improvements.

Best Tips to Improve Your Sleep

Common Mistake #4: Arbitrary Rules

“No food after 6pm.” “No carbs in the morning.” “You must eat grapefruit for breakfast.” These types of black-and-white rules are everywhere in diet culture. Most of them don’t make sense and are impossible to follow long term. A smarter approach is to identify the real issue and fix it logically. For example: “I eat too much between dinner and bedtime. I’ll try eating later and add more protein and fiber to stay fuller.” That’s more effective than banning food after 6pm.

Common Mistake #5: Lack of Protein

A simple but common error. Most people don’t eat enough protein. Increasing protein helps you feel fuller, maintain muscle, and makes dieting more effective.