The Most Common Diet Mistakes That Sabotage Your Progress in the Gym

Whether your goal is fat loss, muscle gain, or improved athletic performance, there’s one factor that underpins it all, your diet. You can have the best training program in the world, but without the right nutrition strategy to support it, your results will plateau—or worse, reverse.

The good news is many of the common dietary mistakes people make are easy to fix once you're aware of them. We’ll break down the most frequent nutritional errors we see as a coaches and health professionals and what the research says you should do instead.

As you read this list, if you find any of these points resonating with your personal experience and your relationship with food, I would STRONGLY SUGGEST making an appointment with our Dietitian Sophie here . A Dietitian is the single best person to help guide you through what can be a tricky and sometimes even emotional landscape. Use our code STUDIO99 for 50% off your initial appointment.

1. Overestimating How Much You Burn, Underestimating How Much You Eat

One of the most pervasive problems in fitness nutrition is the mismatch between perceived effort and actual intake.

People often think:

“I just smashed a HIIT session—I’ve earned this burger and fries.”

But studies show that most of us overestimate calories burned during exercise and underestimate food intake by as much as 20–50%. (Lichtman et al., 1992)

A tough gym session might burn 300–500 kcal. That’s easily wiped out with one smoothie or a handful of nuts. When trying to lose fat, this mismatch can keep you in a calorie surplus without realizing it.

Fix:Use tracking tools (e.g., MyFitnessPal) or measure portion sizes for a few weeks to recalibrate your understanding. You don’t need to track forever, but you do need to get accurate, especially when progress stalls.

2. Cutting Calories Too Aggressively

Yes, you need a calorie deficit to lose fat, but too large a deficit can backfire.

Research shows that drastic calorie cuts (greater than 30% of maintenance) can lead to:

• Loss of lean muscle mass

• Decreased training performance

• Hormonal imbalances

• Reduced metabolic rate (Trexler et al., 2014)

These effects slow down fat loss and can make long-term weight maintenance harder.

Fix:Aim for a moderate deficit of 15–25% below your maintenance calories. This is enough to promote fat loss while preserving performance and muscle mass. Combine it with resistance training and adequate protein intake.

3. Neglecting Protein Intake

Protein is the cornerstone of any physique or performance goal. It’s essential for:

• Muscle protein synthesis (the process of building muscle)

• Satiety, helping you stay fuller for longer

• Recovery after training

Yet many gym-goers, especially those focused on fat loss, under-eat protein.

The evidence suggests:

• For fat loss and muscle preservation: 1.6–2.2g of protein per kg of body weight per day

• For building muscle: closer to 2.0–2.4g/kg/day may be optimal for trained individuals (Morton et al., 2018)

Fix:Prioritise protein at every meal. Good sources include lean meats, dairy, eggs, tofu, legumes, and protein powders. Distribute it evenly across the day for best results.

4. Relying Too Heavily on Supplements

Supplements are marketed as shortcuts, but most do far less than promised.

Let’s be clear, supplements don’t fix poor nutrition habits.

The most reliable performance or body composition supplements consistently supported by peer-reviewed research are:

• Protein powder (for convenience)

• Creatine monohydrate (for strength, power, and muscle gain, and some neurological effects)

• Caffeine (as a pre-workout ergogenic aid)

• Beta-alanine (for high-intensity endurance performance)

Most fat burners, detox teas, and metabolism boosters are either ineffective or marginal at best.

Fix:Treat supplements as a top-up to an already solid diet, not a foundation. Spend your money on whole foods first.

5. Fearing Carbohydrates

Thanks to the popularity of keto, low-carb, and "clean eating" fads, carbs have gained a reputation as the enemy of leanness.

But carbohydrates are the primary fuel source for high-intensity training. Low-carb diets can:

• Impair training performance

• Reduce muscle glycogen stores

• Increase perceived exertion during workouts

• Make recovery slower (Hawley & Leckey, 2015)

For most people who lift weights or train hard, carbs are your friend.

Fix: Don’t cut carbs unless there’s a specific reason (e.g., medical condition or preference). Instead, time them around your workouts to improve training output and recovery.

6. Chasing “Clean Eating” Over Energy Balance

Many people eat what they perceive as "healthy" or "clean" and get frustrated when the scale doesn't move.

But even nutrient-dense foods like avocado, almond butter, granola, or smoothie bowls are calorie-dense. Fat loss requires a calorie deficit, no matter how healthy your food is.

As researchers like Dr. Eric Helms and Dr. Layne Norton often point out:

“You can overeat on chicken breast and quinoa, just like you can on donuts.”

Fix: Focus on both quality and quantity. Build meals around whole foods, but monitor portions, especially for calorie-dense items like oils, nuts, and dressings.

7. Inconsistent Eating Patterns

Many people are “on track” Monday to Thursday and “wing it” on weekends. This inconsistency can erase a week's worth of progress in 2–3 days.

It’s not uncommon to see someone in a 500 kcal deficit five days a week… then go into a 2000 kcal surplus over the weekend.

Fix: Adopt the “80/20 rule”: stick to your plan 80% of the time, but stay within your calorie needs overall—even when indulging. Plan for meals out and include flexibility in your approach.

8. Ignoring Micronutrients and Fibre

While macros (protein, carbs, fats) often take center stage, micronutrients and fibre are crucial for:

• Energy metabolism

• Immune function

• Hormone regulation

• Gut health

Highly processed diets or extreme low-calorie diets can lead to deficiencies, which may manifest as fatigue, poor recovery, or even injury.

Fix: Aim for a wide variety of colourful vegetables, fruits, whole grains, nuts, and seeds. Most adults should consume at least 25–30g of fibre per day, according to NHMRC guidelines.

9. Drinking Your Calories

Liquid calories, such as fruit juice, soft drinks, smoothies, and alcohol often don’t register with our appetite signals. You can consume hundreds of calories without feeling full.

Even "healthy" beverages like green juices or coconut water can be sneaky contributors to excess energy intake.

Alcohol adds another layer, reducing recovery, impairing sleep, and decreasing testosterone levels, all while providing 7 kcal per gram.

Fix: Prioritise water, black coffee, tea, and low/zero-calorie drinks. Save high-calorie drinks for intentional indulgences, not daily habits.

10. Trying to Change Too Much, Too Fast

All-or-nothing approaches rarely last. Whether it’s cutting out entire food groups or jumping on the latest cleanse, overly restrictive diets set you up for burnout and rebound weight gain.

Behavioural science shows that sustainable change happens through small, consistent actions, not massive overhauls.

Fix: Pick 1–2 habits to change at a time. For example:

• Add protein to breakfast

• Swap soft drinks for water

• Include a vegetable at each main meal

Once those become second nature, layer on the next habit.

Takeaway: Precision Beats Perfection

At the end of the day, optimal nutrition for body composition and performance isn’t about perfection—it’s about consistency, self-awareness, and making evidence-based choices.

To recap, here are the top diet mistakes to avoid:

1. Overestimating energy burn, underestimating intake

2. Cutting calories too much

3. Neglecting protein

4. Relying on supplements

5. Fearing carbs

6. Focusing only on “clean eating” instead of energy balance

7. Weekend inconsistency

8. Ignoring micronutrients and fibre

9. Drinking your calories

10. Trying to overhaul everything at once

By focusing on the fundamentals and adjusting as you learn more about your body, you’ll build a nutrition plan that supports your goals and your lifestyle.

Again, BOOK IN TO SEE A DIETITIAN. Just like a trainer takes away the guess work in the gym, a Dietitian gives you reassurance on your fork.

References:

• Lichtman, S. W., et al. (1992). Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. N Engl J Med, 327(27), 1893–1898.

• Morton, R. W., et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains. Br J Sports Med, 52(6), 376–384.

• Trexler, E. T., Smith-Ryan, A. E., & Norton, L. E. (2014). Metabolic adaptation to weight loss: implications for the athlete. J Int Soc Sports Nutr, 11(1), 7.

• Hawley, J. A., & Leckey, J. J. (2015). Carbohydrate dependence during prolonged, intense endurance exercise. Sports Med, 45(Suppl 1), 5–12.