Blog Diets The 30-Day Tuna Diet: How It Works, Benefits, And Dangers

The 30-Day Tuna Diet: How It Works, Benefits, And Dangers

30 day tuna diet

Many fad diets exist out there, each one making big promises. For seafood lovers, the tuna diet might be the most tempting – lose 11 kgs in one week, eating nothing but tuna for a few days, and then transitioning to healthy foods after. Like most restrictive meal plans, the tuna diet is not without its downsides.


So we’ve weighed the benefits and dangers of this diet to answer your most pressing questions – how much tuna can you eat safely? Will eating a lot of tuna make you lose weight?

Tuna Fish Nutrition

First things first, how nutritious is this fish? Here are some of the benefits of tuna

According to the USDA, 1 can (165g) of light tuna, packed in water (without salt) and drained contains (6): 

  • Calories: 191
  • Fat: 1.4g
  • Sodium: 83mg
  • Carbohydrates: 0g
  • Fiber: 0g
  • Sugar: 0g
  • Protein: 42g


Tuna is not a fatty fish – it’s overall fat content is low. However, the fat it has is the good type – omega-3 fatty acids that have proven health benefits including protecting against heart disease and stroke, lowering triglycerides, and helping treat depression (12). The best part is that these benefits are noted at low levels of omega-3’s – you don’t need to eat a lot of tuna to reap the rewards.

tuna diet

Vitamins And Minerals

Tuna has calcium, phosphorus, potassium, zinc, B-vitamins, selenium, and choline. Each of these nutrients perform important functions in our bodies. However, canned tuna can be high in sodium – when you’re eating too much tuna as part of a diet, you need to watch out for this sodium content. It’s worth comparing food labels at the grocery store and choosing a brand with no added salt, or low sodium content.

Read More: Is Raw Salmon Good For You?


Tuna is very high in complete proteins, that is, it has all of the essential amino acids. These are essential for bodybuilding and muscle maintenance. Protein also keeps you feeling full longer so that it is easier to stick to your food plan (3).


Tuna has no carbs, fiber or sugar. This makes it a great addition to any low-carb weight loss plan. Furthermore, tuna fish calorie counts are low enough to help you maintain a calorie deficit.

See also
Hourglass Figure Diet: What To Eat For A Smaller Waist

tuna diet

How Does The Tuna Diet Work?

The tuna diet is based on the idea that eating only tuna and water can help you lose a lot of weight fast. No other vegetables, fruits or protein sources are allowed during the first couple of days – it’s just fish and water. 

The reason why this ‘diet’ works is because your body relies on stored energy (fat) for most of its fuel to survive. By getting rid of all carb rich foods off limits from your diet, such as breads, pasta and starchy vegetables. You cut out calories that you would normally use up throughout the day. This means that after 3 days without these carbohydrates, your body will start breaking down fat cells for fuel instead of glycogen (stored sugar) (11).

How To Do The Tuna Diet for 30 Days:

Day 1-3 Of Dieting

This is the ‘fish and water’ period. You must only eat tuna, at least 1.5 grams of protein per kilogram of body weight (3.3 grams per lb.) each day (10). The fish must be plain, without any added oil, sauces, or spices. 

You’re also required to drink at least 2 liters of water daily, take Metamucil each night to boost your fiber intake, and take vitamins since the diet isn’t balanced.

The Transition Phase 

After 3 days, you can add more foods to your diet such as leafy green vegetables, steamed non-starchy vegetables, fruit, low fat dairy products, and chicken. 

It’s not clear how many days you should eat protein, fruits, and vegetables before starting the 3-day tuna diet again. You can create a plan based on how much weight loss you’d like to achieve. One advantage of this diet is it maintains a calorie deficit throughout the 30 days, whether you’re in the 3-day tuna diet phase or transition phase.

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tuna diet

Potential Benefits Of The Tuna Diet

In moderation, tuna can be part of a healthy diet. Here are some of the health benefits you can gain from eating tuna

See also
7-Day Weight Gain Diet: Tips and Tricks to Maximize Your Gains

Creating A Calorie Deficit

Tuna’s low fat content makes it an ideal choice for anyone looking to lose weight. A 3 oz serving of tuna has about 31 calories, while a similar portion of chicken breast has 95 calories.

A calorie deficit is essential for weight loss. When you eat less calories than what your body needs, you begin to lose weight (16).

Weekly calorie intake depends on a person’s weight and activity level, but 2000 calories is a safe starting point for most men and 1600 calories is a good amount of calories for women. Using tuna as an alternative to other high-calorie foods will help you stick with the diet while keeping your caloric intake lower than before.

Reduced Cholesterol And Blood Pressure

When consumed in moderation, tuna has no adverse effects on cholesterol levels or blood pressure. In fact, it may even reduce both when included in your diet regularly.

tuna diet

Nutrient Boost

Tuna is high in selenium – a trace mineral important for thyroid health and immune function (4). 

Amino Acids Methyl-containing amino acids such as taurine are found in a significant amount of fish, especially tuna and cod. These amino acids help to support eyesight, heart health, brain function and immune system function. 

Another benefit of the tuna diet is that it may improve your cholesterol levels since lean fish contains high quality protein which helps increase good HDL cholesterol while reducing bad LDL cholesterol (8). 

However, you will not gain these benefits if you eat too much tuna – overloading on vitamin A from tuna can cause serious damage to your liver (14). Also, pregnant women or those planning to become pregnant in the next year should eat no more than 12 ounces of fish and seafood as it can be unhealthy (13).

Read More: 30-Day Diet Plan For Weight Loss: The Ultimate Cheat Sheet For Shedding Those Stubborn Pounds

Dangers Of The Tuna Diet

The risks of this restrictive diet outweigh its benefits.

tuna diet

Huge Weight Loss In A Short Time Period

This diet restricts your calorie intake, which can help improve your overall weight loss results. However, as with other fad diets that restrict calories, this one can also cause muscle loss due to the lack of nutrients needed for healthy cell maintenance and repair. Rapid weight loss can also have a negative effect on your metabolism. When your body loses weight quickly, your metabolism slows down (1). 

See also
16 Green Vegetables to Boost Your Health: A Comprehensive Guide

High Mercury Intake

Tuna contains high levels of mercury, a heavy metal chemical element known to alter brain functioning as well as affect growth and development in infants and young children (5). Mercury from seafood is especially dangerous since it tends to accumulate in larger fish such as tuna.

However, you can limit the amount of mercury you intake by choosing canned light tuna instead of albacore or yellowfin tuna. You should also avoid eating large quantities of any type of tuna because seafood has been linked to increased incidence of bowel cancer.

Nutritional Deficiency

Although this fish is nutritious, it doesn’t have all the nutrients necessary to perform daily activities and keep your body functioning optimally. It doesn’t have carbs, and this can trigger intense cravings that might make it harder to stick to the diet (7). You’ll also be lacking energy for workouts. 

Furthermore, nutritionists recommend eating a variety of foods to keep from missing out on any essential vitamins.

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tuna diet

Healthy Weight Loss Alternatives To The Tuna Diet

To lose weight and stay healthy without sacrificing your muscle tone, aim for slow and steady results. Don’t be discouraged if the number on your scale doesn’t drop as quickly or drastically as you hoped it would. Instead, be happy with the changes you see in the mirror – a tighter waistline and firmer biceps is a sign that you’re on your way to a healthier body. 

Portion Control For Weight Loss

We’ve established that reducing your calorie intake is the only way to lose weight. Instead of going from three meals a day to living off tuna, a more sustainable approach is to cut down on your meal portions (15). Eating only what you need, and stopping when you feel full is a skill that is learned over time. However, it pays off greatly and can help you maintain a healthy weight for a longer time than most fad diets can. 

See also
The Best Homemade 3-Day Juice Cleanse Recipe Guide 

Here are some portion control tips that can come in handy: 

Don’t Complicate It

When you’re starting out, focusing on counting every calorie and weighing all foods can make the process overwhelming. Instead, use your hand to estimate the right portion sizes

Know Your Body Type

What works for someone else in terms of portions might not work for you. If you have a carb-sensitive body type, you’ll reduce your carb portions. If you have a non-sensitive body type with a fast metabolism, you’ll increase your portions accordingly (2).

tuna diet

Master Your Hunger And Fullness Cues

Mastering your hunger cues prevents emotional eating, ensuring you eat only when necessary. Mastering your fullness cues prevents you from overeating. 

Build Muscle For Faster Weight Loss

If you want to lose pounds and keep them off, one important way is to build muscle mass. Muscle burns more calories than fat and so building muscle mass helps you to burn more calories at rest – even when you’re not working out!

Another advantage of muscle building workouts is that they help improve your metabolism for three days after your workout ends (9). 

To build muscles and raise your metabolism, aim for weight training exercise sessions 3-4 times a week. You should also incorporate cardiovascular exercise into your program two or three days a week in the form of 30 minute brisk walks, jogging or cycling at least 5 miles per day. Remember to warm up thoroughly before starting any activity.

The Bottom Line

A three-day tuna diet will not cause you to become deficient in any nutrients because your body stores enough protein and other types of tissue for one week without food. 

The problem arises when you continue this diet longer than the recommended 3 days. Long term ingestion of very low calories can affect your metabolism and make it harder for you to lose weight. Furthermore, you won’t be getting all the nutrients your body needs to function properly. 

See also
The Anti-Diet Culture Guide: How To Heal Your Relationship With Food & Break Free From Dieting Forever

While eating tuna can be healthy in moderation, going on a fish-only diet is extreme. Tuna is not the only option for weight loss and should not be consumed every day. It is best for your health to try other options, such as setting goals to walk more steps each day and consuming fewer calories from fat sources. You can also consult with a registered dietitian who will help you choose foods that fulfill all your daily calorie needs while providing enough nutrients to maintain optimal health.



This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!


  1. Adaptive thermogenesis in humans (2010,
  2. Body Type Guide: How to Eat for Your Specific Body Type (n.d.,
  3. Dietary protein intake and human health – Food & Function (2016,
  4. Discovery of the strong antioxidant selenoneine in tuna and selenium redox metabolism (2010,
  5. Environmental Mercury and Its Toxic Effects (2014,
  6. Fish, tuna, light, canned in water, without salt, drained solids (2019,
  7. Food Cravings and Body Weight: A Conditioning Response (2019,
  8. Higher-Protein Diets Are Associated with Higher HDL Cholesterol and Lower BMI and Waist Circumference in US Adults (2015, academic.oup)
  9. Increasing muscle mass to improve metabolism (2013,
  10. Low Carb Dieting – Tuna and Water Diet (n.d.,
  11. Nutrient Metabolism, Human | Learn Science at Scitable (2020,
  12. Omega-3 Fatty Acids – Health Professional Fact Sheet ( (2021,
  13. Questions & Answers from the FDA/EPA Advice on Eating Fish (n.d.,
  14. Vitamin A – LiverTox – NCBI Bookshelf (2020,
  15. What is the role of portion control in weight management? (2014,
  16. “Calories in, calories out” and macronutrient intake: the hope, hype, and science of calories | American Journal of Physiology-Endocrinology and Metabolism (2017,
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