Tuna Diet Risks & Weight Loss Benefits (Beware of Mercury)

Weight Loss & Diets | Written by Nathan Petitpas | Updated on 28 March 2023

A can of tuna with a tuna fish and the word diet in the background.

Despite the levels of mercury in tuna, the tuna diet advices participants to only eat tuna for 3 days straight and promises fair results due to the intense calorie restrictions.

Or in other words, the tuna diet is effective but at what cost?

Before you jump ship to a tuna based diet, it’s important to weight the potential risks vs benefits so let’s dive right in. 

The Tuna and Water Diet Explained

The tuna diet is an extreme, short-term weight loss plan created by bodybuilder Dave Draper and consists of only tuna for 3 days. Draper claims is regularly practiced by athletes to gain more muscularization before a competition. 

To be frank, this diet is is a low-calorie, low-carbohydrate, and high-protein diet that should be used with caution. 

Draper states on his website that following the diet will create a ketone-burning system in your body that causes fast weight loss, similar to how ketogenic diets work. Although there has been little research done to study the effects of this diet on the human body, doctors and scientists have been interested in the benefits of tuna meat itself for decades. 

How Tuna Promotes Weight Loss

For years, tuna has enjoyed the status of super health food, with good reason. It is incredibly nutritious and is associated with many health benefits.

  • High in Protein

Tuna is an extremely great source of protein, with a single 3oz serving of tuna canned in water alone having 19.8g of protein already. This is good if you want to lose weight as protein can prolong your body’s energy expenditure, making the body burn more calories even while at rest. [1]

Additionally, a study in 2012 found that weight loss depends on how much protein is consumed, not how low the amount of carbs there is. This means that eating high-protein foods is superior to cutting down on carbs.

Furthermore, a high-protein diet aids in hunger and appetite suppression, leading to fewer calories consumed. [2] Lastly, protein is essential in muscle building, and this aids in weight loss as muscles burn more calories than fat, even while resting. 

  • Low in Calories

Tuna is more than just a high-protein metabolism booster – it’s also low in calories, generally speaking. Depending on how the tuna is packed, a single 3oz serving would have between 90 to 190 calories:

This low-calorie factor can help you remain in a calorie deficit which is the basic tenet of weight loss – when you burn more calories than you consume in a day, you begin to shed excess weight. [3]  

Tuna is Rich in Other Nutrients

Aside from aiding in weight loss, tuna offers other nutrients that deliver immense health benefits. 

First, the muscle tissues of tuna are rich in selenium, an essential antioxidant that can lower your risks of prostate, lung, and colon cancer, immunodeficiency, and heart diseases. Additionally, selenium also assists with healthy thyroid function. [3] Just one 3oz serving of tuna canned in water has 55g of selenium which is already 100% of the recommended dietary allowance. [4] 

Second, tuna is packed with Vitamin B-12, a nutrient that keeps your blood and nerve cells healthy. It helps in the production of red blood cells, and consuming enough Vitamin B-12 is necessary to prevent blood disorders like anemia. [5]

Third, tuna is famously known as a good source of omega-3 fatty acids that are important to our bodies for disease prevention and health promotion. Omega-3s prevent cardiovascular diseases and fight autoimmune diseases. Moreover, omega-3s also play a key role in brain and eye development. [6]

The Tuna Diet Plan

Now that we’ve established how good tuna is for weight loss, lets discuss the diet in detail. Below, we briefly discuss the rules or lay of the land (ocean) for the diet and and what foods are allowed during it. 

  • The First Three Days

The instructions to follow for the first part of the tuna diet are simple; you are only allowed to eat tuna and you must eat a lot of it. The diet requires that you consume 1.5 grams of protein per kilogram of your body weight each day or 3.3 grams of protein per pound that you weigh. 

In comparison, the recommended protein intake for healthy adults on a regular diet is 0.8 grams per kilogram or 0.36 grams per pound. [7]

Second, besides eating more than your body weight in tuna, Draper also emphasizes the importance of drinking a lot of water. He instructs that you should drink at least 2.4 liters each day that you’re on a diet. 

Third, you have to drink your vitamins and minerals twice a day, your branch-chain amino acid supplements before your workout and immediately after, and your Metamucil, keto Fiber supplements before sleep. And that’s pretty much it for the first three days in terms of food and caloric intake. However, Draper’s instructions also imply that you should be working out despite the low-calorie nature of the tuna diet.

  • Afterwards

After the first three days, the diet is more lenient and allows you to eat protein-rich chicken, low-fat dairy such as cottage cheese, and leafy green vegetables like those often found in salads. During this time, you are encouraged to exercise using moderate weights. 

Draper doesn’t specify how long you should continue with this restrictive diet, but after reading the risks for yourself, you can make the call on whether to try it at all or how long to keep it up. 

Does the Tuna Diet Work?

So, is all of the tuna you ate worth it? Is the tuna diet good for weight loss? Does it work?

Technically, yes, it does work. You will lose weight, maybe even a lot of it, in just a couple of days. 

But why does it work? Along with the fact that the diet is extremely calorie-restrictive, another reason is that it eliminates most of the carbohydrates from your diet. Our bodies break down the carbohydrates we consume into glucose which is the body’s primary source of energy. By removing the bulk of carbs from your diet, your body will eventually adapt by breaking down fat cells instead of glucose for fuel. [8] 

However, it is vital to include all three macronutrients – fat, protein, and carbohydrates – in your regular diet because each one of them performs functions that human bodies depend on. 

Other Single Food Diets:

Benefits of The Tuna and Water Diet

As discussed above, one benefit of the tuna and water diet is that it is technically effective for losing weight. 

  • Fast Weight Loss from Calorie Restriction

Seeing as tuna is a low-calorie food, this diet will place you in an extreme calorie deficit. One to two pounds of loss per week is considered healthy weight loss, so even a loss of just 2 pounds during those three days would be regarded as a lot. [10]

As such, if you’re looking for a quick fix, then the fast weight loss associated with this diet is a benefit for you.

But is this weight loss necessarily a good thing? We will discuss this in the risks and drawbacks section below. 

  • Nutrient Boost from Tuna

As we have already discussed, tuna is rich in nutrients, specifically in selenium, vitamin B-12, and omega-3 fatty acids. Because most of the nutrients you consume will come from tuna while you’re still on the diet, you get a boost of these nutrients in your system.  

This is beneficial, especially if you usually do not consume the daily recommended amount of selenium, vitamin B-12, and omega-3 fatty acids.   

Risks and Drawbacks

By now, you know that the tuna diet works for weight loss. But before you go and buy a dozen of tuna canned in water to start the diet, we first have to explore the risks and drawbacks associated with a tuna diet. 

  • Slows Metabolism and Weight Regain

You will lose a handful of pounds with the tuna diet, and unfortunately, you will likely regain all of it. During weight loss, your body automatically reacts through a phenomenon called adaptive thermogenesis, or when your resting metabolic rate (RMR) slows down to protect the body from starvation. [11] This means that it is entirely normal for your metabolism to slow down while you’re losing weight.

However, studies have shown that crash diets can slow your metabolism down even after you’ve resumed a regular diet. 

In a study done on participants of The Biggest Loser, researchers found that after their rapid weight loss, the RMR of the participants’ also decreased in proportion. More importantly, they discovered that this slower metabolic rate persisted even after the competition ended six years later. [12] 

This study shows that rapid weight loss is counterproductive if your overall goal is to lose weight and keep it off. After your calorie-restrictive diet, your slower metabolic rate can’t burn off all of the calories you usually consume, and this will eventually lead to you regaining all the weight you lost. 

  • Lacks Essential Nutrients

Even though tuna is nutrient-rich, it is still not enough to provide you with all the nourishment your body needs. 

Tuna lacks many essential nutrients like carbohydrates, fiber, and vitamins A, C, and E, just to name a few. Relying on it alone will increase your risk of nutritional deficiency, leading to health issues such as digestive disorders, immunodeficiency, and chronic disease.[13]

Granted, you likely won’t face these problems since you won’t be on the diet for long. Nevertheless, different tuna diet variations can last for up to 30 days long, and these prolonged versions will definitely bring you closer to nutritional deficiency. 

  • High in Mercury

You also run the risk of mercury poisoning with the tuna diet.

Like many other fish, tuna has a high mercury concentration; in fact, fresh tuna is fifth on the list of fish with high mercury levels. [14] This is very dangerous, seeing as mercury is toxic and exposure to it can cause neurological symptoms such as paresthesia, ataxia, dysarthria, hearing defects, and even death. [15]

Due to the risks of mercury poisoning, the FDA recommends limiting the consumption of light, canned tuna to around two to three servings only per week, approximately 8 to 12 ounces. They also suggest women who are pregnant or breastfeeding eat less than 4 ounces a week. In addition, children under ten should consume no more than 3 ounces per week. 

Doing the math, let’s say a 150-pound person wanted to go on the three-day tuna diet.

He’s required to consume 225 grams of protein each day. If one serving of canned tuna has approximately 20g of protein, he would be eating around 12 servings a day, or 38 servings for the duration of the diet– 35 servings more than recommended. 

Evidently, going on this extreme diet even for the minimum amount of three days increases your risk of mercury poisoning. 


Tuna is a nutrient-rich staple food that can be good for weight loss if used appropriately. It is low in calories and very high in protein. However, even healthy food like tuna must be consumed in moderation and within the recommended 8 to 12 ounces a week.

Although the tuna diet seems like a solid diet choice for quick and effective weight loss at face value, a closer examination shows that this diet has more risks than potential benefits. 

The weight you lost after going on the tuna diet is probably not permanently gone and will likely return in a couple of weeks. And, you can damage your metabolism by making it slower long-term, which will only set you back in your weight loss journey.

Lastly, eating only tuna for three days can cause serious medical issues due to tuna’s high mercury content. And if you’re wondering if shrimp is good for weight loss, similar rules about moderating you intake apply across the board for most types of fish. 

Almost every person who has wanted to lose weight has tried to do it quickly, in one way or another. Nevertheless, even if you’re in a time crunch and need to lose weight fast, the tuna diet isn’t a great idea. A healthy weight loss of one to two pounds per week is optimal and is achievable through a balanced diet, some exercise, and a little persistence.


[1] Whitehead, J. M., McNeill, G., & Smith, J. S. (1996). The effect of protein intake on 24-h energy expenditure during energy restriction. International journal of obesity and related metabolic disorders: journal of the International Association for the Study of Obesity20(8), 727–732. Web. https://pubmed.ncbi.nlm.nih.gov/8856395/ 

[2] Veldhorst, M. A., Westerterp, K. R., van Vught, A. J., & Westerterp-Plantenga, M. S. (2010). Presence or absence of carbohydrates and the proportion of fat in a high-protein diet affect appetite suppression but not energy expenditure in normal-weight human subjects fed in energy balance. The British journal of nutrition104(9), 1395–1405. https://doi.org/10.1017/S0007114510002060

[3] Howell, S., Kones, R. (2017). “Calories in, calories out” and macronutrient intake: the hope, hype, and science of calories. American Journal of Physiology-Endocrinology and Metabolism 2017, 313:5, E608-E612. https://doi.org/10.1152/ajpendo.00156.2017  

[4] Yamashita, Y., Yabu, T., & Yamashita, M. (2010). Discovery of the strong antioxidant selenoneine in tuna and selenium redox metabolism. World journal of biological chemistry1(5), 144–150. https://doi.org/10.4331/wjbc.v1.i5.144 

[5] Selenium. The Nutrition Source. (2021, October 19). Retrieved November 15, 2021, from https://www.hsph.harvard.edu/nutritionsource/selenium/

[6] Vitamin B-12: A Fact Sheet for Consumers. National Institutes of Health, Office of Dietary Supplements. Retrieved November 15, 2021, from https://ods.od.nih.gov/factsheets/VitaminB12-Consumer/ 

[7] Calder, P. C., & Yaqoob, P. (2009). Omega-3 polyunsaturated fatty acids and human health outcomes. BioFactors (Oxford, England)35(3), 266–272. https://doi.org/10.1002/biof.42

[8] Institute of Medicine. (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. https://doi.org/10.17226/10490 

[9] Dowis, K., & Banga, S. (2021). The Potential Health Benefits of the Ketogenic Diet: A Narrative Review. Nutrients13(5), 1654. https://doi.org/10.3390/nu13051654 

[10] What is healthy weight loss? Centers for Disease Control and Prevention. Retrieved November 15, 2021, from ​​https://www.cdc.gov/healthyweight/losing_weight/index.html 

[11] Johannsen, D., Knuth, N., Huizenga, R., Rood, J., Ravussin, E., Hall, K. (2012). Metabolic Slowing with Massive Weight Loss despite Preservation of Fat-Free Mass, The Journal of Clinical Endocrinology & Metabolism, Volume 97, Issue 7, 1 July 2012, Pages 2489–2496, https://doi.org/10.1210/jc.2012-1444 

[12] Fothergill, E., Guo, J., Howard, L., Kerns, J. C., Knuth, N. D., Brychta, R., Chen, K. Y., Skarulis, M. C., Walter, M., Walter, P. J., & Hall, K. D. (2016). Persistent metabolic adaptation 6 years after “The Biggest Loser” competition. Obesity (Silver Spring, Md.)24(8), 1612–1619. https://doi.org/10.1002/oby.21538

[13] Institute of Medicine (US) Committee on Military Nutrition Research. Military Strategies for Sustainment of Nutrition and Immune Function in the Field. Washington (DC): National Academies Press (US); 1999. 7, Nutrition and Immune Responses: What Do We Know? Available from: https://www.ncbi.nlm.nih.gov/books/NBK230970/?report=classic

[14] Mercury Levels in Commercial Fish and Shellfish (1990-2012). U.S. Food & Drug Administration. Retrieved November 15, 2021, from ​​https://www.fda.gov/food/metals-and-your-food/mercury-levels-commercial-fish-and-shellfish-1990-2012 

[15] Carrington, C. D., & Bolger, M. P. (2002). An exposure assessment for methylmercury from seafood for consumers in the United States. Risk analysis: an official publication of the Society for Risk Analysis22(4), 689–699. https://doi.org/10.1111/0272-4332.00061 

About the Author

Nathan Petitpas

Nathan has been a fitness enthusiast for the past 12 years and jumps between several types of training such as bodybuilding, powerlifting, cycling, gymnastics, and backcountry hiking. Due to the varying caloric needs of numerous sports, he has cycled between all types of diets and currently eats a whole food diet. In addition, Nathan lives with several injuries such as hip impingement, spondylolisthesis, and scoliosis, so he underwent self-rehabilitation and no longer lives with debilitating pain.