In this article, we take a look at three popular diets: The 5:2 diet, the Paleo diet and the Dukan diet. These diets all restrict calories in some way and they all work. But are these diets healthy for you or do they result in nutritional deficiencies? In addition, are these diets suitable for a people involved in sports? Lets take a closer look.
5:2 Fast Diet
The 5:2 diet, also known as The Fast Diet, is currently the most popular intermittent fasting diet.
It was popularized by British doctor and journalist Michael Mosley. It’s called the 5:2 diet because five days of the week are normal eating days, while the other two restrict calories to 500–600 per day. This diet is not prescriptive about what food to eat but rather, when to eat those foods.
The Principles Behind the Diet
The 5:2 diet is based on a principle known as intermittent fasting (IF). For five days a week, you eat normally and don’t have to think about restricting calories. On the other 2 days, you reduce calorie intake to 500 calories per day, for women and 600 calories per day for men. You can choose how to order the fasting days as long as there is a non-fasting day in between. Eating normally means eating a healthy, balanced diet and not eating literally anything.
There are very few studies that have tested the 5:2 diet specifically and many more studies on intermittent fasting as a whole. One study showed that the 5:2 diet caused weight loss similar to regular calorie restriction. Additionally, the diet was very effective at reducing insulin levels and improving insulin sensitivity (1).
A similar diet, the 4:3 diet was shown to help reduce insulin resistance, asthma, seasonal allergies, heart arrhythmias and menopausal hot flushes. (2,3). In a randomized controlled trial which included both normal weight and overweight individuals showed major improvements in the group doing 4:3 fasting, compared to the control group that ate normally (4).
After 12 weeks, the fasting group had:
The results of this study show that intermittent fasting has some impressive health benefits as well as the ability to reduce weight.
Research shows that intermittent fasting causes a smaller reduction in muscle mass than weight loss with conventional calorie restriction. A review was conducted to examine the effects of a calorie restriction diet versus intermittent fasting diet on weight loss, fat mass loss and lean mass retention in overweight and obese adults. The results showed that similar weight loss and fat mass loss with 3 to 12 weeks' intermittent fasting and daily calorie restriction. In contrast, less fat free mass was lost in response to intermittent fasting versus daily calorie restriction. These findings suggest that these diets are equally as effective in decreasing body weight and fat mass, although intermittent fasting may be more effective for the retention of lean mass.(5)
The Bottom Line
The 5:2 diet may have several impressive health benefits. These include weight loss, reduced insulin resistance and decreased inflammation. Blood lipids may also be improved. For losing weight, the 5:2 diet appears to be effective when done correctly. Results are dependent on eating a healthy, balanced diet on non-fasting days rather just eating anything. Therefore, it is very important not to compensate for the fasting days by eating much more on the non-fasting days.
The downside of this diet is that there is a drastic calorie restriction on 2 days a week and if these calories are not consumed wisely, nutritional deficiencies could result. In addition, such restricted calorie intake can make you feel dizzy, irritable, give you headaches and make it hard to concentrate, which can affect work and other daily tasks. Other reported side effects are difficulties sleeping and daytime sleepiness, bad breath and dehydration. I can also see that there would be a tendency to compensate and overeat on the non-fasting days.
The 5:2 Diet for the Sports Person
The principles of modulating macro nutrients makes sense, however, this diet takes this to an extreme and in my view, it is not a suitable diet for anyone involved in an intense training routine. The 5:2 diet provides no guidance on macro nutrient balance and with only 500 to 600 calories on fasting days, there is not a lot of room for manoeuvre. Personally, 500 to 600 calories a day would not provide enough energy to support my training and recovery.
The fasting days would not provide enough protein, carbohydrates or micro nutrients to support high intensity training. With such a low level of calories and nutrients on the fasting days, there is a possibility of feeling faint and weak which is clearly not conducent to optimal training. I also see the lack of calories and potential lack of nutrients on the fasting days as a major challenge for sustainability.
The Paleo Diet
The paleo diet emulates the diet of our hunter-gatherer ancestors, based on the premise that they did not suffer from the same diseases as modern humans. In the year 2013, the paleo diet was the world’s most popular diet. However, it is still very controversial among health professionals and mainstream nutrition organizations.
The Principles Behind the Diet
The core of paleo is the diet is that it bans the consumption grains, sugars, and modern vegetable oils in favour of high-quality meat, fish, eggs, and vegetables. Basically, the paleo diet suggests that if it looks like it was made in a factory, don’t eat it!
Several studies suggest that this diet can lead to significant weight loss (without calorie counting) and major improvements in health. In one study 29 men with heart disease and elevated blood sugars or type 2 diabetes, were randomized to either a Palaeolithic diet or a Mediterranean-like diet. Neither group was calorie restricted. This study was conducted over 2 weeks and measured glucose tolerance, insulin levels, weight and waist circumference.
The results show that the group on the paleo diet group saw a significant improvement in glucose tolerance. The glucose tolerance test measures how quickly glucose is cleared from the blood. It is a marker for insulin resistance and diabetes. Those on the Mediterranean Diet did not did not see a significant improvement in glucose tolerance. Both groups lost a significant amount of weight. The paleo group lost 11lbs and the control group lost 8.4 lbs. However, the difference was not statistically significant between groups. There was a statistically significant reduction in waist circumference in the paleo diet group (2.2 inches) compared to the control group (1.1 inches).
Every patient in the paleo group ended up having normal blood sugars, compared to 7 of 15 patients in the control group. The paleo group ended up eating 451 fewer calories per day (1344 compared to 1795) without intentionally restricting calories or portions. In conclusion, a Palaeolithic diet lead to greater improvements in waist circumference and glycemic control, compared to a Mediterranean-like diet.(6)
In another study 13 individuals with type 2 diabetes were placed on either a Palaeolithic diet or a typical Diabetes diet in a cross-over study. They were on each diet for 3 months at a time. On the paleo diet, the participants lost 6.6 lbs more weight and lost 1.6 inches more off of their waistlines, compared to the Diabetes diet. HbA1c was measured, which is a marker for 3-month blood sugar levels. This decreased by 0.4% more on the paleo diet. HDL increased by 3 mg/dL on the paleo diet compared to the Diabetes diet. Triglycerides went down by 35 mg/dL on the paleo diet compared to the Diabetes diet. In conclusion, over a 3-month study period, a Palaeolithic diet improved glycemic control and several cardiovascular risk factors compared to a Diabetes diet in patients with type 2 diabetes. (7)
A study was conducted with 10 healthy women with a BMI over 27, They consumed a modified Palaeolithic diet for 5 weeks and measurements for weight loss, liver fat, muscle cell fat and insulin sensitivity were taken. The results showed that the women lost an average of 9.9 lbs and reduced their waist circumference by 3.1 inches. The fat content of liver and muscle cells are a risk factor for metabolic disease and the results of this study showed a reduction in liver fat of 49%, but no significant effect on the fat content of muscle cells. The women who had a lot of liver fat at the baseline had the most significant decrease. In addition, blood pressure went down from an average of 125/82 mmHg to 115/75 mmHg, although it was only statistically significant for diastolic blood pressure (the lower number). Total cholesterol decreased by 33 mg/dL and Triglycerides went down by 35 mg/dL. In conclusion, during the 5 week trial, the women lost weight and had major reductions in liver fat. They also had improvements in several important health markers.(8)
The studies had statistically significant reductions in waist circumference, which should translate to a reduced risk of diseases like diabetes and cardiovascular disease. The limitation of these studies are that they were short term and based on few participants. Longer term studies are required and a greater number of people.
The Bottom Line
The good thing about the paleo diet is that encourages you to eat less processed food and more fruit and vegetables. This diet is also simple and doesn’t involve counting calories. On the downside, this diet bans the consumption of dairy and wholegrain foods which form part of a healthy, balanced diet. It is a high protein diet with a lot of meat and not a lot of complex carbohydrates which,
in my view for a sports person, is not an optimum diet. In addition, this diet is based on a lot of assumptions made about our ancestor’s diets from the Palaeolithic era and the lack of disease that we see in modern society. I think we need to remember that our Palaeolithic ancestors had a much shorter life expectancy than we enjoy today. Palaeolithic skeletons indicated a life expectancy of 35.4 years for men and 30.0 years for women! Having said that, the research above does show some compelling evidence that the paleo diet has a beneficial effect on several health markers.
The Paleo Diet for the Sports Person
While some of the principles of the paleo make sense for the sports person such as avoiding processed foods, eating more fruit and vegetables etc., my personal view is that there are not enough complex carbohydrates in the diet to provide the necessary energy for intense training. In addition, it seems that there is a potentially a lot of saturated fat in this diet from the consumption of larger amounts of meat.
Once again, calories are not counted in this diet and therefore, you don’t know where you stand with your macro nutrient breakdown, other than you are eating a fair bit of protein and fat. The exclusion of grains and dairy foods groups also limits micro-nutrients from these important food groups that are a good source of calcium and vitamin B complex for example.
Due to the low carbohydrate intake, the body must shift to a process called ketosis. This is a normal metabolic process under certain circumstances. When the body doesn't have enough carbohydrates to use for its energy requirement, it must burn fat for energy. As part of this process, it makes ketones. Ketosis can become dangerous when ketones build up. High levels lead to dehydration and change the chemical balance of your blood. You will also feel pretty lousy during the transition. This has come to be known as ‘Low-Carb Flu’. Some people experience lethargy, fatigue, irritability and shakiness when first eliminating starches, grains and legumes from their diet. The low-carb flu symptoms usually last up to a month and during this time as the body transitions to burning fats as a fuel source instead of carbohydrates. This is not a good scenario for a sports person engaged in intense training and I see this as a major challenge for sustainability. In addition, as the brain also uses glucose as its primary fuel source, depriving oneself of the very foods that provide this energy does not make a lot of sense.
The Dukan Diet
The Dukan Diet was created by Dr. Pierre Dukan, a French general practitioner who specializes in weight management. Dr. Dukan published the book, The Dukan Diet in 2000. This diet has reportedly helped people achieve rapid, easy weight loss without hunger.
The Principles Behind The Diet
The Dukan diet is a low-carb, high-protein diet that is split in to 4 phases. There is no limit to how much you can eat during the plan's four phases, provided you stick to the rules of the plan. The four phases are as follows:
Phase 1 - Attack phase (1-7 days): You start the diet by eating unlimited lean protein plus 1.5 tablespoons of oat bran per day.
Phase 2 - Cruise phase (1-12 months): Alternate lean protein one day with lean protein and non-starchy vegetables the next, plus 2 tablespoons of oat bran every day.
Phase 3 - Consolidation phase (variable): Unlimited lean protein and vegetables, some carbs and fats, one day of lean protein only per week, and 2.5 tablespoons of oat bran per day. You should do this for 5 days for every pound lost in phases 1 and 2.
Phase 4 - Stabilization phase (indefinite): This is the final phase and follows basic Consolidation phase guidelines, but rules can be loosened as long as weight remains stable. Oat bran is increased to 3 tablespoons per day. This phase is all about maintaining the improvements achieved during the earlier phases of the diet. No foods are strictly off-limits.
There is very little research specifically on this diet. However, there is one study on Polish women who followed the Dukan Diet. These women ate about 1,000 calories and 100 grams of protein per day and lost 33 pounds in 8–10 weeks. (9)
The Bottom Line
In summary, the Dukan Diet allows protein-rich foods in Phase 1 and protein with vegetables in Phase 2. It adds limited portions of carbs and fats in Phase 3, with looser guidelines in the final phase. The aim is gradual weight loss of up to 2lb a week and to promote long-term weight management. There's no time limit to the final phase, which involves having a protein-only day once a week and taking regular exercise. The high-protein Dukan Diet can produce fast weight loss and losing weight quickly can be motivating. However, it seems a very unbalanced and unhealthy diet, particularly in the early phases. The Dukan Diet is different from many related high-protein diets in that it restricts both carbs and fat. It is a high-protein, low-carb and low-fat diet.
The Dukan Diet for the Sports Person
Once again, this diet is low in complex carbohydrates that would not provide the necessary energy for intense training. Calories are not counted in this diet and therefore, you don’t know where you stand with your macro nutrient breakdown, other than you are eating a lot of protein and low carbs as well as low fat.
As so many food groups are excluded from this diet in the early phases, there is the possibility of nutrient deficiency in the early stages. In fact, the diet recommends taking a multi-vitamin to compensate for this.
In a similar fashion to the Paleo Diet, the body must shift to the process of ketosis to burn fat for its energy requirement. Once again, during this transition, you will feel pretty lousy and feel the effects of the so called ‘Low-Carb Flu’ with lethargy, fatigue and irritability. This is not a good scenario for a sports person engaged in intense training. I see this as a major challenge for sustainability. In addition, as the brain also uses glucose as its primary fuel source, depriving oneself of the very foods that provide this energy does not make a lot of sense. I view this diet as unhealthy and it is certainly not a diet that would support an endurance athlete or indeed someone engaged in an anaerobic sport in their training.
Comparison of The 3 Diets
These diets all restrict calories in some way, otherwise, they wouldn’t work. However, the macro-nutrient composition of the Paleo and Dukan diet are simply not balanced for the sports person and the lack of complex carbs will be a real issue when it comes to energy requirements. The 5:3 diet seems a little better in that it is not restricting a particular macronutrient and therefore, this can be modulated based on your sport. However, my concern on this diet is simply the lack of calories on the fasting days which will impact training and recovery. Macronutrients can be determined on the fasting days but there is very little room for manoeuvre with only 500-600 calories. In summary, I would not recommend any of these diets for a sports person.
1. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women.
Int J Obes (Lond). 2011 May;35(5):714-27. doi: 10.1038/ijo.2010.171. Epub 2010 Oct 5.
2. The effect on health of alternate day calorie restriction: eating less and more than needed on alternate days prolongs life.
Med Hypotheses. 2006;67(2):209-11. Epub 2006 Mar 10.
3. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma.
Free Radic Biol Med. 2007 Mar 1;42(5):665-74. Epub 2006 Dec 14.
4. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial.
Nutr J. 2013 Nov 12;12(1):146. doi: 10.1186/1475-2891-12-146.
5. Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?
Obes Rev. 2011 Jul;12(7):e593-601. doi: 10.1111/j.1467-789X.2011.00873.x. Epub 2011 Mar 17.
6. A Palaeolithic diet improves glucose tolerance more than a Mediterranean-like diet in individuals with ischaemic heart disease.
Diabetologia. 2007 Sep;50(9):1795-807. Epub 2007 Jun 22.
7. Beneficial effects of a Paleolithic diet on cardiovascular risk factors in type 2 diabetes: a randomized cross-over pilot study.
Cardiovasc Diabetol. 2009 Jul 16;8:35. doi: 10.1186/1475-2840-8-35.
8. A Palaeolithic-type diet causes strong tissue-specific effects on ectopic fat deposition in obese postmenopausal women.
J Intern Med. 2013 Jul;274(1):67-76. doi: 10.1111/joim.12048. Epub 2013 Mar 11.
9. Assessment of food intakes for women adopting the high protein Dukan diet.
Rocz Panstw Zakl Hig. 2015;66(2):137-42.
Sports & fitness nutritionist, researcher and author on a mission to improve the human condition. Focusing on evidence-based and outcome-based nutrition, training, mindset & environment