Over the past few decades, there has been an abundance of research examining the consequences that food choices can have on human health. The preponderance of scientific evidence has established that the more a diet gravitates toward plant-sourced foods, the greater the beneficial health effects will be. These include lowering mortality from heart disease; reversing advanced coronary artery disease; lowering the risk of most chronic diseases including cardiovascular disease, type-2 diabetes and cancer; and decreasing the incidence and severity of high-risk conditions such as obesity, hypertension, hyperlipidemia and hyperglycemia; reducing medication needs; and supporting sustainable weight management. If you are interested in more detail on these benefits, check out a small sampling of this science in the sources for this article (1,2,3,4,5,6,7).
However, the majority of investigations to date only have the power to show links between diet and health, and, strong as these associations are, they cannot prove cause and effect. Moreover, there are problems in this area of study with the definitions of a “Mediterranean diet” and a “low-fat vegan diet”: there is no “standard Mediterranean diet” nor is there a level of fat in a vegan diet generally recognized as “low-fat”.
The Study (8)
A recent study, published in February 2021, was designed to overcome such scientific confusion (8). It is a randomized, cross-over study which has the strong statistical power necessary to show cause and effect. Another advantage of this type of study is that the participants act as their own controls because they receive both dietary interventions over the course of the study. In so doing, the amount of confounding* in the investigation is decreased.
* Confounding occurs when extra variables that are related to the outcome of interest, but not in a causal way, must be accounted for in the results of the study. A confounding variable might suggest an association where none exists or mask a true association. Some examples of confounding variables are the BMI of participants; age; ethnicity; alcohol intake; healthy lifestyle habits such as physical exercise; and unhealthy lifestyle habits such as smoking. (9,10)
Because a cross-over trial removes the variability between participants, fewer participants are required to come to a clear conclusion. But the researchers involved in this study also acknowledged that the cross-over study design requires careful analysis to make sure that the physical effects of the first diet period does not unduly influence the second diet experience. Moreover, they noted that it was essential in this study to adjust for any changes in body weight and other variables that occurred during the course of the study.
To further ensure that the study results accurately reflected the outcomes of ingesting a truly low-fat vegan diet and a healthy Mediterranean diet, the investigators were very careful to specifically define to study participants the foods that were to be consumed and those to be avoided.
Study Method (8)
62 overweight participants, with BMIs of 28 to 40, were involved in this study. Excluded were those with type 1 diabetes; smokers; those who abused alcohol or drugs; those pregnant or lactating; and those currently eating a vegan or Mediterranean diet. Four participants dropped out during a vegan phase and six dropped out during a Mediterranean phase, mostly for reasons unrelated to these trials, leaving 52 who completed the study.
Several measurements were recorded for each subject. These included body weight, blood lipids, blood pressure, body composition, insulin resistance, oral glucose insulin sensitivity and predicted insulin sensitivity. All measurements were performed, after a 10 hour overnight fast, at baseline (before the study commenced) and during week 16.
Participants were randomly assigned in a 1:1 ratio to either a Mediterranean diet or a low-fat vegan diet for 16 weeks. They returned to their normal diet for 4 weeks and then began the opposite diet for a further 16 weeks. No meals were provided to participants.
Participants attended weekly classes while they were on each diet. The classes covered food preparation, how to maintain the diet while travelling or dining out and other health topics. Nutritional experts with expertise in the diet they were overseeing, including registered dieticians, physicians and other study personnel, managed the implementation and on-going adherence to each diet.
Participants kept a 3-day dietary record for each diet at the beginning of each trial and during the last week. These were analyzed by Registered Dieticians or other staff members certified in Nutrition Data System for Research, Version 2018 (Nutrition Coordinating Center, University of Minnesota, Minneapolis, MN).
Physical activity was assessed through the International Physical Activity Questionnaire.
The Mediterranean Diet
The Mediterranean diet followed the one used in the PREDIMED study (11).
Participants were asked to consume the following foods;
- At least 2 daily servings of vegetables
- At least 2 to 3 daily servings of fresh fruits
- At least 3 weekly servings of legumes
- At least 3 weekly servings of fish or shellfish
- At least 3 weekly servings of nuts and seeds
- When eating meat, they were asked to choose white meats (with visible fat removed) instead of red meats.
- When using oil or fat in food preparation, participants were asked to choose extra virgin olive oil and requested to aim for an intake of 50 gm per day.
- Nuts, eggs, fish and shellfish, low-fat cheese, chocolate (≥50% cocoa) and whole-grain cereals could be consumed ad libitum (ie: eaten as much as participants wanted).
Participants were asked to eliminate;
- Cream, butter and margarine
- Processed meats
- Sweetened beverages
- Processed snacks
- Cured ham, red meat and fatty cheeses were limited to 1 or fewer servings a week
The Low-Fat Vegan Diet
The aim of the low-fat vegan diet was to derive 75% of its energy from carbohydrates, 15% from protein and 10% from fat. Participants were asked to eat only vegetables, grains, legumes and fruit and avoid animal products and added fat. Vitamin B12 supplements of 500 mcg per day were provided during this phase of the study.
For Both Diets
Participants were not restricted in the quantities they ate of the foods allowed on the diet they were following.
Alcoholic beverages were limited to 1 per day for women and 2 per day for men.
All participants were asked that they not alter their normal exercise habits and to continue any current medications for the duration of the study (unless changes were initiated by their personal physicians).
Study Results (8)
Adoption of and adherence to both diets was high.
These trials were performed in at-home settings with meals prepared by the participants, so their results relate directly to people making their own food choices.
Positive outcomes from both diets were greater during the first study period than during the second study period, reflecting the changes that occurred during consumption of the first diet and the washout period. A test for the carryover effect was not statistically significant, indicating that results from both treatments were statistically valid outcomes.
Comparing the Results of the Two Diets:
| FACTOR ||MEDITERRANEAN DIET||VEGAN DIET|
|Total energy intake||No change|| Energy intake decreased by 500 kcal/day
This effect was likely due to the satiety effect
of high fiber intake.
|Percentage of energy from fat|| Increased|
This effect was due mainly to increased consumption of monounsaturated fat in the Mediterranean diet.
|Percentage of energy from carbohydrate||Decreased||Increased|
|Change in fiber intake||Increased by 16%||Increased by 50%|
|Types of foods consumed||73% of participants consumed 2 or more servings of vegetables daily.|
94% of participants ingested olive oil as their main dietary fat
|81% of participants avoided all animal products
94% of participants consumed less than 50 mg of cholesterol daily
86% of participants derived less than 25% of their energy from fat and less than 5% from saturated fat
|Overall body weight changes||No change in body weight||Average body weight decreased by 13 pounds
This change was attributed to an average loss of 7.5 pounds of fat mass and 315 cc of visceral fat (fat in the abdominal area)
(Considering only those participants who had no changes made in their lipid medications)
|Total cholesterol decreased by 0.08 mmol/L|
LDL - cholesterol decreased by 0.01 mmol/L
|Total cholesterol decreased by 0.5 mmol/L
LDL - cholesterol decreased by 0.4 mmol/L
|No significant change in insulin sensitivity or insulin resistance||Insulin sensitivity increased
Insulin resistance decreased
(Considering only those participants who had no changes made in their blood pressure medications)
|Systolic blood pressure dropped by 9.3mmHg|
Diastolic blood pressure dropped by 7.3mmHg
|Systolic blood pressure dropped by 3.4mmHg
Diastolic blood pressure dropped by 4.1mmHg
|Medication changes||2 participants reduced or discontinued lipid-lowering meds|
1 participant increased lipid-lowering meds
6 participants reduced or discontinued blood pressure meds
1 participant increased blood pressure meds
|7 participants reduced or discontinued lipid-lowering meds
0 participants increased lipid-lowering meds
7 participants reduced or discontinued blood pressure meds
1 participant increased blood pressure meds
Study Discussion (8)
Out of the 52 study completers, 26 lost weight during the Mediterranean phase and 48 lost weight during the vegan phase.
The low-fat vegan diet resulted in an average overall body weight loss of 13 pounds.
There was no overall weight change from the Mediterranean diet.
Weight loss from a Mediterranean diet might prove to be more favourable if a true traditional Mediterranean diet was used in which only 23% of its consumed energy is derived from fat (12). The PREDIMED Mediterranean diet, with its inclusion of fish and small amounts of other animal products, oil and nuts, has an average fat intake of 42%. It is hypothesized that the high fat content of this Mediterranean diet counteracted any weight loss that might have occurred.
Despite the fact that neither diet was restricted in amounts of food eaten, energy intake was decreased on the low-fat vegan diet by 500 kcal/day but did not change significantly on the Mediterranean diet.
Plant-based diets reduce body weight in two main ways;
- Reducing the energy density of the diet through lowering calorie intake, increasing fiber intake, and decreasing total fat and saturated fat consumption. Though calories are reduced, a vegan diet is more satisfying. The main dynamic here is the large increase in fiber in the vegan diet which fills up the stomach and causes it to send out signals that it is full earlier than in those eating less fiber (13).
- Increasing after meal energy expenditure – The thermic effect of food (the increased energy expenditure that comes from the processing and storage of food in the body) can account for 10% to 16% of total daily energy expenditure (14,15). In addition, the downregulation of mitochondria in skeletal muscles by diets containing high amounts (40% and more) of fat disrupts the digestive process of oxidative phosphorylation (16) and high-fat diets facilitate the passage of bacterial endotoxins from the intestine, through the gut wall into the bloodstream. Endotoxins appear to impair glucose oxidation (17).
Insulin sensitivity was increased and insulin resistance decreased in the vegan diet. High insulin sensitivity and low insulin resistance allow the cells of the body to use blood glucose effectively, reducing blood sugar. The Mediterranean diet resulted in no changes in insulin sensitivity or insulin resistance.
The vegan diet decreased total cholesterol levels by 0.5 mmol/L and LDL-cholesterol levels by 0.4 mmol/L. There were no significant cholesterol changes from the Mediterranean diet.
This difference may be due to the greater reduction in saturated fat through the vegan diet as well as its effective elimination of cholesterol intake.
Blood pressure decreased on both diets, but both systolic and diastolic blood pressure decreased more on the Mediterranean diet (by 5.9 mmHg and 3.2 mm Hg respectively) than on the vegan diet. The larger decrease from the Mediterranean diet may be due to its high content of monounsaturated fat, vitamin E, and the ability of extra virgin olive oil to increase antioxidant capacity (18,19).
Final Takeaways from this study
This well-designed study illustrates that a vegan diet is very effective for weight loss while a Mediterranean diet may result in no overall loss of weight. The vegan diet also caused reductions in fat mass and visceral fat volume as well as better outcomes for insulin sensitivity and cholesterol levels (8).
As the scientists in charge of this study stated, “Mediterranean and vegan diets are variations on a theme: one favors plant-based foods; the other consists of plant products exclusively.” They also noted that previous studies had reported that the only aspect of a Mediterranean diet that was associated with preventing weight gain was the avoidance of meat products (20). As well, they referenced the PREDIMED study in which a sub-analysis revealed that the more participants followed a plant-based diet, the lower their risk of cardiovascular events (21).
A Mediterranean diet includes copious amounts of plant-based foods. Why not take it a few steps further and turn it into a plant-based Mediterranean diet?
Here’s what you can do to optimize the potential health benefits of a Mediterranean diet;
- Increase your intake of fruits, vegetables, legumes and whole grains.
- Avoid all meats. If you feel you simply must eat fish, limit it to one serving a week.
- Eat no eggs or cheese, low-fat or otherwise.
- Use olive oil sparingly, only when you feel an added fat is needed.
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2 Orlich, M.J., Singh, P.N., Sabaté, J., et al. Vegetarian dietary patterns and mortality in Adventist Health Study 2. JAMA Intern Med. 2013 Jul 8; 173(13): 1230–1238. Doi:http://dx.doi.org/10.1001/jamainternmed.2013.6473.
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8 Barnard, N.D., Alwarith, J., Rembert, E., Brandon, L., Nguyen, M., Goergen, A. Horne, T., et al. A Mediterranean Diet and Low-Fat Vegan Diet to Improve Body Weight and Cardiometabolic Risk Factors: A Randomized, Cross-over Trial. Feb 5, 2021. Journal of the American College of Nutrition, DOI: 10.1080/07315724.2020.1869625.
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