Which weight loss diets work best?

It seems like everyone has a “magic diet plan” these days. Hollywood is promoting intermittent fasting, CrossFit gyms are pushing ketogenic diets and your friends are all going paleo or vegan.

Do any of these diets work? Let’s see what emerging research shows and what we can learn from each. 

Calories in, calories out  

The theory behind low-calorie diets has to do with how much you eat (calories in) compared to how much energy you use (calories out). All weight loss revolves around this theory, and no diet plan will work if you violate this principle. However, it is important you understand two key ideas before starting any diet plan: the thermic effect of food and how losing weight affects your metabolism.

The thermic effect of food

When you eat food, your body burns calories just to digest and process it. This is called the thermic effect of food. According to a few studies, the thermic effect of protein is 25-35% of calories, but it is only 5-15% for carbohydrates. Fat is about equal to or less than carbs depending on the type.1, 2 This means if you eat 100 calories of protein, your body burns 25-35 calories just to process it. But if you eat 100 calories of carbs, you will burn only 5-15 calories to process it. The end result is you’ll lose more weight when eating a higher protein diet consisting of the same total calorie levels as a higher carb or higher fat diet.


Weight loss slows your metabolism

The human body is great at keeping body weight consistent. Researchers originally expected that if someone lost 10% of their body weight, they would burn 10% fewer calories. In reality, people burn about 20-25% fewer calories after losing 10% of their weight.3 In one study this lower-than-expected energy expenditure was still present after a full year!4

Fortunately, some researchers have been able to design studies where participants who lose weight can actually increase their energy expenditure.5 The difference? They included high protein levels in their diet and performed resistance training. This extra protein and weight training allowed them to overcome the typical slower metabolism that accompanies calorie-restricted diets.


Intermittent fasting vs. eating less throughout the day

More recently, instead of eating less throughout the day, some people have decided to stop eating completely for specific periods of time. This is referred to as intermittent fasting. There are three types of intermittent fasting: alternate-day fasting, whole-day fasting and time-restricted feeding.

Alternate-Day Fasting: This diet typically involves a one-day fast every other day.

Whole-Day Fasting: In this diet, fasting takes place 1-2 days each week when you either don’t eat at all on those days, or when you severely reduce your calories.

Time-Restricted Feeding: This diet allows you to eat every day, but only during a 4-8 hour period. The other 16-20 hours are fast periods.A

We could review these types of fasting in more depth, but the results from each are similar. In the largest review of fasting research to date, including over 40 studies, researchers determined that intermittent fasting and eating less throughout the day were equivalent in their effects on weight loss and body composition. But, they did mention that intermittent fasting seemed to be better at suppressing hunger.6 At the end of one study, whole-day-fasting subjects completed 70% of their fasting days without cheating while the eating-less-throughout-the-day group only reached their low-calorie goal 39% of the time.7


Low fat

In a review of studies covering 54,000 people, researchers found that reducing fat intake can help decrease body weight.8 Very little research goes beyond weight loss such as what type of weight was lost (muscle vs. fat). Many people who are vegan and vegetarian dieters generally follow a low-fat diet without even realizing it. However, research shows that reducing fat intake below normal levels may not be the best way to lose weight.13 When consuming fat, pick the healthier forms found in foods like fish, avocado, nuts, flaxseed, olive and coconut oil and avoid trans fat or hydrogenated fat.


Low carb, ketogenic and paleo

In a meta-analysis of 1,416 subjects, low-carb diets (50-150g of carbs per day) led to more fat loss compared to placebo.9 The ketogenic diet or keto (maximum of 50g of carbs per day) is an extreme version of the low-carb diet.A

Multiple scientists have pointed out that it appears the higher-protein content of low-carb diets is the crucial factor in promoting weight loss, rather than the low-carb or high-fat aspects.10,11 Researchers also point out that when people switch to a ketogenic diet, they eat less.A One study showed ketogenic dieters, without purposeful calorie restriction, consumed 294 fewer calories each day.12 This is likely related to the satiating effect of fat and protein.

The paleo diet is another type of low-carb diet. It lets you eat anything that a caveman could hunt or gather. The focus is on fruits, vegetables, meats, seafood and nuts. It’s essentially a low calorie and low carb diet because you eliminate all carbs from grains and legumes (no bread, pasta or rice allowed). The result is a higher protein and higher fat diet.

Lower-carb-type diets also may be better for your overall health. In a recently-published, large cohort study spanning 18 countries over 7.4 years with 135,335 individuals, doctors found that high carbohydrate intake was associated with a 30% higher risk of total mortality compared to those on a low carb diet. Those eating higher-fat diets had a 23% lower chance of dying compared to those who ate less fat.13


High protein diets

The general consensus of the researchers reviewing these types of diets was that protein intake seems to be the best indicator of successful weight loss.A Protein consumed at double the Recommended Daily Amount (RDA) has repeatedly outperformed the RDA when it comes to preserving muscle and reducing body fat during weight loss.14,15 For a 150-pound person, that means consuming 110g of protein a day instead of the RDA of 54g per day.

If more protein is better, what is the limit? Increasing protein intake beyond double the RDA has shown mixed results. One study showed that triple the RDA had the same results as double the RDA.16

In a more recent study, dieting was combined with resistance and sprint training. Triple the protein RDA (162g for a 150-pound person) led to muscle gains and fat loss, while 1.5x the protein RDA only preserved muscle (no gains) and resulted in less fat loss.17 (see Figure 1)

Too Much Protein? 

Another researcher had subjects ingest either 5.5x or 2.25x the RDA for protein. Both groups had similar body composition results, even though the super-high protein group consumed an extra 800 calories per day.18 The same researcher later did another study that included an exercise plan and found that individuals consuming 4.25x the RDA had a similar increase in lean mass, but a significantly greater decrease in fat mass, even with an extra 374 calories per day, compared to those consuming 2.9x the protein RDA.19 He also eased any safety concerns about long-term high protein intakes (3-4x the RDA) over a 1-year study with a comprehensive list of measured clinical markers.20 

Protein clearly has a thermic, satiating and lean-mass preserving effect that may be enhanced in individuals who exercise compared to sedentary (non-active) people.A


Keys to a successful diet program

1. The science is clear – any meaningful weight loss will be driven by eating fewer calories than you burn.

2. Take advantage of the thermic effect of protein, which will allow you to eat more total calories. 

3. If you struggle with sticking to an “eating less throughout the day” low-calorie plan, consider one of the three major types of intermittent fasting.

4. Try to eat fewer carbs and especially avoid the sugary carbs. Don’t reduce fat intake below normal levels.

5. Consume higher protein levels in order to retain muscle and lose more body fat. It is also the most satiating macronutrient, so it will help you control your appetite. Try to eat at least double the RDA of protein. There is a chart above to help you calculate this is based on your weight. Protein shakes are a great way to boost your protein intake while reducing your calorie intake.

6. Lastly, consider a whole-body exercise plan that incorporates weight training. It can help you combat a diet-induced slow metabolism, and it can increase your lean muscle and eliminate that flabby look.

Before you start a diet program, consult your doctor.


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