Diets for weight loss: Different types
Obesity is a worldwide, multifactorial disease defined as abnormal or excessive fat accumulation that presents a risk to health. The disease is associated with several chronic morbidities, such as cardiovascular diseases (CVDs), diabetes, and cancer. Prevalence of overweight and obesity has tripled since 1975, reaching 39% and 13% of the world's population, respectively . Because of its significant effects on health, medical costs, and mortality, obesity has become a public health concern.
The fundamental cause of obesity is an energy
imbalance between calories consumed and calories expended; however, this
involves a complex interplay of biological, genetic, and psychosocial factors.
Evidence has shown that a weight loss of 5% to 10% within 6 month is necessary
to reduce risk factors of comorbidities and to produce clinically relevant
health improvements such as reductions in blood glucose, triacylglycerols, and
blood pressure.
To achieve successful weight loss and sustain
it over time, the Academy of Nutrition recommends changes in lifestyle
behavior; a diet that reduces excessive energy intake and enhances dietary
quality; and an increase in energy expenditure. Furthermore, the successful
treatment of overweight and obesity could require adjuvant therapeutics such as
cognitive-behavioral therapy, pharmacotherapy, and even bariatric surgery.
These therapies are indicated for specific conditions and should be
individually analyzed, which is a topic that goes beyond the scope of this
review.
Regarding dietary interventions for weight
loss, an individualized diet that achieves a state of negative energy balance
should be rescribed.
Many dietary approaches can generate this
desired reduction in caloric intake.
Diets are usually based on the inclusion or exclusion of different foods or
food groups .
Historically, several diets have become popular
and then faded owing to a lack of reliable scientific support. In this context,
this review aimed to provide scientific evidence to support the adoption of
dietary strategies to promote weight loss.
We classified these strategies into three main
categories:
- Diets based on the manipulation of macronutrient content (i.e., low-fat [LF], high-protein [HP], and low-carbohydrate diets [LCDs]).
- Diets based on the restriction of specific foods or food groups (i.e., gluten-free, Paleo, vegetarian/vegan, and Mediterranean diets).
- Diets based on the manipulation of timing (i.e., fasting).
Diets based on the manipulation of macronutrient content
The manipulation of macronutrient content in
isocaloric diets has been studied to determine which composition best promotes
weight loss while including other metabolic benefits.
Increased protein and decreased carbohydrates
are the most common modifications and have resulted in several popular diets
created over time.
Changes in the macronutrient composition affect
hormones, metabolic pathways, gene expression, and the composition and function
of the gut microbiome that might effect fat storage.
Metabolically, carbohydrates elevate insulin
secretion, thereby directing fat toward storage in adipose tissue, described as
the carbohydrateinsulin model of obesity . In this context, LCDs ranging from
20 to 120 g of carbohydrates claim to treat obesity because they promote
reduced insulin secretion and increased glucagon, which cause a metabolic shift
to higher fat oxidation
LCDs can be designed to be either normal-fatHP
or high-fat HF normal-protein. However,
despite the theory of the carbohydrateinsulin model, clinical trials comparing
LCDs with low-fat diets (LFDs) in isoprotein diets reported similar weight
loss and even higher body fat loss when
reducing fat but not carbohydrates.
Moreover, an important meta-analysis of 32
controlled studies concluded that energy expenditure and fat loss were more
significant with LFDs when compared with isocaloric LCDs.
Finally, individuals with insulin resistance
(IR), glucose intolerance,or both may benefit from a LCD , although this has
not been confirmed in a recent study with 609 individuals.
Furthermore, observational data have
demonstrated an increase in mortality associated with the long-term intake of
both LCDs and high-carbohydrate diets (HCDs) with minimal risk at 50% to 55%
(energy derived from carbohydrates). They also reported that animal-derived
protein and fat were associated with higher mortality, whereas plant-derived
protein and fat were associated with lower mortality.
Finally, high-protein diets (HPDs), in which 20% of energy is derived from protein, appear to offer advantages regarding weight loss and body composition in the short term.
Popular HPHF diets, such as Atkins or Zone,
promoted significant weight loss for short periods . HP intake acts on relevant
metabolic targets, increasing satiety and energy expenditure . Conversely, in
clinical trials >1 to 2 year, evidence indicated no significant differences in
weight loss. Moreover, HP-HF diets are often associated with a high intake of
animal products and saturated fat, causing detrimental effects of increased
low-density lipoprotein cholesterol.
In conclusion, in the short term, HP-LCDs are
suggested to present benefits for weight-loss. However, owing to their major
effects on metabolism and gut health, they should be considered as a jump-start
weight loss tool rather than a diet for life. In the long term, current
evidence indicates that a different ratio of macronutrients associated with a
caloric restriction in healthy diets promotes similar weight loss.
Diets based on the restriction of specific foods or food groups
Different foods and food groups have emerged as
villains and have been removed from specific diets to promote weight loss. The
long list includes a vegetarian diet, which excludes all animal products; the
Paleo diet, which restricts many food groups including grains, dairy, and
legumes; and the popular gluten-free diet (GFD).
The Mediterranean diet is not based on the
complete restriction of a specific food group, but instead is characterized by
a richness of plant-based food and moderation of refined grains, red meat, and
dairy.
Plant-based diet for weight-loss
Vegetarian dietary patterns are very diverse
due to the different reasons for its adoption and the wide variety of available
food choices.
A vegetarian plan can range from the simple
exclusion of meat products to the raw vegan plan, which only includes raw
vegetables, fruits, nuts, seeds, legumes, and sprouted grains . Exclusion of
animal products can reduce the intake of certain nutrients, which might lead to
nutritional deficiencies of protein, iron, zinc, calcium, and vitamins D and
B12.
Adoption of plant-based diets is growing
because evidence has shown some health benefits when compared with omnivorous
diets. They can protect against chronic diseases, such as CVDs , hypertension
and type 2 diabetes, and some cancers.
Further
research will clarify whether these benefits are related to the reduction of
animal products or the increased intake of fruits, vegetables, and fibers.
In observational studies, individuals on a
plant-based diet usually present a lower body mass index (BMI) than
non-vegetarians.
In interventional studies, prescription of
vegetarian diets was well accepted and
was associated with weight loss.
Two meta-analyses reported a significant
reduction of body weight after the adoption of vegetarian diets . Subgroup
analysis observed a higher reduction in weight loss with vegan diets compared
with lacto-ovo-vegetarian diets.
It is likely that this reduction is due to the
typically low energy density, LF and HF intake associated with plant-based
diets.
In summary, evidence has supported the
therapeutic use of plant-based diets as an effective treatment of overweight
and obesity. However, further long-term trials are required to confirm the
relevance of results, as some studies did not report differences in weight loss
. The adoption and implementation of a well-designed vegetarian diet require
effective counseling and adequate nutritional supplementation.
Paleo diet for weight-loss
The Paleolithic diet, also called Paleo, is
based on everyday foods that mimic the food groups of our preagricultural,
hunter gatherer ancestors.
The diet claims to help optimize health,
minimize risks for chronic disease, and result in weight loss. These statements
are supported by the theory that the hunter gatherer diet and lifestyle
sustained humanity for 2.4 million
years, causing humans to be genetically adapted to it.
According to proponents of the Paleo diet,
profound changes in diet and other lifestyle conditions after the introduction
of agriculture and animal husbandry 10,000 years ago have been too recent on
an evolutionary time scale for an adjustment of the human genome.
Only foods that were available to hunter
gatherers are included in the diet. These include meat, nuts, eggs, healthy
oils, and fresh fruits and vegetables. Cereal grains, legumes, dairy, and other
processed/refined products are excluded.
The Paleo diet features characteristics such as
a lower ratio of v-6 to v-3 fatty acids and lower sodium, along with a high
content of unsaturated fatty acids, antioxidants, fiber, vitamins, and
phytochemicals that operate synergistically to promote health benefits.
The diet is high in protein (20-35% of energy)
and moderate in fat and carbohydrates (22-40% of energy, specifically restricting
a high glycemic index).
Finally, the Paleo diet yields a healthier net
alkaline load compared with the net acid load estimated for the typical Western
diet.
Much has been studied about the beneficial
metabolic outcomes of the Paleolithic diet. Evidence has demonstrated several
improvements such as ameliorations in metabolic syndrome (MetS) , increase in
insulin sensitivity , reduction of cardiovascular risk factors , increased
satiety , and beneficial modulation of intestinal microbiota.
Specifically, regarding Paleo diet for weightloss, scientific evidence points toward consistent reduction of body weight and
body fat mass either in short- or
long-term studies. Low adherence, poor palatability, and high costs are common
issues reported by those who follow the Paleo diet.
In summary, although evidence suggests general
health benefits and weight loss, further research is needed to support the
popular claims of the Paleo diet. As an important limitation, the Paleo diet
presents a potential deficiency risk that includes vitamin D, calcium , and
iodine.
Gluten-free diet for weight-loss
Gluten is a protein complex found in cereals
such as wheat, rye, barley, and oats . Studies have shown that the main
fraction of gluten, namely gliadin, cannot be completely digested by the
gastrointestinal (GI) tract, triggering an intestinal inflammatory response in
susceptible individuals.
Celiac disease, wheat allergy, and non-celiac gluten sensitivity represent the main gluten reactions mediated by the immune system. The treatment for these disorders is based on the complete dietary exclusion of all gluten-containing food, which is well established by the scientific literature . However, the market for gluten-free products has been growing for the past 15 year, mainly due to individuals who adhere to a GFD to reduce body weight or improve diet quality . Despite the popular association of gluten and weight loss, controlled studies are scarce in the scientific literature.
Evidence supports a possible obesogenic effect
of gluten. First, a cereal-based diet impaired insulin sensitivity and blood
pressure control and increased the levels of C-reactive protein in pigs.
In rodents, two pioneer studies reported
obesogenic effects of gluten using the nutritional model of obesity: An HFD
added with gluten induced higher weight gain, adiposity, blood glucose,
inflammation, and IR, partly by reducing the thermogenic capacity of adipose
tissues.
In summary, to date, little has been studied
about gluten and body weight evolution. It is well known that gluten intake can
increase inflammatory status, cause intestinal dysbiosis , and increase
intestinal permeability . However, it is not known whether gluten presents
obesogenic properties and, if it does, the metabolic mechanism involved is
unknown.
Mediterranean diet for weight-loss
The Mediterranean diet is a balanced diet
characterized by high consumption of vegetables, fruits, legumes, whole-grain
cereals, seafood, olive oil, and nuts. Red meat, dairy and alcohol are recommended
in moderation . The Mediterranean diet is rich in plant-based foods, having
high levels of antioxidants and dietary fiber, and low glycemic load compared
with other diets . It also has an adequate ratio of monounsaturated to
saturated fatty acids.
Nevertheless, the main relevance of the
Mediterranean diet has been related to its strong evidence-based health and
metabolic benefits. Due to the high nutrition quality of its food composition,
the Mediterranean diet has been considered a healthy eating pattern for many
conditions.
Studies have demonstrated that the
Mediterranean diet improves outcomes for glycemic control in patients with type
2 diabetes. Hence it is one of the diets recommended by the American Diabetes
Association. The Mediterranean diet has been associated with a reduction of
inflammatory markers, and important reduction of cardiovascular risk factors
and mortality.
Moreover, this diet was efficient in decreasing
inflammation and cardiovascular risk
even in the absence of meaningful weight loss . More recently, the association
with amelioration of non-alcoholic liver disease and cancers has been promising but requires
further investigation.
Diets based on the manipulation of timing (fasting)
To achieve the negative energy balance required
for weight loss, most weight control programs use a 20% to 40% continuous
(daily) calorie restriction. However, more recently the manipulation of timing,
namely intermittent calorie restriction or intermittent fasting (IF), has
received considerable interest as an alternative strategy.
Intermittent fasting consists of abstaining from food and caloric beverages for a certain period of time alternated with normal eating.
Several variations of IF differ in length and frequency of the
fast cycles. Moreover, modified IF allows a small amount of intake to avoid
persistent hunger . IF is often combined with regular exercise and even other diets. The most
common types of IF include periodic fasting or 5:2 diet, alternate-day fasting,
time restricted feeding, and religious fasting.
The basic premise of fasting is to promote
changes in metabolic pathways, cellular processes, and hormonal secretions .
Major physiologic responses of fasting on health indicators include greater
insulin sensitivity and reduced levels of
blood pressure , body fat , glucose, atherogenic lipids, and inflammation .
In animals, fasting ameliorated
functional outcomes of diseases including cancer , type 2 diabetes , and CVD.
Emerging findings in rodents also observed the
potential of fasting to delay aging, although the magnitude of the effects
remains controversial. Depending on the species, the age at regimen initiation
and the fasting cycle, the results ranged from a negative effect to as much as
30% life span extension.
In humans, 12 to 24 hours of fasting typically
results in a 20% significant decrease in serum glucose and depletion of the
hepatic glycogen. Under these circumstances, the body switches to a ketogenic
metabolic mode using non-hepatic glucose, fat-derived ketone bodies, and free
fatty acids as energy sources . The restriction of carbohydrates also triggers
these metabolic changes.
Regarding the effects of IF on weight loss, the
average reported weight loss in cohorts with overweight and obesity has ranged
between »4% and 10% over dieting periods of 4 to 24 week . Nevertheless, some
contradictory effects of IF were observed. In rats, alternate-day fasting (a
type of IF) was unable to eliminate HF, diet-induced, muscle IR and did not promote changes in body weight .
In several clinical studies, the lack of an appropriate control group might
suggest that the beneficial improvements of IF could be comparable to other
types of calorie restriction.
Fasting is also adopted in several religious
and spiritual traditions, and thus is intensely studied in this population.
Ramadan is a month during which healthy adult Muslims fast for an average of 12
to 16 hours/day . Results describing the effects of Ramadan on body weight have
been inconclusive. Several studies reported a weight loss, whereas many others
showed no significant changes. Very often, a weight regain is observed a few
weeks after the fasting period.
In summary, there is growing evidence demonstrating the metabolic health benefits of IF. In rodents, these appear quite profound, whereas in humans they are sparse and need further investigation, especially in long-term studies.
It has been suggested that IF
does not produce superior weight loss in comparison with continuous calorie
restriction plans , and there are limited data regarding other clinical
outcomes such as diabetes, CVD, and cancer. IF diets seem safe and tolerable
for adults, but it is unclear if periods of fasting and hunger lead to
overeating . Adverse effects of fasting, which are similar to the KD, are often
moderate and include halitosis, fatigue, weakness, and headaches . It is also
important to emphasize that fasting diets might be harmful to specific
populations such as children, the elderly, and underweight individuals.
Alternative dietary approaches
There are several alternative dietary
approaches with promising favorable outcomes in patients with overweight and
obesity. For instance, the replacement of two to three meals a day with “meal
replacements” that contain all recommended nutrients has been described as an
approach that promotes significant weight loss . Studies also reported
reductions in cardiovascular risk factors
and improvement in metabolic parameters associated with meal
replacements. However, use of meal replacement products is not sustainable in
the long term due to severe energy restriction.
Furthermore, the benefits and disadvantages of
a long list of other weight loss diets, including the Whole30 diet, the Dukan
diet, the South Beach diet, the HCG diet, and the Detox Diet, and commercial
weight loss programs such as Weight Watchers require deeper scientific
investigation. A complete review of all weight loss dietary approaches would
not be feasible, giving the plethora of studies and trials. Hence, a few diets
of primary importance were selected and discussed in this review.
Adherence
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