Despite efforts to the contrary, body sizes and waistlines continue to grow in the United States and worldwide. Approximately two thirds of the U.S. population has excess body fat and more than half of them store excess fat around their abdomen (1). According to the most recent data, in approximately 12 years, from 1999–2000 to 2011–2012, the prevalence of abdominal obesity increased in the United States from 46.4% to 54.2%. Furthermore, the estimated mean waist circumference (WC) in the United States increased during that time span by 3 cm (~1.2 in) (1). Although risk increases with age, abdominal obesity is not just a condition of older adulthood. Data from 2003–2004 NHANES, when total prevalence was estimated to be 52.1%, estimated that 24% or greater of men and 41% of women between the ages of 20 and 29 years were abdominally obese (1,2). Thus, overall, not only do more people of all ages have large waistlines, but their waistlines are larger than ever before.
…not only do more people have large waistlines, but their waistlines are larger than ever before.
Although larger body sizes and waistlines are becoming normalized in society (3), the health consequences cannot be denied. The relative risk for increased body fat is especially high if the fat is in the abdomen. Independent of body weight, a larger WC increases risk for cardiovascular disease, diabetes, and metabolic syndrome (4). In fact, it has been estimated that approximately 84% of people with abdominal obesity are metabolically unhealthy (5). Therefore, it is paramount that health and fitness professionals stay up to date on assessment methods, risk factors, and the diets that have been evaluated for abdominal obesity if they are on the front lines “battling the bulge.”
Although larger body sizes and waistlines are becoming normalized in society (3), the health consequences of excess body fat cannot be denied.
HOW IS ABDOMINAL OBESITY ASSESSED AND DIAGNOSED?
Abdominal obesity is categorized by excess fat stored in two different abdominal regions: subcutaneous and visceral. Subcutaneous adipose tissue (SAT) is stored above the abdominal muscles and below the surface of the skin; visceral adipose tissue (VAT) is stored below the abdominal muscles and surrounds major internal organs of the body, including the liver, pancreas, and heart (Figure 1). Compared with SAT, VAT has greater innervation and vascularization, is more metabolically active, sensitive to lipolysis, and insulin resistant. It has been hypothesized that the proximity and characteristics of VAT lead to increased release of inflammatory messengers and free fatty acids into portal circulation (6). Highly lipolytic VAT also may provide more free fatty acids to the liver, via portal circulation, where a series of negative downstream effects result, including reduced blood glucose regulation, elevated blood lipids, elevated blood pressure, increased release of inflammatory markers, and more. Research also has shown that VAT has an altered production of adipokines and cytokines that favors inflammation and insulin resistance (7). Research is ongoing to clarify these proposed mechanisms, but they are currently supported by extensive evidence (6,7). Thus, the real danger from abdominal obesity seems to stem from excess storage of VAT.
Current methods to determine the existence and extent of abdominal obesity range from inexpensive field-ready measures, such as WC, to far costlier clinic-based measures, such as computed tomography (CT) scans (Table 1). WC is an inexpensive, safe, and reliable field method, but it cannot distinguish the difference between lean body mass and fat (8). CT scans and magnetic resonance imaging (MRI) are considered criterion standards for assessing abdominal obesity and can provide separate estimates of skeletal, bone, and fat tissue as well as distinguishing VAT from SAT. Unfortunately, access to CT scans and MRIs are limited and costly, and CT scans expose individuals to a significant amount of ionizing radiation. Dual-energy x-ray absorptiometry (DXA) scans are less expensive and involve much less radiation than CT scans, but similar to WC, they cannot distinguish VAT from SAT. Despite these disadvantages, DXA and WC are more accessible, safe, and less costly than MRI and CT scans and have strong correlations to CT estimations of VAT.
Measuring Abdominal Fat With DXA
Given its strengths, DXA has taken over as the criterion standard for whole-body composition assessment and provides excellent estimates of regional body fat. In general, abdominal fat is measured in a specified region above the iliac crest and correlates well with single-slice CT scans of VAT at the level of the fourth and fifth lumbar vertebrae (9). At this time, however, there are no universally accepted cut-points for VAT area or volume to diagnose abdominal obesity.
Given the variables to assess abdominal obesity using DXA scans, it is important to compare an individual’s scans from similar machines, analyzed scans in the same regions of interest, and using similarly trained technicians. Scans from different machines (even if the same brand) or different regions of interest can give a false impression of changes, especially after efforts to change body composition or bone health.
Using WC to Measure Abdominal Obesity
Overall, research has shown that WC is the most convenient method of assessment for abdominal obesity and the best anthropometric surrogate marker for VAT (10). The 2013 American College of Cardiology, American Heart Association, and The Obesity Society Guideline for the Management of Overweight and Obesity in Adults recommends measuring WC specifically in individuals within the body mass index (BMI) range of 25 to 34.9 kg/m2 for determining cardiometabolic risk, but additional risk information is gleaned by measuring WC in individuals with BMI greater than 35 kg/m2 (10).
When measuring WC, the anatomical site is important to compare results to evidence-based cut-points that indicate cardiometabolic risk level. In the United States, the National Institutes of Health (NIH)/National Heart, Lung, and Blood Institute (NHLBI) recommends measuring WC at the top of the iliac crest for most populations (11), but international health organizations have different WC protocols. The World Health Organization (WHO) and the International Diabetes Federation (IDF), two leading health organizations focused on cardiometabolic health, recommend measuring WC midway between the lower rib and the top of the iliac crest (Figure 2) (4,11,12). Organizations also define abdominal obesity and health risk level differently (Table 2). Thus, it is helpful to routinely measure WC using both the NIH/NHLBI and WHO/IDF protocols to compare results over time to the cut-points by the leading health organizations.
Who Is at Risk for Developing Abdominal Obesity?
With the emergence of research specifically linking abdominal obesity and VAT to multiple comorbid conditions, efforts have increased to determine an individual’s likelihood of developing abdominal obesity and effective prevention and reduction methods. For example, smoking and excessive alcohol consumption have been repeatedly shown to increase risk for cardiometabolic disease and abdominal obesity, but other risks are still emerging.
Age and hormones are factors that also have been found to increase risk of abdominal obesity (13,14). As men and women age, they both experience increases in VAT, possibly related to overall weight gain and shifting fat distribution from peripheral to visceral storage. Research has shown that between 25 and 65 years, VAT increases more than 200% in men and nearly 400% in women. The hormonal milieu also has been linked to VAT deposition through a number of different hormones and receptors. In terms of sex-hormones, bioavailable testosterone levels strongly influence VAT deposition independent of age, race, total body fat, and other cardiometabolic risk factors such as insulin resistance. In men, lower bioavailable testosterone increases VAT, but in women, higher bioavailable testosterone increases VAT. Furthermore, VAT significantly increases during pregnancy and the menopause transition, beginning with perimenopause (~4 years before menopause) and extending through the end of menopause (14). Although some of the mechanisms have been identified, much of the research in this area is ongoing.
It has been recognized that different races and ethnicities may be at higher risk for abdominal obesity. In fact, some races and ethnicities have different levels of VAT per WC, but it is still difficult to determine relative risk of developing abdominal obesity because race- and ethnicity-based WC cut-points have not yet been established (4). More research on risk factors is certainly warranted.
CAN DIET BE USED TO TARGET ABDOMINAL FAT?
Given that abdominal obesity, especially VAT, is associated with so many comorbidities, targeted weight loss approaches are of interest. It seems that every day, a new “expert” has found the “cure” to “belly fat” and is willing to share this “new discovery” — the secret is waiting for you if you just “click here.” Because of the abundance of unqualified or unethical nutrition sources, it is imperative that all health, fitness, and nutrition professionals be on the same front to correct the viral misinformation that spreads.
It is well established that the real winner in the battle against the bulge is still an energy deficit from a healthy diet and regular exercise, alone or combined. However, are there any dietary approaches, foods, or nutrients that can preferentially target abdominal fat? What follows is a brief summary of a few of the diet patterns, foods, and specific nutrients that have surfaced in scientific literature or media as potential weight loss or belly fat diet strategies.
In the “battle against the bulge” it is critical that health and fitness professionals are on the same front screening for health risk and recommending evidence-based diets while correcting the abundance of misinformation.
Timing of Energy Intake: Intermittent Fasting
A recent trending diet pattern is intermittent fasting (IF), an approach that is based on timing of energy intake. Currently, there is no standardized method to IF, but common approaches include some level of fasting or energy restriction for 1 to 3 days/week, with or without restrictions on the remaining days. Alternate-day fasting is a form of IF that alternates fast (~25% of energy needs met) and feed (eating ad libitum) days. This method of energy restriction has been proposed as an alternative weight loss method based on proposed metabolism alterations and improved diet adherence. However, according to a systematic review of 12 studies comparing IF with continuous energy restriction diets, all diet patterns resulted in similar weight loss and reductions in waist circumference, regardless of fasting or energy intake timing (15).
Macronutrient Distribution of Energy: High-Protein Diet
High-protein diets are still very popular both in scientific literature and media outlets. Protein has been shown to assist weight loss efforts because of its potential to increase satiety and resting energy expenditure. In addition, when combined with lower carbohydrate intake, weight loss can be amplified by the loss of glycogen and associated water molecules. The current recommendation for daily protein intake is 10% to 35% of total energy, or 0.8 g/kg of body weight; high-protein diets that have been studied extensively typically range from approximately 25% to 40% of total energy and 1.2 to 2.0 g/kg of body weight. When comparing energy-matched high- and low-protein diets, higher protein has been found to enhance satiety and reduce the loss of lean body mass (10). However, although higher dietary protein may be an option for assisting with weight loss in general, there is insufficient evidence from these studies that higher protein diets reduce abdominal obesity greater than other energy-restricted diets.
Food Sources: Paleolithic-Style Diet
One diet continuing to trend is the Paleo-style diet, a dietary approach that attempts to mimic the eating patterns of our Paleolithic ancestors. The difference between the modern interpretation of the Paleo-style diet and current dietary recommendations is the elimination of specific food groups and foods not available during the Paleolithic era, such as dairy, grain-based foods, legumes, added sugars, and any industry-made product. Although there are many books and media sources of information, there is a paucity of high-quality research on the Paleo-style diet and abdominal obesity. In fact, only four randomized, controlled trials were identified to be included in a 2015 systematic review and meta-analysis of Paleo-style diets on cardiometabolic risk factors. In this review, authors reported that the Paleo-style diet elicited greater changes in WC compared with diets following current dietary guidelines worldwide. However, on careful review, abdominal obesity was not a primary outcome identified in these studies and the studies had small samples sizes, thus reducing the confidence in the statistical power of the findings. Furthermore, three of the four studies did not design or control the diets — participants were only given information about their assigned diets (16). Thus, given the small number of studies, and study design limitations, it is too soon to definitively say that a Paleo-style diet can target abdominal obesity.
Bioactive Compounds: Green Tea
Drinking green tea also has been identified in literature as a possible way to enhance weight loss and reduce cardiometabolic risk factors — including abdominal obesity. In particular, green tea catechins (GTC) and caffeine have been proposed to have separate and synergistic effects on energy expenditure, fat absorption, and fat oxidation (17). Because of these bioactivities, green tea and GTC (with and without caffeine) have been studied for their role in health and weight management, with conflicting results (18). Although GTC has shown potential in animal studies, the dose needed to see a clinically significant difference in WC (~3 cm or 1.2 in) in humans may translate to unrealistic servings of GTC. However, it is possible that green tea incorporated into a healthy diet may work synergistically with other dietary factors, but more research is needed (19).
Heart-Healthy: Dietary Approaches to Stop Hypertension and Mediterranean Diets
Aside from general weight loss efforts, research has shown that dietary trans and saturated fats are positively associated with abdominal obesity, whereas fiber, especially soluble, is negatively associated (20). Thus, a great start to preventing and reducing abdominal obesity is to follow a diet that encourages foods high in fiber such as vegetables, fruits, legumes, and whole grains and limits foods high in trans and saturated fats, such as fatty meats, baked goods, and highly processed foods. These recommendations are consistent with heart-healthy diets like the NIH-developed Dietary Approaches to Stop Hypertension (DASH) diet and the Mediterranean-style diet. Very recently, these two diets were selected by health experts consulting with the U.S. News and World Report as the “2018 Best Overall Diets” (21) and may inspire more people to ask about incorporating DASH or Mediterranean-style diets into their lifestyle. The DASH diet, summarized in Table 3, is routinely recognized as an ideal diet for weight loss and reduction of cardiometabolic disease risk and was originally developed to manage hypertension. The Mediterranean-style diet, summarized in Figure 3, has been recognized for its association with lower cardiometabolic disease rates. This way of eating originates from the Mediterranean region and emphasizes a plant-based diet with fresh foods and ingredients that incorporate higher fiber and healthy fats, fewer sources of saturated fats and added sugars, and healthy bioactive compounds. Given the nutrient profile of the DASH and Mediterranean-style diets, both are great options to encourage clients/patients to safely incorporate while following modest energy restriction and/or exercise for weight loss as per the 2009 ACSM Position Stand (22).
SUMMARY & TAKEAWAY MESSAGE
In the battle against the bulge, it is critical that health and fitness professionals are on the same front screening for health risk and recommending evidence-based diets while correcting the abundance of misinformation. Teaming up with a Registered Dietitian Nutritionist would be beneficial for your clients to prevent unintended nutrient deficiencies and toxicities from poorly designed diets or inclusion of special supplements. To find a Registered Dietitian Nutritionist near you, go to www.eatright.org and use the “Find an Expert” tool. Additional key takeaway points from this article are as follows:
- Assess clients for abdominal obesity, and monitor changes monthly, using the two sites suggested by the NIH/NHLBI and the WHO/IDF (Figure 1).
- Evaluate your client’s cardiometabolic health risk using appropriate population-specific cut-points for abdominal obesity (Table 2).
- Educate your clients/patients on the health risks associated with excess abdominal fat and encourage them to take steps toward preventing or reducing abdominal obesity.
- Correct misinformation about trending diets — there is still no miracle diet, food, nutrient, or bioactive component that will target abdominal fat.
- Recommend only evidence-based diets that have repeatedly been shown to be heart-healthy, such as the DASH and Mediterranean-style diets.
- Work with a Registered Dietitian Nutritionist to manage your client’s body weight to achieve healthy weight loss through diet and exercise. Refer to the 2009 ACSM Position Stand on Appropriate Physical Activity for Weight Loss and Prevention of Weight Regain for Adults (22).
BRIDGING THE GAP
The prevalence of abdominal obesity was estimated in 2012 to be approximately 54%, reflecting an 8% increase in approximately 12 years. Because of the cardiometabolic risks associated with excessive abdominal fat, it is imperative that health and fitness professionals: 1) appropriately screen for, and understand, the health consequences associated with abdominal obesity; 2) correct misinformation about trending diets for belly fat, and 3) work with Registered Dietitian Nutritionists and encourage evidence-based diet patterns that address individual needs. This article discusses the risks of abdominal obesity, the importance of proper assessment and tracking, and research evidence on trending diets for abdominal obesity.
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