What is metabolic health?
Metabolism refers to the chemical reactions that occur in our body. These reactions turn energy in food into energy for moving about and regulate blood sugar among countless other things. Metabolic health refers to maintaining certain factors in ranges that foster proper functioning. Poor metabolic health predicts increased risk of chronic diseases including coronary heart disease, stroke, high blood pressure, type 2 diabetes, and chronic kidney disease. The metabolic syndrome refers to a collection of unhealthy metabolically related factors including large waist circumference, high blood pressure, low levels of HDL-cholesterol, and high levels of triglycerides and blood sugar.
Metabolic health of hunter-gatherers
Hunter-gather populations offer glimpses into the causes of chronic diseases of modern Western civilization. A recent review showed that hunter-gatherers have far lower incidence of cardiovascular disease, type 2 diabetes, and obesity than people in Western countries. Plus, longevity of hunter-gatherers approaches that of Western countries. What factors might account for their reduced risk of chronic diseases? Detailed investigations of two groups of hunter-gatherers in Africa of revealed that their level of daily physical activity (about 120 minutes per day) more than tripled that of typical Americans (about 35 minutes per day). Total energy expenditure was similar for hunter-gatherers and Americans, because hunter-gatherers have much higher basal metabolism. Thus, daily total energy expenditure can’t account for the large difference in risk of chronic diseases between hunter-gatherers and modern Americans. The energy density of hunter-gatherer diets is modestly lower but far higher in fiber and micronutrients than those of Americans.
Hunter-gatherers eat little or no ultra-processed food, which accounts for over half of a typical American’s food consumption. The ratio of carbohydrate to fat in hunter-gatherer diets probably doesn’t explain their resistance to chronic diseases, either. The proportions of plant and animal food sources in hunter-gatherer diets vary widely depending on latitude and local factors. From the Equator to about 30 degrees latitude, diets of hunter-gatherers and Americans have similar relative amounts of plant and animal foods. The animal fraction increases as latitude increases above 30 degrees. Aside from high levels of dietary fiber and micronutrients and a lack of ultra-processed food, higher levels of social bonding, lower inequality, and more time spent in nature may help hunter-gatherers avoid chronic diseases.
Caloric restriction
Cross-sectional studies of humans suggest that caloric restriction predicts better cardiovascular function. Would a long-term randomized, controlled trial—the gold standard in research studies—confirm these results? CALERIE is a multi-centered, two-year trial in which 218 young and middle-aged, evidently healthy US adults were randomized to either a calorie restriction group (target of 25 percent calorie reduction) or a control group (asked to maintain their current dietary habits). After two years, participants in the calorie restriction group actually achieved a 12 percent calorie reduction, while the control group participants reduced their caloric intake by 0.8 percent. Participants in the calorie restriction group had significant reductions in conventional cardiometabolic risk factors. Improvements included factors related to reduced hypertension, chronic inflammation, arterial damage, and oxidative stress, all of which are associated with improved metabolic health. Specific examples included lower levels of the following: body weight, body fat, LDL-cholesterol, total cholesterol to HDL-cholesterol ratio, systolic and diastolic blood pressure, C-reactive protein, and metabolic syndrome score, along with increased insulin sensitivity. All this independent of weight loss. These impressive results suggest that young and middle-aged adults who embrace a healthy, modestly calorie restricted diet might greatly reduce their risk of cardiometabolic disease and improve their metabolic health.
Prevalence of optimal metabolic health in America
Better cardiometabolic health predicts lower risk of cardiovascular and other chronic diseases. What proportion of Americans exhibit optimal cardiometabolic health? Researchers addressed this question with data from 8,721 participants in the National Health and Nutrition Examination Survey 2009–2016. Researchers defined optimal metabolic health as the following: 1) waist circumference (WC <102/88 cm or 40/35 inches for men/women), 2) blood glucose (fasting glucose <100 mg/dL and HbA1c <5.7%), 3) blood pressure (systolic <120 and diastolic <80 mm Hg), 4) triglycerides (<150 mg/dL), and 5) high-density lipoprotein cholesterol (≥40/50 mg/dL for men/women), and 6) not taking any related medications. According to this definition, 20 percent of Americans had optimal cardiometabolic health. But using more recently recommended cut-points for the above factors reduced the proportion to 12 percent. Yikes! Participants with optimal cardiometabolic health tended to exhibit the following characteristics: female gender, youth, more education, never smoking, practicing vigorous physical activity, and low body mass index. Less than one-third of even normal-weight American adults had optimal metabolically health.
Prevalence of the metabolic syndrome in America
The metabolic syndrome predicts increased risk of cardiovascular disease and premature death. This condition can be defined as having at least three of the following factors: 1) waist circumference more than 88 cm (35 inches) for women and 102 cm (40 inches) for men, 2) triglycerides more than 40 mg/dL, 3) high-density lipoprotein cholesterol less than 40 mg/dL, 4) systolic blood pressure more than 130 mm Hg or diastolic blood pressure more than 80 mm Hg or taking hypertensive medication, 5) fasting plasma glucose more than 100 mg/dL or taking diabetes medication. A report in the Journal of the American Medical Association in 2015 found that 33 percent of American adults had the metabolic syndrome as of 2012. An updated study found that the prevalence increased somewhat to 34.7 percent as of 2016, statistically indistinguishable from the previous estimate. The prevalence of the metabolic syndrome more than doubled with age, rising from 20 percent for persons age 20-39 to 49 percent for persons age 60 or older.
Fructose sugars
Sugar-sweetened drinks comprise the major source of fructose sugars in American diets and predict increased risk of the metabolic syndrome. Canadian researchers conducted a systematic review and meta-analysis of 13 longitudinal studies that investigated links between food sources of fructose sugars and risk of developing the metabolic syndrome. Each 355 ml (12 ounce can) per day increase in sugar-sweetened beverage intake showed a 14 percent increase in risk of metabolic syndrome. Small intakes (125 ml per day or 4.2 fluid ounces) of 100 percent fruit juice or mixed fruit juice predicted 42 and 23 percent lower risks of metabolic syndrome, respectively. But daily intakes of either 100 percent fruit juice or mixed fruit juice above about 175 ml (6 fluid ounces) predicted increased risk of metabolic syndrome. Daily consumption of 240 grams (three servings) of whole fruit predicted a 39 percent reduced risk of metabolic syndrome. Consuming 85 grams of yogurt (one-third cup) daily predicted a 34 percent lower risk of developing metabolic syndrome. This study supports the recommendation to avoid or at least greatly limit intake of sugar-sweetened beverages and to eat more whole fruit. Drinking small amounts of 100 percent fruit juice or mixed fruits juice may be beneficial, but foregoing the juice and eating more whole fruit would likely be better. And why not leave the sweetened yogurt at the store and eat unsweetened yogurt instead?
Greater physical activity AND better diet
Previous research suggests that greater physical activity and better diet each predict lower risk exhibiting or developing the metabolic syndrome. But do greater physical activity and better diet together further lower the odds of having or developing the metabolic syndrome? Researchers in Boston used cross-sectional and longitudinal data from the Framingham Heart Study to find out. The cross-sectional data came from 2,379 participants with an average age of 47 years. Factors of the metabolic syndrome included high waist circumference, high blood pressure, high fasting glucose, high blood triglycerides, and low HDL-cholesterol. Accelerometers measured physical activity over the course of a week. Adherence to 2018 physical activity guidelines was defined as at least 150 minutes per week of moderate- or vigorous-intensity movement. Participants reported dietary intake on a food-frequency form. Adherence to a quality diet was defined at scoring above the median value on the 2015 Dietary Guidelines for Americans Adherence Index. Greater adherence to physical activity and dietary guidelines showed 51 and 33 percent lower odds of having the metabolic syndrome, respectively. Participants adhering to both guidelines had 65 percent lower odds.
The longitudinal part of the above study used data from 1,562 participants who were followed for an average of 8 years. Adherence to physical activity and dietary guidelines showed 34 and 32 percent lower risk of developing the metabolic syndrome, respectively. Participants adhering to both guidelines had 52 percent lower risk. Thus, simultaneously meeting physical activity and dietary guidelines may help middle-aged Americans avoid the metabolic syndrome.
Stair climbing
Researchers in the UK used cross-sectional data from 782 participants with an average of 58 years in the Dutch Famine Birth Cohort Study to determine if any amount of stair climbing would predict lower risk of manifesting the metabolic syndrome. Any three of the following four factors (elevated blood glucose, triglycerides, blood pressure, and waist circumference) defined the presence of the metabolic syndrome. Participants responded with either yes or no to the question, Do you climb stairs daily? Compared to participants who reported no, those who reported yes had significant 72 percent lower risk of having the metabolic syndrome, after adjusting for confounding factors including physical activity level. Furthermore, not climbing stairs predicted an increased number of metabolic syndrome factors. The energetic demands of stair climbing could possibly lead to reduced blood sugar or blood pressure levels thereby lowering risk of having the metabolic syndrome.
Sedentary behavior
Increased amounts of sedentary behavior predicts increased risk of cardiovascular disease. Could an intervention designed to decrease daily sedentary behavior by one hour help sedentary, middle-aged adults (average age 58 years) eliminate or lessen the metabolic syndrome? Researchers in Finland recruited 64 participants for a three-month randomized, controlled trial to find out. Participants in the experimental group were offered suggestions for reducing sedentary time, such as using stairs rather than taking the elevator, using a stand-up desk, or standing while talking on the phone. Participants in the control group were asked to continue their usual routines. After three months, participants in the experimental decreased their daily sedentary time by 50 minutes and increased their light- and moderate-intensity physical activity and number of daily steps compared to participants in the control group. Participants in the experimental group showed significantly smaller increases in blood insulin, insulin resistance, and HbA1c compared to participants in the control group. The intervention moderated but did not reverse all of the biomarkers associated with the metabolic syndrome. Either greater intensity of physical activity and/or further reduction in sedentary time may be required to prevent cardiometabolic disease in these middle-aged people.
Metabolically healthy obesity
The prevalence of obesity in US adults climbed from 29 percent in 1999-2002 to 41 percent in 2015-2018, according to nationally representative data from the National Health and Nutrition Examination Survey (NHANES). That sounds ominous, give the links between obesity and ill health. Researchers used cross-sectional NHANES data from 20,403 participants with an average age of 47 years to determine if so-called metabolically healthy obesity (MHO) also rose during the above time frame, thereby tempering some of the adverse effects of increased obesity. Participants had MHO if they avoided four components of the metabolic syndrome: 1) elevated systolic blood pressure (130 mm Hg or more) and diastolic blood pressure (85 mm Hg or more); 2) elevated fasting blood glucose (100 mg/dL or more), 3) reduced HDL-cholesterol (less than 40 mg/dL for men and less than 50 mg/dL for women), and 4) elevated triglycerides (150 mg/dL or more). Happily, the age-adjusted prevalence of MHO doubled from 3.2 to 6.6 percent, driven mostly by reduced triglycerides (blood fat). In addition, the increase in MHO prevalence accrued largely to participants with higher socio-economic status. Alas, the prevalence of obesity increased faster than did the prevalence of metabolically healthy obesity.
Nonalcoholic fatty liver disease
Nonalcoholic fatty liver disease (NAFLD) is related to the metabolic syndrome. NAFLD refers to the accumulation of at least 5 percent fat in the liver unrelated to alcohol intake. The prevalence of NAFLD is growing globally and predicts increased risks of type 2 diabetes, hypertension, and cardiovascular disease. Insulin resistance (the reluctance of cells to take up glucose from the blood stream) appears to cause NAFLD. Recent research shows that ratio of two common blood fats, triglyceride (TG) and high-density cholesterol (HDL) may serve as an easily measured surrogate for NAFLD. Generally, lower TG and higher HDL levels predict better cardiometabolic health. Thus, increasing values of the TG/HDL ratio predict worse cardiometabolic health.
Researchers in China used cross-sectional data for 18,061 apparently healthy patients in a Chinese health check-up program and determined NAFLD with ultrasonography. A quarter of the patients had NALFD. When the patients were classified in 4 groups of increasing TG/HDL ratio, the prevalence of NAFLD increased in a step-wise manner. Compared to patients in the lowest (healthiest) quartile of TG/HDL ratio, the patients in the three successively higher quartiles had 2.1, 3.6, and 9.2 (!) times higher odds of having NAFLD. The TG/HDL ratio predicted the risk of NAFLD better than any of the other types of blood fats investigated. In this study, the optimal cut-off point for the TG/HDL ratio was 0.9 for women and 1.4 for men. You can ask your primary medical care provider to measure your TG and HDL in a blood sample to see if your TG/HDL ratio falls in the safe zone or not.
What to do
Increase your metabolic health by increasing your daily level of physical activity and improving your diet with more fruits and vegetables, legumes, nuts, seeds, plain yogurt, and whole grains.
