The Impact of Diet on Obesity-Related Diseases
The prevalence of obesity continues to increase worldwide, as obesity is associated with an increased risk of hypertension, dyslipidaemia, metabolic syndrome, Type II diabetes mellitus, and cardiovascular disease. [1,2]
During the past few decades, the alteration of fat and carbohydrate intake may have also encouraged unwanted dietary changes that may have contributed to the increase in obesity. [3]
This has stimulated a considerable amount of interest and controversy in the management of dietary consumption. Therefore, recommending the appropriate diet in clinical practice is vital to limiting the obesity epidemic that is associated with the morbidity and mortality of diet-related diseases.
Low-Carbohydrate – High-Fat Diets
Short-term exposure to a (Low Carbohydrate – High Fat) diet has been shown to result in weight loss. This may suggest that altering the macronutrient composition is sufficient to achieve changes in body weight. [4]
Low-carbohydrate diets have appeared to provide superior weight loss, glycemic control, and lipid profile when compared to low-fat diets, which may be effective in improving various cardiovascular risk markers in people with diabetes. [5]
In addition, Betz et al. (2012) demonstrated similar findings, showing significant weight loss when comparing Low Carbohydrate-High Fat diets to High Carbohydrate-Low Fat diets.
Several mechanisms have been suggested, including the production of ketosis with the loss of energy via ketone bodies excreted via the urine, the suppression of appetite due to circulating ketones, and/or increased energy expenditure due to adaptive thermogenesis. [6]
According to Caton et al. when the (Low Carbohydrate – High Fat) diet is discontinued and the habitual diet or even a low-fat diet is resumed in the absence of no increase in energy intake resulted in an increase in body fat that exceeded the original weight.
Furthermore, Caton et al. also demonstrated that a (Low Carbohydrate – High Fat) diet had no effect on exercise capacity and adverse effects upon both growth hormone and IGF-I concentrations. It is suggested that this may have implications for the development and maintenance of lean body mass, normal growth rate, and overall metabolic health. [4]
According to Bielohuby et al. 2012 (Low Carbohydrate – High Fat), diets have shown unfavourable effects on bone growth and structure, alongside a decrease in bone mineralisation density and mechanical strength in rats. In addition, a significant increase in the levels of visceral fat and leptin serum levels.
Leptin has been suggested to control bone resorption by controlling the expression of RANKL and CART (cocaine amphetamine-regulated transcript), as these two factors are thought to regulate osteoclast differentiation.
It is thought that the increase in leptin and decrease in IGF-1 and IGFBP-3 may be responsible for mediating the poor bone quality. These results were supported by the transcription factors that influence osteoblastogenesis in bone marrow (Runx2, osterix, and C/EBPb). Thus, reduced bone formation occurs through a lower rate of mesenchymal cells differentiating into osteoblasts. [7]
Therefore, it can be hypothesised that the use of (Low Carbohydrate – High Fat) diets in the adult population already suffering from bone metabolism impairments such as osteoporosis should not be recommended a (Low Carbohydrate – High Fat) diet. [7]
The risk factors of overweight and obesity are important in diabetes, cardiovascular disease, cancer and premature death.5 The cardiovascular benefits of (Low Carbohydrate – High Fat) diets have been controversial because high-fat diets have traditionally been associated with obesity and metabolic syndrome. [8,9,10]
Nevertheless, in a study conducted by Bosse et al. (2013), low-carbohydrate, high-fat diets were found to lower blood pressure and improve the endothelial function of mesenteric arteries with no significant difference in the homeostasis of systemic glucose, and insulin-mediated signal transduction in the heart or skeletal muscle of hypertensive rats. [8]
However, a study conducted by Holloway et al. (2012) associated low-carbohydrate, high–fat diets with the promotion of atherogenesis, heart failure, insulin resistance, and glucose intolerance. Furthermore, the effects of short-term (Low Carbohydrate – High Fat) diets increased circulating free fatty acid (FFAs) concentrations and impaired myocardial function. [11,12]
In addition, it was associated with cognitive impairment. Epidemiologic studies have shown that cognitive deficits after the ingestion of chronic high-fat diets may be linked to a risk factor for developing Alzheimer’s disease. [12]
Bisschop et al. 2004 found that both low-fat and high-fat diets rapidly reduce the fractional turnover rate (FTR) and, therefore, may have significant effects on human bile salt metabolism. This reflected in a decrease in the turnover and synthesis of primary bile salts and consequently resulted in the impaired removal of cholesterol from the body. [13]
Furthermore, low-carbohydrate diets have statistically demonstrated a significant reduction in total cholesterol and low-density lipoproteins when compared to low-fat diets. However, has been shown to be correlated with a greater increase in high-density lipoproteins and a decrease in triglyceride levels. [14,15]
High Carbohydrate – Low-Fat Diets
The consumption of a (High Carbohydrate – Low Fat) diet may trigger undesirable changes in hepatic metabolism due to the high content of simple sugars, which may cause hypertriglyceridemia and insulin resistance. [16]
Furthermore, since humans have a limited capacity to store carbohydrates as energy, the excess storage and oxidation capacities are converted into fat by de novo lipogenesis (DNL). The liver plays a crucial role in glucose homeostasis by converting excess carbohydrate and excess gluconeogenic substrates to triacylglycerol. [16]
According to the results in the study conducted by Schwartz et al. 2003 both hyperinsulinemia and low-fat diets increase (DNL), and that (DNL) contributes to hypertriglyceridemia. Therefore, (DNL) may have an important role in maintaining glucose homeostasis and preventing diabetes. Furthermore, the increase in triacylglycerol concentrations may have detrimental effects by increasing the risk of cardiovascular diseases. [16]
The consumption of a (High Carbohydrate – Low Fat) diet may have different effects in younger populations than in middle-aged or older populations. In a study by Huang X et al. (2011), the High Carbohydrate Low Fat diet was associated with lower HDL cholesterol levels in males and higher TG levels in females. The results revealed that LPL-Ser447Stop and Hind III polymorphisms have different effects on serum glucose, insulin, and the lipid profile and that the response of serum lipids may be gender-specific. [17]
The impact of low-carbohydrate diets of various fat content compared to (High Carbohydrate – Low Fat) diets has no advantage on metabolic parameters in obese adolescents, including weight loss, BMI, or body fat percentage. [18]
However, there was a significant drop in baseline insulin and homeostasis model assessment levels after the twelve-week intervention and nine-month follow-up period in the low-carbohydrate diet groups. Therefore, suggesting that low-carbohydrate diets may have a role in the prevention of insulin resistance. [18]
Limitations of Dietary Modifications
A common problem with carbohydrate modification diets is the lack of consideration in the source of carbohydrates. Furthermore, the importance of consuming plant-based foods, including fruits, vegetables, whole grains, and fibre, is often overlooked or underestimated. [19]
Importantly, fibre is the most crucial dietary substrate for the colonic microflora, as plant foods such as whole grains contain a multitude of indigestible constituents beneficial for gut health. [19]
Subsequently, low-carbohydrate diets have also been shown to be associated with a decline in stool weight and frequency of bowel movements. Increased stool bulk is associated with intestinal motility, and therefore increases the frequency of bowel movements and shortens colonic transit time.
This has been postulated to reduce the colonic epithelium’s exposure to potentially harmful agents, including mutagens, carcinogens, and pro-carcinogens. Therefore, there is an increased risk of harmful agents and bowel-related diseases.
Furthermore, it is believed that the fermentation of undigested proteins by the large-bowel microflora generates metabolites, such as ammonia, hydrogen sulphide, and phenols, which are potential cytotoxins and carcinogens. [19]
Fat is an important source of energy, facilitates the absorption of fat-soluble vitamins, and is essential for cellular development and function. Humans cannot synthesise crucial fatty acids and must be supplied through the diet. A concerning issue with modifying fat composition diets is the undue importance of the n-6 and n-3 fatty acid ratio, as the current population intake of PUFA and especially n-3 PUFA are insufficient for optimal health. [3]
There is good evidence to suggest that hypercaloric hyperlipidic diets, particularly rich in saturated fatty acids, promote obesity, insulin resistance and metabolic syndrome. [3,12,20]
Furthermore, the consumption of n-3 PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) has demonstrated physiological benefits on blood pressure, heart rate, triglycerides, anti-inflammatory effects, endothelial function, and cardiac diastolic function. In addition, DHA also plays a significant role in the development of the brain and retina during foetal development. [12]
Conclusion
The literature on (Low Carbohydrate - High Fat) diets or (High Carbohydrate - Low Fat) diets has revealed inconsistent results and potentially numerous unfavourable effects. Furthermore, much of the current research reviewed overlooks examining the importance of nutrients and sources of fats, especially essential fatty acids.
Likewise, in the quality of carbohydrates and quantity of protein, with the importance of plant-based foods such as fruits, vegetables and whole-grain foods. Even though the short-term use of (Low Carbohydrate – High Fat) diets has been shown to result in weight loss.
The unlimited long-term clinical effects of these diets are essentially unknown. Therefore, increasing the risks and probability of morbidity and mortality of cardiovascular diseases, metabolic syndrome, and bowel-related or dietary-related diseases.
There is a need for larger and long-term studies of (Low Carbohydrate – High Fat) diets comparing (High Carbohydrate – Low Fat) diets, with follow-up periods needed to examine further the potential health effects in the general and/or specific populations.