Effects of Saturated Fat, Polyunsaturated Fat, Monounsaturated Fat, and Carbohydrate on Glucose-Insulin Homeostasis: A Systematic Review and Meta-analysis of Randomised Controlled Feeding Trials

BACKGROUND

Effects of major dietary macronutrients on glucose-insulin homeostasis remain controversial and may vary by the clinical measures examined. We aimed to assess how saturated fat (SFA), monounsaturated fat (MUFA), polyunsaturated fat (PUFA), and carbohydrate affect key metrics of glucose-insulin homeostasis.

METHODS AND FINDINGS

We systematically searched multiple databases (PubMed, EMBASE, OVID, BIOSIS, Web-of-Knowledge, CAB, CINAHL, Cochrane Library, SIGLE, Faculty1000) for randomised controlled feeding trials published by 26 Nov 2015 that tested effects of macronutrient intake on blood glucose, insulin, HbA1c, insulin sensitivity, and insulin secretion in adults aged ≥18 years. We excluded trials with non-isocaloric comparisons and trials providing dietary advice or supplements rather than meals. Studies were reviewed and data extracted independently in duplicate. Among 6,124 abstracts, 102 trials, including 239 diet arms and 4,220 adults, met eligibility requirements. Using multiple-treatment meta-regression, we estimated dose-response effects of isocaloric replacements between SFA, MUFA, PUFA, and carbohydrate, adjusted for protein, trans fat, and dietary fibre. Replacing 5% energy from carbohydrate with SFA had no significant effect on fasting glucose (+0.02 mmol/L, 95% CI = -0.01, +0.04; n trials = 99), but lowered fasting insulin (-1.1 pmol/L; -1.7, -0.5; n = 90). Replacing carbohydrate with MUFA lowered HbA1c (-0.09%; -0.12, -0.05; n = 23), 2 h post-challenge insulin (-20.3 pmol/L; -32.2, -8.4; n = 11), and homeostasis model assessment for insulin resistance (HOMA-IR) (-2.4%; -4.6, -0.3; n = 30). Replacing carbohydrate with PUFA significantly lowered HbA1c (-0.11%; -0.17, -0.05) and fasting insulin (-1.6 pmol/L; -2.8, -0.4). Replacing SFA with PUFA significantly lowered glucose, HbA1c, C-peptide, and HOMA. Based on gold-standard acute insulin response in ten trials, PUFA significantly improved insulin secretion capacity (+0.5 pmol/L/min; 0.2, 0.8) whether replacing carbohydrate, SFA, or even MUFA. No significant effects of any macronutrient replacements were observed for 2 h post-challenge glucose or insulin sensitivity (minimal-model index). Limitations included a small number of trials for some outcomes and potential issues of blinding, compliance, generalisability, heterogeneity due to unmeasured factors, and publication bias.

CONCLUSIONS

This meta-analysis of randomised controlled feeding trials provides evidence that dietary macronutrients have diverse effects on glucose-insulin homeostasis. In comparison to carbohydrate, SFA, or MUFA, most consistent favourable effects were seen with PUFA, which was linked to improved glycaemia, insulin resistance, and insulin secretion capacity.

[Effect of fats on cardiovascular disease prevention in Denmark]

In Denmark death from cardiovascular disease (CVD) has decreased, mainly due to a 72% reduction since 1990 in death from ischaemic heart disease from reduced smoking, elimination of industrial trans fatty acids in the diet, and more effective medical treatment. Replacement of saturated fat by carbohydrate and/or n-6 polyunsaturated fat may increase CVD, but it is reduced by substitution with n-3 fats, monounsaturated fat, or low glycaemic index carbohydrates. Despite a high saturated fat content dark chocolate and cheese may reduce CVD and diabetes risk and eggs may be neutral, and less restrictive dietary recommendations are indicated.

Saturated fat, carbohydrates and cardiovascular disease

The dietary intake of saturated fatty acids (SAFA) is associated with a modest increase in serum total cholesterol, but not with cardiovascular disease (CVD). Replacing dietary SAFA with carbohydrates (CHO), notably those with a high glycaemic index, is associated with an increase in CVD risk in observational cohorts, while replacing SAFA with polyunsaturated fatty acids (PUFA) is associated with reduced CVD risk. However, replacing a combination of SAFA and trans-fatty acids with n-6 PUFA (notably linoleic acid) in controlled trials showed no indication of benefit and a signal toward increased coronary heart disease risk, suggesting that n-3 PUFA may be responsible for the protective association between total PUFA and CVD. High CHO intakes stimulate hepatic SAFA synthesis and conservation of dietary SAFA . Hepatic de novo lipogenesis from CHO is also stimulated during eucaloric dietary substitution of SAFA by CHO with high glycaemic index in normo-insulinaemic subjects and during hypocaloric high-CHO/low-fat diets in subjects with the metabolic syndrome. The accumulation of SAFA stimulates chronic systemic low-grade inflammation through its mimicking of bacterial lipopolysaccharides and÷or the induction of other pro-inflammatory stimuli. The resulting systemic low-grade inflammation promotes insulin resistance, reallocation of energy-rich substrates and atherogenic dyslipidaemia that concertedly give rise to increased CVD risk. We conclude that avoidance of SAFA accumulation by reducing the intake of CHO with high glycaemic index is more effective in the prevention of CVD than reducing SAFA intake per se.

Dietary fat, fat subtypes, and breast cancer in postmenopausal women: a prospective cohort study

BACKGROUND

The intake of total dietary fat and of certain fat subtypes has been shown to be strongly associated with breast cancer in international comparisons and in animal experiments. However, observational epidemiologic studies have generally reported either weak positive or no associations. To extend the prospective epidemiologic evidence on this question, we examined the association between adult dietary intake of fat, fat subtypes, and breast cancer in a large, prospective cohort of postmenopausal women.

METHODS

Participants were selected from a national breast cancer mammography screening program conducted from 1973 through 1981 at 29 centers throughout the United States. From 1987 through 1989, 40022 postmenopausal women satisfactorily completed a mailed, self-administered questionnaire that included a 60-item National Cancer Institute/Block food-frequency questionnaire. Women were then followed for an average of 5.3 years; 996 women developed breast cancer. Risk was assessed by use of Cox proportional hazard regression, with age as the underlying time metric. All statistical tests were two-sided.

RESULTS

Compared with women in the lowest quintile (Q1) of percentage of energy from total fat, the adjusted risk ratio (RR) and 95% confidence interval (CI) for women in the highest quintile (Q5) was 1.07 (95% CI = 0.86-1.32). In analyses stratified by history of benign breast disease (BBD), a positive association was observed among only women with no history of BBD (RR (Q5 versus Q1) = 2.20; 95% CI = 1.41-3.42; test for trend, P =.0003). The increased risk in these women appeared to be attributable to unsaturated fat intake and oleic acid in particular.

CONCLUSIONS

In this study, there was no overall association between fat intake during adulthood and breast cancer risk; however, among women with no history of BBD, there appeared to be a positive association between total and unsaturated fat intake and breast cancer risk.

A prospective study of association of monounsaturated fat and other types of fat with risk of breast cancer

BACKGROUND

Animal studies suggest that monounsaturated and polyunsaturated fat may have opposite effects on the risk of breast cancer.

METHODS

We performed a population-based prospective cohort study, including 61,471 women aged 40 to 76 years from 2 counties in central Sweden who did not have any previous diagnosis of cancer; 674 cases of invasive breast cancer occurred during an average follow-up of 4.2 years. All subjects answered a validated 67-item food frequency questionnaire at baseline. Cox proportional hazards models were used to obtain adjusted rate ratio (RR) estimates with 95% confidence intervals (CIs).

RESULTS

After mutual adjustment of different types of fat, an inverse association with monounsaturated fat and a positive association with polyunsaturated fat were found. The RR for each 10-g increment in daily intake of monounsaturated fat was 0.45 (95% CI, 0.22-0.95), whereas the RR for a 5-g increment of polyunsaturated fat was 1.69 (95% CI, 1.02-2.78); the increments correspond to approximately 2 SDs of intake in the population. Comparing the highest quartile of intake with the lowest, we found an RR of 0.8 (95% CI, 0.5-1.2) for monounsaturated fat and 1.2 (95% CI, 0.9-1.6) for polyunsaturated fat. Saturated fat was not associated with the risk of breast cancer.

CONCLUSIONS

Our results indicate that various types of fat may have specific opposite effects on the risk of breast cancer that closely resemble the corresponding effects in experimental animals. Research investigations and health policy considerations should take into account the emerging evidence that monounsaturated fat might be protective for risk of breast cancer.

[Physico-chemical characteristics of different types of vegetable fats and oils used in the manufacture of candies]

The quality of three vegetable fats (cocoa butter and two commercial fats) and three roasted nut oils (almond, hazelnut and peanut) used as raw material in the chocolate products manufacturing was studied. The hydroperoxide content, oxidative stability and fatty acid composition were determined and its health repercussion by atherogenicity and thrombogenicity indexes. Two commercial fats and cocoa butter showed higher oxidative stability, atherogenic and thrombogenic properties than oils because of its different fatty acid profiles. Peroxide value was a low reliability parameter of raw material shelf live. Rancimat presented a good correlation with the unsaturation index of different fats and oils, it was a better index than peroxide value. In the chocolate products manufacturing it would be advisable a good raw material selection and formulation in order to get a balance between technological properties, organoleptic qualities and the influence on the health. Those raw material with less primary oxidation and higher oxidative stability were also those of higher atherogenicity and thrombogenicity indexes.

A fish oil diet inhibits colon cancer in mice

High-fat, high-cholesterol diets have been linked to colon cancer in both epidemiological and animal studies. Saturated and N-6 polyunsaturated fats have both been implicated as playing causative roles. Epidemiological studies have shown a reduced colon cancer incidence in populations consuming a large quantity of N-3 polyunsaturated fat. This study asked whether N-3 polyunsaturated fat found in fish oil would also be associated with reduced colon cancer in animal studies. 1,2-Dimethylhydrazine was used to induce colon cancer in mice fed three high-fat, high-cholesterol diets (beef tallow, safflower oil, and fish oil) and one low-fat, cholesterol-free diet (soybean oil). Colon adenocarcinomas developed in 55% of mice fed tallow, 48% of those fed low-fat diets, 33% of those fed safflower oil, and 18% of those fed fish oil (p less than 0.05). Tumors per animal were also greatest in mice fed tallow and fewest in those fed fish oil (p less than 0.05). Plasma cholesterol levels were significantly higher in mice fed tallow than in mice fed fish oil, but this did not show any association with tumors. Plasma triglyceride levels were not significantly different among groups but were strongly correlated with colon cancers (r = 0.90, p less than 0.025). In both plasma and colon mucosa cells, saturated fat levels were similar. Monounsaturated fat was highest in plasma of mice fed tallow and correlated strongly with colon cancers (r = 0.84, p less than 0.005). N-6 polyunsaturated fat was highest in plasma and colon mucosa cells of both mice fed safflower oil and those fed low-fat (soybean oil) diet, but there was no association with tumors. N-3 polyunsaturated fat was highest in plasma and colon mucosa cells of mice fed fish oil and showed a negative association with tumors (r = -0.57, p less than 0.05). Thus, in this model of colon cancer, a diet high in fish oil was associated with less colon cancer. Monounsaturated fat was most strongly correlated with tumors. This study suggests monounsaturated fat promotes colon tumors and N-3 polyunsaturated fat inhibits colon tumors.

Melanoma and dietary lipids

Samples of subcutaneous adipose tissue were taken from 100 melanoma patients and 100 matched controls in Sydney in 1984-1985 and were analyzed for constituent fatty acids. The mean percentage of linoleic acid in the triglycerides of the subcutaneous adipose tissue (PLASAT) of these subjects was substantially higher than that in a similar group examined in 1975-1976. In addition, the percentage of polyunsaturated fatty acids was found to be higher in the melanoma patients than in the controls (p less than 0.01), and there were significantly more controls than patients who had a low PLASAT (p less than 0.01). Relevant literature is quoted and the suggestion is made that increased consumption of dietary polyunsaturates may have a contributory effect in the etiology of melanoma.

Fat and cancer

Experimental studies and epidemiologic data both indicate that high-fat diets increase the risk of cancer at sites such as breast, colon, and pancreas. Dietary polyunsaturated vegetable oils promote tumorigenesis in animals whereas saturated fats and polyunsaturated fish oils either have little effect or are inhibitory. A blend of fats typical of the American diet enhanced mammary tumorigenesis effectively when fed as 40% of calories. After 9 to 10 weeks on this diet, reducing the fat to 10% of calories inhibited subsequent tumor development. Although dietary fat appears to act as a promoter, the exact mechanisms are still unknown. Cancer mortality in humans correlates better with total dietary fat than with degree of unsaturation, perhaps because most diets contain more than the amount of polyunsaturated fatty acids required for maximum effect on tumorigenesis in animals. A reduction in total dietary fat is recommended to reduce the current high cancer mortality.

The advisability of the prudent diet in adolescence

Risk factor status for cardiovascular disease is affected by life style. Adolescence is a time during which long term life-style habits, including dietary habits, are established. Physicians who treat adolescent patients have a responsibility to be aware of the scientific evidence on the diet-heart question so that they can provide their patients with sound dietary advice. The American Heart Association has recommended that Americans consume a "prudent diet" in which daily consumption of cholesterol is no more than 300 mg with up to 30-35% of calories derived from fat, and less than 10% of calories derived from saturated fat and less than 10% from polyunsaturated fat. This paper reviews this recommendation with particular reference to studies of adolescents. This review centers around four main issues: 1) the estimated effect on serum cholesterol levels of a switch from the usual American diet to the prudent diet; 2) the effect of a predicted decrease in serum cholesterol on the risk of developing cardiovascular disease; 3) evaluation of the evidence of possible adverse effects of the prudent diet; 4) feasibility of the prudent diet. Based on a review of these four issues, the authors feel that the American Heart Association's prudent diet should be strongly recommended for all healthy adolescents.

Cholesterol controversy--where do we go from here? 'Toward healthful diets' reevaluated

The document, entitled 'Toward Healthful Diets' that was developed by the Food and Nutrition Board of the National Academy of Sciences last year, has again stirred major controversy among the general public and in the medical community regarding the diet-heart hypothesis as a basis for efforts to favorably modify the natural history of atherosclerotic vascular disease in the American population. In this rebuttal to the conclusion drawn in 'Toward Healthful Diets', we review recent evidence in support of the diet-heart hypothesis. We conclude that continued efforts to reduce dietary cholesterol and saturated fat, in the context of efforts to eliminate other reversible risk factors as well, should remain an important goal for individuals and physicians seeking to limit premature death or disability fom atherosclerotic vascular disease.

Metabolic activation of DMH by colonic microsomes: a process influenced by type of dietary fat

A study was conducted to determine whether 1,2-dimethylhydrazine (DMH), a potent colon carcinogen, is activated to a more potent mutagen by the drug-metabolizing system of the colonic mucosa and to determine the extent to which this metabolism is modified by lipids in the diet. DMH-treated rats fed a diet enriched with 10% corn oil exhibited markedly elevated colonic enzyme levels for mutagen production. This diet also produced the greatest number of animals with colon tumors, when compared with diets containing other levels and sources of lipid. The potential risk of a diet in which unsaturated fat is the sole source of lipid is underlined.

Diet and coronary disease: a survey of mortality rates and food consumption statistics of 24 countries

Direct, linear and reasonably accurate correlation has been found between coronary heart disease (CHD) mortality rates and the consumption of unfermented milk proteins--namely the protein content of all dairy products with the only important exception of cheese--in a study of male mortality rates and food consumption statistics of 24 countries. Thus in Finland, with the highest CHD mortality amongst these countries, the consumption of unfermented mile proteins is also higher than in the other countries, 30.4 g/day, corresponding to the protein content of 0.85 l of whole milk. In Germany, Yugoslavia and Japan male CHD mortality is approximately a half, a quarter and a tenth of that in Finland. So is the consumption of milk proteins, 14.1, 8.1 and 2.5 g respectively. Multivariate analysis on the combined effect of milk proteins and other possible atherogenic agents, like sugar, saturated fats and cigarette smoking, is performed, the results tending to confirm the leading role of milk proteins, attributing only an adjuvant effect to co-factors. Possible theoretical interpretation of the results is briefly discussed.

Dietary polyunsaturated fats and malignant melanoma

The consumption of polyunsaturated fat of a group of patients with malignant melanoma was compared to that of various groups of control subjects to determine whether such dietary fats predispose to the development of malignant melanoma. The results are tabulated with reference to various associated factors, and a statistical assessment of their significance is presented. The writers were unable to agree on the interpretation of the results, and present two opposing views on whether the ingestion of polyunsaturated fat is associated with the incidence of malignant melanoma.

Fats and atheroma: a retrial

The controversy over medical endorsement of dietary measures to reduce cholesterol intake has been reconsidered. The results of several published reports that apparently do not confirm the association between diet, cholesterol concentrations, and ischaemic heart disease (IHD) were found to be largely inapplicable to the argument. Results of primary prevention trials, however, suggested that lowering the cholesterol concentration had a beneficial effect in reducing morbidity from IHD. The "average Western diet" is particularly associated with accelerated or premature atherosclerotic disease, yet the saturated fatty acid component of the diet may be only one of several factors relevant to IHD. Such diets are usually high in refined carbohydrate and total energy intake. Disordered nutrition generally, and other environmental and constitutional factors seem to be important in the aetiology of IHD. A prudent diet, incorporating decreased intake of fats, simple sugars, and refined carbohydrate, with polyunsaturated fats comprising less than 25% of total energy intake, may be the best method of reducing the incidence of IHD and other diseases of overnutrition.

Fats and atheroma: an inquest

All well-controlled trials of cholesterol-reducing diets and drugs have failed to reduce coronary (CHD) mortality and morbidity. Nevertheless, commercial, professional, and even government-sponsored propaganda continues. Experimentally some vegetable oils and hardened fats can be more damaging to arteries than butter. There are other hazards to heart muscle from vegetable oils. Israelis consume a high polyunsaturated fat diet equal to that recommended for prevention of CHD in USA but their CHD incidence is very high. Urban Bedouins are also affected. The primary clofibrate prevention trial underlines unacceptable risks which could apply also to diets. Official medical endorsement of these cholesterol reducing measures should be withdrawn.

Carcinogenesis induced by 7,12-dimethylbenz[a]anthracene in c3H-A vyfB mice: influence of different dietary fats

The effect of a diet containing either sunflower-seed oil (polyunsaturated fat diet) or tallow (saturated fat diet) on 7,12-dimethylbenz[a]anthracene (DMBA)-induced carcinogenesis in C3H-A vyfB mice was examined. After receiving either diet for 28 days, some of the mice were given an intragastric dose of 5 mg DMBA. To identify the stage of carcinogenesis that might be influenced by dietary fat, the diets of half of the mice were then interchanged so that those previously fed the saturated fat diet were fed the polyunsaturated fat diet and vice versa. The cumulative incidence of tumor-bearing mice was significantly greater among the females fed the polyunsaturated fat diet compared to those fed the saturated fat diet. This enhancement of carcinogenesis was observed only when the mice were fed the polyunsaturated fat diet after DMBA administration. Similar trends were observed in the male mice, but these mice developed fewer tumors and none of the differences between the tumor incidences were statistically significant. The most common sites for tumors in the male mice were the liver, lungs, and skin, whereas those for tumors in the females were the mammary glands and ovaries. The differences in tumor incidence suggest that carcinogenesis was enhanced by the polyunsaturated fat diet during the promotion stage of carcinogenesis.

The nutritional epidemiology of cardiovascular disease

A basic review of the extensive literature focusing on the major risk factors of atherosclerotic coronary heart disease and stroke, i.e., elevation of blood lipids related to diet, blood pressure elevation, and genetic factors using the traditional epidemiological model of interaction between host, agent, and environment, has strongly supported the concept that diet and particularly saturated fat and/or cholesterol are significant contributors to the elevation of blood lipids, especially cholesterol, and contribute importantly to the premature development and mortality of atherosclerotic coronary heart disease. Certainly genetics exert an important impact on this process. To date it remains unclear whether or not major changes in the dietary pattern of huge population groups can be practically effected. The minor dietary modifications so far studied in the average atherosclerosis-prone population cannot be anticipated to make a major dent in the epidemic proportions of atherosclerotic coronary heart disease. It is quite clear that prospective preventive medicine must be implemented at a very early age in the pediatric age group, in which atherosclerosis is now recognized by many as the number one pediatric problem. Tremendous biochemical advances have provided new insights in knowledge regarding the transport of blood lipids, particularly cholesterol, and the regulatory mechanisms at the cellular level for cholesterol under normal circumstances and in the genetic influenced hyperlipidemias (TABLE 4). A bright future lies ahead for the reduction of the epidemic of atherosclerosis which could be greatly enhanced by a greater personal responsibility for health care and a much more careful and prudent diet selection and exercise managment.

Effect of dietary polyunsaturated fat on the growth of a transplantable adenocarcinoma in C3HAvyfB mice

Weaning male and female C3HAvyfB mice were fed a low-fat (4.5%) diet until they were 60-70 days of age when they were fed high-fat (18.6%) diets containing either sunflower-seed oil (polyunsaturated fat diet) or tallow (saturated fat diet). After receiving either of the high-fat diets for 4 weeks, each mouse received an inoculum of approximately 1,700 single cells from a transplantable mammary adenocarcinoma. The cumulative incidence of tumor-bearing mice was significantly greater among both males and females fed the polyunsaturated fat diet than among males and females fed the saturated fat diet. The mean times elapsed before palpable tumors developed were less when mice were fed the polyunsaturated fat diet than when mice were fed the saturated fat diet, but these differences were not statistically significant. The cumulative incidence of tumor-bearing mice was also significantly greater among females than males. The results supported the suggestion from previous work in this laboratory that the polyunsaturated fat diet exerts its effect on the promotional stage of carcinogenesis rather than on the initial event of neoplastic transformation.

Do polyunsaturated fats predispose to malignant melonoma?

The consumption of polyunsaturated fats of patients with malignant melanoma and of a control group of patients with benign naevi has been compared in order to determine whether polyunsaturated fats predispose to the development of malignant melanoma. Patients with malignant melanoma had not consumed excessive quantities of polyunsaturated fats. Thus, there was no evidence to indicate that ingestion of polyunsaturated fats is associated with an increase in incidence of melanoma.

Effect of dietary fats on the incidence of 7,12-dimethylbenz(a)-anthracene-induced tumors in rats

Female rats have been fed high fat diets containing either polyunsaturated or saturated fat. After being fed either of the diets for 4 weeks, some of the animals received an intragastric dose of 7,12-dimethylbenz(a)anthracene (DMBA). At this point, the diets of half of the animals were interchanged so that animals previously fed the polyunsaturated fat diet were fed the saturated fat diet and vice versa. The cumulative incidence of tumor-bearing rats among DMBA-dosed rats was greater when the polyunsaturated fat diet was fed. The mean induction time of tumors decreased and the proportion of tumor-bearing rats which developed malignant tumors increased when the polyunsaturated fat diet was fed. This promotional effect of the polyunsaturated fat diet was exerted only when the diet was fed after DMBA administration.

The limitations of statistical correlations in pathology

Many hypotheses as to the causes of disease, for example some of those for coronary heart disease, depend on statistical correlations. It is well known that correlations do not prove a cause-and-effect relationship, yet it is often tempting to ignore this axiom.