Background:
Being overweight or obese can be linked with multiple chronic conditions including cancers, coronary heart disease and stroke. Due to the rising levels of overweight and obesity, particularly in low- and middle-income countries undergoing rapid nutrition transition, there is a need to determine the effect of total fat intake on an individual’s body weight and how it may contribute to the development of obesity. Previous studies have investigated the causative relationships between total fat intake and body weight; however, these studies have been restricted by the duration (length of dietary fat restriction), quality and specificity (dairy versus non-dairy) of such dietary recommendations or have included individuals with the goal of weight loss who might already be limiting their overall energy intake. This hampers the overall understanding of how high- or low-fat intake in our diet influences the development of obesity or body fatness in the general population. Therefore, the role of long-term intake of dietary fat on the overall body fatness needs to be further examined and reviewed.
Purpose: This review assessed the effects of high or low fat-based energy intake on the measures of body fatness (including body weight, waist circumference, body mass index and percentage of body fat) in adults not aiming to lose weight.
Methods:
Search methods: Studies from several databases including CENTRAL, MEDLINE, Embase, Clinicaltrials.gov and the WHO International Clinical Trials Registry Platform (ICTRP) up to October 2019 were included.
Study Selection Criteria: Randomized controlled trials of at least six months duration which included participants not intending to lose weight. Studies were included if they had no confounding effects of non-nutritional interventions and assessed a measure of body fatness at 6 months.
Participants: Adults (at least 18 years of age) at any risk of cardiovascular disease (with or without existing cardiovascular disease). Participants who were acutely ill, pregnant or lactating were excluded.
Interventions: Studies comparing reduced dietary fat intake with that of higher (usual or modified fat) intake were included. A low-fat intake was defined as reduced fat intake to ≤ 30% energy (≤ 30%E) from fat, and at least partially replace the energy lost with carbohydrates (simple or complex), protein or fruit and vegetables. A high-fat diet included > 30% energy from total fats (usual diet or increasing modified types of fat- mono-unsaturated or poly-unsaturated fats).
Outcomes:
Primary outcome measures included body weight, body mass index, waist circumference, skinfold thickness and percentage fat. To understand these measures of body composition in detail, click here.
Secondary outcome measures included:
- Cardiovascular risk factors: systolic or diastolic blood pressure; serum total, low density lipoprotein (LDL) or high-density lipoprotein (HDL) cholesterol, and triglyceride
- Quality of life measures: informal outcomes such as feelings of health and time off work
Data collection and analysis: Data on the population, intervention, control and outcome measures were extracted. Random-effects meta-analyses, meta-regression, subgrouping, sensitivity, funnel plot analyses and GRADE assessment was conducted.
Results:
Main results table
| Main Outcomes (≥ 6 months) | Mean change in High-fat intake group (>30% energy from fat) | Mean change in Low-fat intake group (≤30% energy from fat) | Number of participants (studies) | Quality of evidence | Comments and recommendations | |
| 1. | Body weight (in kgs) | ↑ by 0.04 kg | ↓ by 1.42 kg (1.73 to 1.10) | 53,875 (26 RCTs) | High | Reducing total fat intake causes a small reduction in body fatness (weight, BMI, and waist circumference) |
| 2. | Body mass index, BMI (kg/m2) | ↓ by 0.14 kg/m2 | ↓ by 0.47 kg/m2 (0.64 to 0.30) | 46,604 (15 RCTs) | ||
| 3. | Waist circumference (cm) | ↑ by 0.6 cm | ↓ by 0.47 cm (0.73 to 0.22) | 16,685 (4 RCTs) | ||
| 4. | Body fat % | ↓ by 0.7 % | ↓ by 0.28%(0.57 to 0) | 2415 (3 RCTs) | ||
| 5. | Serum lipids mmol/L | Baseline: Total cholesterol = 5.5; LDL cholesterol = 4.0; HDL cholesterol = 1.4, Triglyceride (TG) = 1.3 | Relative to controls, total cholesterol ↓ by 0.23 mmol/L (95% CI -3.2 to -0.14), LDL cholesterol ↓ by 0.13 mmol/L (95% CI -0.21 to -0.05), HDL cholesterol ↓ by 0.02 mmol/L (95% CI -0.03 to 0.00), & TG ↑ by 0.01 mmol/L (95% CI -0.05 to 0.07). | Total chol: 9812 (22 RCTs) LDL chol: 8137 (19 RCTs) HDL chol: 8268 (20 RCTs) TG: 8672 (18 RCTs) | High | Reducing total fat intake leads to small reductions in total and LDL cholesterol, with little change in HDL cholesterol or TG. |
| 6. | Blood pressure (BP) mmHg | Systolic BP -1.2; Diastolic BP -0.9 | Relative to controls, SBP ↓ by 0.75 mmHg (95% CI -1.42to-0.07) & DBP ↓ by 0.52 mmHg (95% CI -0.95to-0.09) | SBP: 6078 (10 RCTs) DBP: 6078 (10 RCTs) | High | Reducing fat intake leads to small reductions in systolic and diastolic BP |
| 7. | Quality of life (QoL) | 0.03 | Relative to controls, QOL ↑ by 0.04 (95% CI 0.01to0.07) on a scale of 0 (worst) to 10 (best QOL) | 40,130 (1 RCT) | low | Reducing fat intake may lead to small rises in QoL |
- A total of 37 RCTs (57,079 participants) were included.
- High-quality evidence consistently suggests that reducing total fat intake results in small reductions in body fatness. Therefore, restricting energy from fat reduces energy intake slightly resulting in lower body weight.
- The effect of eating less fat (compared with higher fat intake) is a mean body weight reduction of 1.4 kg. Sub-analyses revealed that greater weight loss results from even lower fat consumption by people with lower fat intake at baseline, and people with higher body mass index (BMI) at baseline.
- Similar reductions in BMI (mean difference (MD) -0.5 kg/m2), waist circumference (MD -0.5 cm) and percentage body fat (MD -0.3%) were noted with less fat intake.
- Eating less fat also resulted in small reductions in LDL (-0.13 mmol/L), & total cholesterol (-0.23 mmol/L)
- Little or no effect on HDL cholesterol (-0.02 mmol/L), triglycerides (0.01 mmol/L), systolic (-0.75 mmHg) or diastolic blood pressure (-0.52 mmHg) were noted.
- Due to the selective study criteria most studies had low risk of bias providing confidence in the results of this review.
- Limitations of this review include eliminating combined dietary and exercise interventions, eliminating studies investigating weight loss, disregarding non-RCT types of study designs.
Conclusions:
- Studies suggest that a low-fat diet intake for ≥ 6 months leads to a consistent, stable but small effect on body fatness (slightly lower weight, BMI, waist circumference and body fat %) compared with high-fat intake studies.
- Greater diet fat reduction, lower baseline fat intake and higher baseline BMI were all associated with greater reductions in body weight.
- Small to no changes were seen in serum lipids, blood pressure or quality of life.
Citation:
Hooper L, Hooper L, Abdelhamid AS, Jimoh OF, Bunn D, Skeaff CM. Effects of total fat intake on body fatness in adults. Cochrane Database of Systematic Reviews. (6)
Reviewed by: Hina Garg, PT, MS, PhD (Physical therapist, MS PT & Wellness Director)