What do low triglycerides mean?

Guidelines draw only upper lines. The European consensus calls fasting triglycerides under about 100 mg/dL (1.2 mmol/L)1.2 mmol/L (about 100 mg/dL) optimal, because below that, remnant particles do not build up [5]. The AHA also calls under 100 mg/dL optimal [6].

Population levels span a huge range, from about 30 mg/dL (0.33 mmol/L)0.33 mmol/L (30 mg/dL) at the bottom [5]. No guideline treats a low value as a problem by itself.

• Efficient clearance. Insulin resistance raises VLDL output and slows clearance [5], so insulin-sensitive, active people tend to run low; aerobic training lowers triglycerides on average [7].
• Carbohydrate restriction. In a survey of 548 adults on carbohydrate-restricted diets, a subgroup of lean people had triglycerides of 70 mg/dL or less together with HDL of 80 mg/dL or more [3].
• Protective genes. Carriers of APOC3 loss-of-function mutations had 39% to 44% lower triglycerides [2] [1]. Loss of ANGPTL3 lowers triglycerides, LDL and HDL together [5].
• Thyroid. An overactive thyroid is associated with low cholesterol and triglycerides, though treatment changed triglycerides far less than cholesterol [8]. TSH is the screening test.
• Rare inherited disorders. Mutations in APOB, PCSK9 or MTTP cause familial hypobetalipoproteinemia and abetalipoproteinemia, with low or absent ApoB and LDL [4].

Reading the whole panel is what separates a healthy low from an unusual one:

• Low triglycerides, normal or high LDL, high HDL. The common pattern in lean, active people and on low-carbohydrate diets [3]. Track LDL and ApoB as usual.
• Low triglycerides with very low LDL and ApoB. The pattern of familial hypobetalipoproteinemia, which can come with fatty liver [4]. A clinician can decide whether liver tests or genetic testing make sense.
• Low triglycerides with signs of an overactive thyroid. Hyperthyroidism is associated with low cholesterol and triglycerides [8], and TSH is the first test.

Because triglycerides move with meals, a single low value after a long fast or a low-fat day says less than a pattern across several tests [9].

Cohort and genetic data agree. People with nonfasting triglycerides under 90 mg/dL (1 mmol/L)1 mmol/L (90 mg/dL) had a hazard ratio of 0.40 for ischemic heart disease compared with 350 mg/dL (4 mmol/L)4 mmol/L (350 mg/dL) or more [1]. Carriers of APOC3 mutations had 40% lower coronary risk [2]. ApoC-III is now a drug target for that reason [10].

The exceptions are rare. In familial hypobetalipoproteinemia and abetalipoproteinemia, fatty liver, cirrhosis and liver cancer have been reported, probably because the liver cannot export triglycerides; the same truncated-ApoB variants carry a 72% lower coronary risk [4]. These disorders show up as very low LDL and ApoB, not just low triglycerides. Clinical features range from none to severe [4], so a clinician is the right person to judge an unusual pattern.

Researchers described a "lean mass hyper-responder" pattern in people on carbohydrate-restricted diets: LDL of 200 mg/dL or more, HDL of 80 mg/dL or more and triglycerides of 70 mg/dL or less. Lower body mass index and a lower triglyceride to HDL ratio predicted larger LDL rises, and reintroducing some carbohydrate lowered LDL in a case series [3].

Whether this LDL carries the usual risk is debated. Mainstream guidance treats LDL and ApoB as causal regardless of triglycerides [11]. The LDL cholesterol guide and ApoB guide explain what to track, and the keto and low-carb protocol keeps the LDL trend in view.